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Heat Transfer - Questions & Answers

A. Objective Questions :


1. Write true or false for each statement :

(a) Evaporation is rapid on a wet day.
Answer: False

(b) Evaporation takes place only from the surface of a liquid.
Answer: True

(c) All molecules of a liquid take part in the process of evaporation.
Answer: False

(d) Temperature of a liquid rises during boiling or vaporization.
Answer: False

(e) All molecules of a liquid take part in boiling.
Answer: True

(f) Boiling is a rapid phenomenon.
Answer: True

(g) All solids expand by the same amount when heated to the same rise in temperature.
Answer: False

(h) Telephone wires are kept tight between the two poles in winter.
Answer: True

(i) Equal volumes of different liquids expand by different amounts when they are heated to the same rise in temperature.
Answer: True

(j) Solids expand the least and gases expand the most on being heated.
Answer: True

(k) A mercury thermometer makes use of the property of expansion of liquids on heating.
Answer: True

(l) Kerosene contracts on heating.
Answer: False

2. Fill in the blanks :

(a) Boiling occurs at a fixed temperature.

(b) Evaporation takes place at all temperatures.

(c) The molecules of liquid absorb heat from surroundings in evaporation.

(d) Heat is internal energy during boiling.

(e) Cooling is produced in evaporation.

(f) A longer rod expands more than a shorter rod on being heated to the same temperature.

(g) Liquids expand more than the solids.

(h) Gases expand more than the liquids.

3. Match the following :

(a) Blowing air
Answer: (v) evaporation

(b) Increase in pressure increases
Answer: (iv) boiling point

(c) Thermal expansion
Answer: (i) increase in inter-molecular separation

(d) Invar
Answer: (ii) pendulum of a clock

(e) Pyrex glass
Answer: (iii) cooking utensils

4. Select the correct alternative :

(a) In evaporation :
(i) all molecules of liquid begin to escape out
(ii) only the molecules at the surface escape out
(iii) the temperature of liquid rises by absorbing heat from surroundings.
(iv) the molecules get attracted within the liquid.
Answer: (ii) only the molecules at the surface escape out

(b) The rate of evaporation of a liquid increases when :
(i) temperature of liquid falls
(ii) liquid is poured in a vessel of less surface area
(iii) air is blown above the surface of liquid
(iv) humidity increases.
Answer: (iii) air is blown above the surface of liquid

(c) During boiling or vaporization :
(i) all molecules take part
(ii) temperature rises
(iii) no heat is absorbed
(iv) the average kinetic energy of molecules increases.
Answer: (i) all molecules take part

(d) The boiling point of a liquid is increased by:
(i) increasing the volume of liquid
(ii) increasing the pressure on liquid
(iii) adding ice to the liquid
(iv) decreasing pressure on liquid.
Answer: (ii) increasing the pressure on liquid

(e) Two rods A and B of the same metal, but of length 1 m and 2 m respectively, are heated from 0°C to 100°C. Then :
(i) both the rods A and B elongate the same
(ii) the rod A elongates more than the rod B
(iii) the rod B elongates more than the rod A
(iv) the rod A elongates, but the rod B contracts.
Answer: (iii) the rod B elongates more than the rod A

(f) Two rods A and B of the same metal, same length, but one solid and the other hollow, are heated to the same rise in temperature. Then :
(i) the solid rod A expands more than the hollow rod B
(ii) the hollow rod B expands more than the solid rod A
(iii) the hollow rod B contracts, but the solid rod A expands
(iv) both the rods A and B expand the same.
Answer: (iv) both the rods A and B expand the same.

(g) A given volume of alcohol and the same volume of water are heated from the room temperature to the same temperature then :
(i) alcohol contracts, but water expands
(ii) water contracts, but alcohol expands
(iii) water expands more than alcohol
(iv) alcohol expands more than water.
Answer: (iv) alcohol expands more than water.

(h) The increase in length of a metal rod depends on :
(i) the initial length of the rod only
(ii) the rise in temperature only
(iii) the material of rod only
(iv) all the above three factors.
Answer: (iv) all the above three factors.

(i) The correct statement is :
(i) Iron rims are cooled before they are placed on the cart wheels.
(ii) A glass stopper gets tightened on warming the neck of the bottle.
(iii) Telephone wires sag in winter, but become tight in summer.
(iv) A little space is left between two rails on a railway track.
Answer: (iv) A little space is left between two rails on a railway track.

B. Short/Long Answer Questions :

1. What is matter ? What is it composed of ?
Answer: Matter is the substance which occupies space and has mass. It is composed of tiny particles called molecules.

2. Name the three states of matter and distinguish them on the basis of their (i) volume, and (ii) shape
Answer: The three states of matter are solid, liquid, and gas.
(i) Volume: Solids and liquids have a definite volume, whereas gases do not have a definite volume.
(ii) Shape: Solids have a definite shape. Liquids and gases do not have a definite shape; they take the shape of the container.

3. Distinguish between liquid and vapour (or gas) states of matter on the basis of following factors :
(a) Arrangement of molecules
(b) Inter-molecular separation
(c) Inter-molecular force, and
(d) Kinetic energy of molecules
Answer:
(a) Arrangement: In liquids, molecules are less tightly packed than solids but can move within the boundary. In gases, molecules are free to move anywhere in the available space.
(b) Separation: Inter-molecular separation is large in liquids compared to solids, but it is largest (maximum) in gases.
(c) Force: Inter-molecular force is weak in liquids, while it is negligible (weakest) in gases.
(d) Kinetic Energy: Molecules in a liquid have higher kinetic energy than solids, but gas molecules have the highest kinetic energy.

4. What is evaporation ? Explain it on the basis of molecular motion.
Answer: The change of state from liquid to gas at all temperatures from the surface of the liquid is called evaporation. On the basis of molecular motion: Molecules in a liquid are in continuous motion. Some molecules with higher kinetic energy reach the surface and overcome the attractive forces of other molecules, escaping into space to form vapour.

5. Do all the molecules of a liquid take part in evaporation ? If not, explain your answer.
Answer: No, all molecules do not take part. Evaporation is a surface phenomenon. Only the molecules near the surface of the liquid that acquire sufficient kinetic energy to overcome the attractive forces of other molecules escape out into space.

6. No heat is supplied to a liquid during evaporation. How does then the liquid change into its vapour ?
Answer: During evaporation, the molecules at the surface absorb the necessary heat (latent heat) from the liquid itself or its surroundings to increase their kinetic energy and escape as vapour.

7. Comment on the statement ‘evaporation is a surface phenomenon’.
Answer: Evaporation is called a surface phenomenon because it occurs only at the surface of the liquid. Only the molecules present at the surface which possess high kinetic energy are able to break the forces of attraction and escape into the atmosphere.

8. Why is cooling produced when a liquid evaporates ?
Answer: In the process of evaporation, liquid changes to vapour and requires heat energy. If no external heat is supplied, the liquid draws the necessary heat from its surroundings (or the liquid itself). This loss of heat from the surroundings causes the temperature to drop, producing cooling.

9. Give reason for the increase in rate of evaporation of a liquid when
(a) air is blown above the liquid
(b) surface area of liquid is increased
(c) temperature of liquid is increased.
Answer:
(a) Blowing air takes away the molecules of liquid escaping out of the surface, allowing more molecules to escape, thus increasing the rate.
(b) Increasing surface area increases the number of molecules exposed to the surface, allowing more molecules to escape simultaneously.
(c) Increasing temperature increases the average kinetic energy of the molecules, so more molecules acquire enough energy to escape the surface.

10. What is boiling ? Explain it on the basis of molecular motion.
Answer: The change from liquid state to gaseous state on heating at a constant temperature is called boiling.
Basis of molecular motion: When heated, the average kinetic energy of liquid molecules increases. Molecules throughout the whole volume of the liquid acquire sufficient energy to overcome the force of attraction and form bubbles of vapour that rise to the surface and escape.

11. Why does bubbles appear when a liquid is heated ?
Answer: When a liquid is heated, the average kinetic energy of the molecules increases. This leads to the rapid formation of vapour from all parts of the liquid (not just the surface), indicated by the presence of visible bubbles of vapour which grow in size and rise to the surface.

12. What is the change in average kinetic energy of molecules of a liquid during boiling at its boiling point ?
Answer: During boiling at the boiling point, the temperature remains constant. Since average kinetic energy is a measure of temperature, there is no change in the average kinetic energy of the molecules; instead, the heat energy increases the potential energy as the state changes.

13. How is the heat energy supplied to a liquid used during boiling at a fixed temperature ?
Answer: The heat energy supplied during boiling at a fixed temperature is used as latent heat to change the state of the substance from liquid to vapour. It works to overcome the inter-molecular forces of attraction and increase the potential energy of the molecules, without increasing the temperature.

14. Name two ways of changing liquid state to the vapour state and distinguish them.
Answer: The two ways are Evaporation and Boiling.
Distinction:
1. Evaporation takes place at all temperatures, while boiling takes place at a fixed temperature (boiling point).
2. Evaporation is a surface phenomenon, while boiling involves the entire volume of the liquid.
3. Evaporation is a slow process, while boiling is a rapid process.

15. What do you understand by thermal expansion of a substance ?
Answer: Thermal expansion is the increase in the size (length, area, or volume) of a substance (solid, liquid, or gas) on heating. When a substance is cooled, it contracts.

16. Give two examples of the substances which expand on heating.
Answer: Iron and Mercury.

17. Describe an experiment to demonstrate the thermal expansion in solids.
Answer: (Ball and Ring Experiment): Take a metal ball and a ring where the ball just passes through the ring at room temperature. Heat the ball over a burner. Try to pass the ball through the ring again; it will not pass. This shows that the ball expanded on heating. Allow the ball to cool, and it will pass through the ring again, showing contraction on cooling.

18. State three factors on which depend the linear expansion of a metal rod on heating.
Answer: The linear expansion depends on:
1. The original length of the rod.
2. The increase in temperature.
3. The material of the rod.

19. Two iron rods — one 10 m long and the other 5 m long, are heated to the same rise in temperature. Which will expand more ?
Answer: The 10 m long rod will expand more. (Reason: Linear expansion is directly proportional to the original length).

20. Two identical rods of copper are heated to different temperatures — one by 5°C and the other by 10°C. Which rod will expand more ?
Answer: The rod heated by 10°C will expand more. (Reason: Linear expansion is directly proportional to the rise in temperature).

21. One rod of copper and another identical rod of iron are heated to the same rise in temperature. Which rod will expand more ? Give reason.
Answer: The copper rod will expand more. Reason: Copper has a higher coefficient of linear expansion than iron.

22. Two identical rods — one hollow and the other solid, are heated to the same rise in temperature. Which will expand more ?
Answer: Both rods will expand to the same extent. The expansion depends on the length, temperature rise, and material, but does not depend on whether the rod is solid or hollow.

23. In the ball and ring experiment, if the ball after heating is left to cool on the ring for some time, the ball again passes through the ring. Explain the reason.
Answer: On cooling, the metal ball contracts (its size decreases). As it loses heat to the surroundings, its diameter reduces back to its original value, allowing it to pass through the ring again.

24. Explain the following :
(a) The telephone wires break in winter.
(b) Iron rims are heated before they are fixed on the wooden wheels.
(c) The gaps are left between the successive rails on a railway track.
(d) A glass stopper stuck in the neck of a bottle can be removed by pouring hot water on the neck of the bottle.
(e) A cement floor is laid in small pieces with gaps in between.
Answer:
(a) In winter, the atmospheric temperature falls, causing the wires to contract. If they are already tight, this contraction can produce enough tension to break them.
(b) The iron rim is made slightly smaller than the wooden wheel. On heating, the rim expands and fits over the wheel. On cooling, it contracts and grips the wheel firmly.
(c) This is to allow space for the expansion of rails during summer due to the rise in atmospheric temperature. If no gap is left, the rails may bend sideways and cause accidents.
(d) Pouring hot water heats the neck of the bottle, causing it to expand. This creates more space, loosening the stopper so it can be removed.
(e) The gaps allow space for the expansion of the cement floor during hot weather, preventing cracking or buckling.

25. Why is one end of a steel girder in a bridge kept on rollers instead of fixing it in pillar ?
Answer: One end is kept on rollers to allow the girder to expand (in summer) or contract (in winter) freely with changes in temperature without damaging the pillars or the bridge structure.

26. A metal plate is heated. State three factors on which the increase in its area will depend.
Answer: The increase in area (superficial expansion) depends on:
1. The initial area of the plate.
2. The increase in temperature.
3. The material of the plate.

27. A cubical metal solid block is heated. How will its volume change ?
Answer: Its volume will increase. This is known as cubical or volume expansion.

28. Describe an experiment to show that liquids expand on heating.
Answer: Take a bottle filled completely with water and fitted with a cork having a drinking straw. Add a few drops of ink to make the water visible. Mark the initial water level in the straw. Heat the bottle using a burner. You will observe that the water level in the straw rises. This shows that the water (liquid) expands on heating.

29. State one application of thermal expansion of liquids.
Answer: The working of a mercury thermometer is based on the thermal expansion of liquids (mercury expands when in contact with a hot body).

30. Describe an experiment to show that air expands on heating.
Answer: Take an empty bottle and attach a deflated rubber balloon to its mouth. Place the bottle in a water bath containing boiling water. The heat from the water causes the air inside the bottle to expand, which inflates the balloon. This demonstrates that air (gas) expands on heating.

31. An empty glass bottle is fitted with a narrow tube at its mouth. The open end of the tube is kept in a beaker containing water. When the bottle is heated, bubbles of air are seen escaping into water. Explain the reason.
Answer: When the bottle is heated, the air inside it expands. Since the volume of the air increases, the excess air escapes through the tube into the water in the form of bubbles.

32. Which of the following will expand more, when heated to the same temperature : (a) solid (b) liquid and (c) gas ?
Answer: (c) gas. Gases expand much more than solids and liquids for the same rise in temperature.

33. Describe an experiment to show that same volume of different liquids heated to same rise in temperature expand by different amounts.
Answer: Take identical glass bottles fitted with narrow tubes and fill them with different liquids (e.g., water, alcohol, kerosene, benzene) to the same level. Place them in a common boiling water bath. After some time, observe the levels. You will notice that the levels rise by different amounts (e.g., benzene expands more than water). This shows that different liquids expand by different amounts for the same temperature rise.

34. 100 ml of each of the following liquids is heated from 10°C to 50°C. Which will expand more : (a) water (b) benzene (c) alcohol ?
Answer: (b) benzene. (Benzene has a higher coefficient of expansion than alcohol and water).

35. Water is heated from 0°C to 4°C. Will it expand ?
Answer: No, it will contract. Water shows anomalous behavior; it contracts when heated from 0°C to 4°C and expands only on heating above 4°C.

36. What do you mean by anomalous behavior of water ?
Answer: Anomalous behavior of water refers to the unusual property where water contracts on heating from 0°C to 4°C, unlike most substances which expand. Consequently, its density increases from 0°C to 4°C, reaching a maximum at 4°C.

37. How does the density of a substance (solid, liquid and gas) change on heating ?
Answer: On heating, the volume of a substance generally increases while its mass remains constant. Therefore, the density (mass/volume) decreases on heating. (Exception: Water density increases between 0°C and 4°C).

38. An iron washer is heated. State the effect on it (i) mass, (ii) internal diameter, (iii) external diameter, and (iv) density.
Answer:
(i) Mass: Remains unchanged.
(ii) Internal diameter: Increases.
(iii) External diameter: Increases.
(iv) Density: Decreases (as volume increases).

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Multiple Choice Questions - Test your understanding with these MCQs. Click "Show Answer" to reveal the correct answer.

Question 1What is the approximate size of a molecule?

Correct Answer: 10^-10 m

Question 2The total internal energy of a substance is the sum of which two energies of its molecules?

Correct Answer: Kinetic and Potential

Question 3What determines the temperature of a body on a molecular level?

Correct Answer: Average kinetic energy

Question 4What is the relationship between calorie and joule?

Correct Answer: 1 cal = 4.2 J

Question 5Which of the following is NOT a main effect of heat?

Correct Answer: Change in mass

Question 6The process of a solid changing into a liquid at a fixed temperature is called:

Correct Answer: Melting

Question 7What is the reverse process of melting?

Correct Answer: Freezing

Question 8At what temperature does pure water freeze?

Correct Answer: 0°C

Question 9The process of a liquid changing into vapour at a fixed temperature is called:

Correct Answer: Boiling

Question 10The change of state from gas to liquid is known as:

Correct Answer: Condensation

Question 11Which process involves a solid changing directly into vapour?

Correct Answer: Sublimation

Question 12Which substance is an example of one that undergoes sublimation?

Correct Answer: Camphor

Question 13What is the process called when vapour changes directly into a solid?

Correct Answer: Solidification (Deposition)

Question 14The heat absorbed or rejected during a change of state at constant temperature is called:

Correct Answer: Latent heat

Question 15Does the temperature of a substance change during its change of state?

Correct Answer: No, it remains constant

Question 16Evaporation takes place at:

Correct Answer: All temperatures

Question 17Boiling is a phenomenon that occurs:

Correct Answer: Throughout the whole volume

Question 18Which phenomenon causes cooling in the surroundings?

Correct Answer: Evaporation

Question 19Why does putting ether on your palm make it feel cold?

Correct Answer: Ether absorbs heat from the palm to evaporate

Question 20How does an increase in surface area affect the rate of evaporation?

Correct Answer: Increases it

Question 21How does blowing air (wind) affect the rate of evaporation?

Correct Answer: Increases it

Question 22What effect does high humidity have on the rate of evaporation?

Correct Answer: Decreases it

Question 23What happens to the boiling point of a liquid when pressure is increased?

Correct Answer: It increases

Question 24Why is it difficult to cook vegetables at high altitudes (mountains)?

Correct Answer: Water boils at a temperature lower than 100°C

Question 25What is the approximate temperature inside a pressure cooker?

Correct Answer: 125°C

Question 26One kg of water at 100°C absorbs how much heat to convert to steam at 100°C?

Correct Answer: 2.26 x 10^6 J

Question 27Which process is slow and silent?

Correct Answer: Evaporation

Question 28Which process is rapid and violent (produces bubbles)?

Correct Answer: Boiling

Question 29In which state of matter is thermal expansion the maximum?

Correct Answer: Gases

Question 30The increase in the length of a solid on heating is called:

Correct Answer: Linear expansion

Question 31The increase in the area of a solid on heating is called:

Correct Answer: Superficial expansion

Question 32The increase in the volume of a body on heating is called:

Correct Answer: Cubical expansion

Question 33In the 'Ball and Ring' experiment, why does the ball not pass through the ring after heating?

Correct Answer: The ball expands

Question 34Linear expansion depends on original length, increase in temperature, and:

Correct Answer: The material of the rod

Question 35If two rods of the same material have different lengths, which one expands more for the same temperature rise?

Correct Answer: The longer one

Question 36Comparing Iron and Copper, which expands more for the same temperature rise?

Correct Answer: Copper

Question 37What is the ratio of the coefficients of linear (alpha), superficial (beta), and cubical (gamma) expansion?

Correct Answer: 1:2:3

Question 38The mathematical relation for linear expansion is:

Correct Answer: Lt - L0 = L0 * alpha * t

Question 39A bimetallic strip is made of two bars of:

Correct Answer: Different materials, same length

Question 40In a bimetallic strip of Brass and Iron, which way will it bend on heating?

Correct Answer: Towards the Iron side

Question 41Why does a Brass/Iron bimetallic strip bend towards the Iron side on heating?

Correct Answer: Brass expands more than Iron

Question 42What device uses a bimetallic strip to control temperature?

Correct Answer: Thermostat

Question 43Invar is an alloy known for:

Correct Answer: Negligible expansion on heating

Question 44Why is the pendulum of a clock made of Invar?

Correct Answer: To prevent time loss/gain due to expansion/contraction

Question 45Why are gaps left between successive rails on a railway track?

Correct Answer: To allow for expansion in summer

Question 46How is one end of a steel girder in a bridge fixed?

Correct Answer: One end is fixed, the other is on rollers

Question 47Why are telephone wires kept slightly loose in summer?

Correct Answer: To prevent breaking due to contraction in winter

Question 48When fitting a steel rim on a wooden cart wheel, the rim is initially made:

Correct Answer: Slightly smaller than the wheel

Question 49Why is the steel rim heated before fitting it onto a cart wheel?

Correct Answer: To expand its diameter to fit over the wheel

Question 50Why does thick glass crack when hot liquid is poured into it?

Correct Answer: Unequal expansion of inner and outer surfaces
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True or False Questions - Determine whether each statement is true or false. Click "Show Answer" to reveal the correct answer.

Statement 1Matter is composed of tiny particles called molecules.

Correct Answer: True

Statement 2A molecule is large enough to be seen with the naked eye.

Correct Answer: False

Statement 3The approximate size of a molecule is 10^-10 meters.

Correct Answer: True

Statement 4Molecules in a substance possess kinetic energy due to their motion.

Correct Answer: True

Statement 5Potential energy in molecules arises from the forces of repulsion between them.

Correct Answer: False

Statement 6Heat energy is the sum of the internal kinetic and potential energy of molecules.

Correct Answer: True

Statement 7The SI unit of heat is the calorie.

Correct Answer: False

Statement 81 calorie is approximately equal to 4.2 Joules.

Correct Answer: True

Statement 9Temperature is a measure of the average potential energy of molecules.

Correct Answer: False

Statement 10When a substance absorbs heat, the motion of its molecules generally becomes more rapid.

Correct Answer: True

Statement 11Melting is the change of state from liquid to solid.

Correct Answer: False

Statement 12Freezing occurs at a fixed temperature.

Correct Answer: True

Statement 13Boiling is the rapid change of state from liquid to gas at a fixed temperature.

Correct Answer: True

Statement 14Evaporation takes place only at the boiling point.

Correct Answer: False

Statement 15Condensation is the reverse process of vaporization.

Correct Answer: True

Statement 16Sublimation is the process where a solid changes directly into a liquid.

Correct Answer: False

Statement 17Camphor changes directly from solid to vapour on heating.

Correct Answer: True

Statement 18Evaporation is a surface phenomenon.

Correct Answer: True

Statement 19Boiling takes place throughout the entire volume of the liquid.

Correct Answer: True

Statement 20Evaporation produces heating in the surroundings.

Correct Answer: False

Statement 21The rate of evaporation increases with an increase in temperature.

Correct Answer: True

Statement 22Blowing air over a liquid surface decreases the rate of evaporation.

Correct Answer: False

Statement 23The presence of humidity increases the rate of evaporation.

Correct Answer: False

Statement 24Latent heat causes a rise in temperature during a change of state.

Correct Answer: False

Statement 25The boiling point of water is 100°C at standard atmospheric pressure.

Correct Answer: True

Statement 26The boiling point of a liquid decreases with an increase in pressure.

Correct Answer: False

Statement 27Water boils at a temperature lower than 100°C on mountains.

Correct Answer: True

Statement 28In a pressure cooker, water boils at approximately 125°C.

Correct Answer: True

Statement 29Cooking vegetables is slower in a pressure cooker than in an open pot.

Correct Answer: False

Statement 30Thermal expansion is the increase in size of a body due to heating.

Correct Answer: True

Statement 31Solids expand more than liquids for the same temperature rise.

Correct Answer: False

Statement 32Gases expand the most among the three states of matter.

Correct Answer: True

Statement 33Absolute zero is the temperature at which molecular motion theoretically ceases.

Correct Answer: True

Statement 34In liquids, molecules are held in fixed positions.

Correct Answer: False

Statement 35Gases have a definite volume but no definite shape.

Correct Answer: False

Statement 36Linear expansion refers to the increase in the length of a solid.

Correct Answer: True

Statement 37The increase in length of a rod depends on its original length.

Correct Answer: True

Statement 38Copper expands more than iron for the same rise in temperature.

Correct Answer: True

Statement 39A bimetallic strip is made of two strips of the same metal.

Correct Answer: False

Statement 40In a brass-iron bimetallic strip, brass expands more than iron.

Correct Answer: True

Statement 41Thermostats are used to measure temperature in a laboratory.

Correct Answer: False

Statement 42Thermostats are used to control temperature in electric irons.

Correct Answer: True

Statement 43Hollow rods expand differently than solid rods of the same material and length.

Correct Answer: False

Statement 44The coefficient of linear expansion is denoted by the symbol alpha (α).

Correct Answer: True

Statement 45Invar is an alloy that expands significantly on heating.

Correct Answer: False

Statement 46Pendulums of clocks are often made of Invar.

Correct Answer: True

Statement 47Superficial expansion refers to the increase in volume of a solid.

Correct Answer: False

Statement 48The coefficient of superficial expansion (β) is approximately equal to 2α.

Correct Answer: True

Statement 49The coefficient of cubical expansion (γ) is approximately equal to 3α.

Correct Answer: True

Statement 50Steel girders in bridges are fixed rigidly at both ends.

Correct Answer: False
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Fill in the Blanks - Complete the sentences by filling in the missing words. Click "Show Answer" to check your response.

Question 1Matter is composed of tiny particles called ______.

Correct Answer: molecules

Question 2The approximate size of a molecule is nearly ______ meters.

Correct Answer: 10^-10

Question 3The sum of internal kinetic energy and internal potential energy is called the total ______ energy of the substance.

Correct Answer: internal

Question 4Heat is the internal energy of a substance and is measured in the unit ______.

Correct Answer: joule

Question 51 calorie is approximately equal to ______ joules.

Correct Answer: 4.2

Question 6The average kinetic energy of the molecules of a substance is a measure of its ______.

Correct Answer: temperature

Question 7When a substance changes its state from solid to liquid at a fixed temperature, the process is called ______.

Correct Answer: melting

Question 8The process where a liquid changes into a solid at a fixed temperature is called ______.

Correct Answer: freezing

Question 9______ is the process where a solid changes directly into vapour on heating.

Correct Answer: Sublimation

Question 10The process of a vapour changing directly into a solid on cooling is called ______.

Correct Answer: solidification

Question 11The change of state from liquid to gas at all temperatures is called ______.

Correct Answer: evaporation

Question 12The heat absorbed or rejected during a change of state, which does not result in a temperature change, is called ______ heat.

Correct Answer: latent

Question 13During a change of state, the heat absorbed changes the average ______ energy of the molecules.

Correct Answer: potential

Question 14The change in size of a body due to heating is called ______ expansion.

Correct Answer: thermal

Question 15On heating, the average inter-molecular separation between molecules ______.

Correct Answer: increases

Question 16The temperature at which molecular motion completely ceases is known as ______ zero.

Correct Answer: absolute

Question 17Evaporation is essentially a ______ phenomenon as it occurs at the boundary of the liquid.

Correct Answer: surface

Question 18The rate of evaporation increases with an increase in the ______ of the liquid.

Correct Answer: temperature

Question 19When air is blown above the surface of a liquid, the rate of evaporation ______.

Correct Answer: increases

Question 20In the presence of high ______, the rate of evaporation becomes slow.

Correct Answer: humidity

Question 21Evaporation produces ______ in the surroundings because the liquid absorbs necessary heat to change state.

Correct Answer: cooling

Question 22Boiling takes place at a fixed temperature called the ______ point.

Correct Answer: boiling

Question 23Boiling is a ______ process, whereas evaporation is a slow process.

Correct Answer: rapid

Question 24The boiling point of a liquid ______ with an increase in pressure.

Correct Answer: increases

Question 25At mountains, atmospheric pressure is low, so water boils at a temperature ______ than 100°C.

Correct Answer: lower

Question 26In a pressure cooker, water boils at a temperature nearly equal to ______ °C.

Correct Answer: 125

Question 27One kg of water at 100°C absorbs ______ joules of heat to convert into steam at 100°C.

Correct Answer: 2.26 x 10^6

Question 28Water contracts on heating from 0°C to ______ °C.

Correct Answer: 4

Question 29Silver iodide contracts on heating from 80°C to ______ °C.

Correct Answer: 141

Question 30When a solid is heated, it expands in all directions, so its length, area, and ______ all increase.

Correct Answer: volume

Question 31The increase in length of a solid on heating is called ______ expansion.

Correct Answer: linear

Question 32The increase in area of a solid on heating is called ______ expansion.

Correct Answer: superficial

Question 33The increase in volume of a solid on heating is called ______ expansion.

Correct Answer: cubical

Question 34In the ball and ring experiment, the metal ball does not pass through the ring after ______.

Correct Answer: heating

Question 35The increase in length of a metal rod depends on its original length, increase in temperature, and the ______ of the rod.

Correct Answer: material

Question 36For the same rise in temperature, a copper rod expands ______ than an iron rod of the same length.

Correct Answer: more

Question 37A bimetallic strip is used in a ______, a device to control temperature.

Correct Answer: thermostat

Question 38The increase in length of a rod is given by the formula L_t - L_0 = L_0 x ______ x t.

Correct Answer: alpha

Question 39The symbol alpha represents the coefficient of ______ expansion.

Correct Answer: linear

Question 40______ is an alloy which almost does not expand on heating.

Correct Answer: Invar

Question 41The pendulum of a clock is often made of Invar to prevent time loss in summer due to ______.

Correct Answer: expansion

Question 42In a bimetallic strip made of brass and iron, the strip bends because ______ expands more than iron.

Correct Answer: brass

Question 43The coefficient of linear expansion of brass is about ______ times that of iron.

Correct Answer: 1.5

Question 44If beta is the coefficient of superficial expansion, then beta is equal to ______ times alpha.

Correct Answer: 2

Question 45If gamma is the coefficient of cubical expansion, then gamma is equal to ______ times alpha.

Correct Answer: 3

Question 46The ratio of the coefficients of linear, superficial, and cubical expansion is 1 : 2 : ______.

Correct Answer: 3

Question 47In the construction of a bridge, one end of a girder is supported on ______ to allow for expansion.

Correct Answer: rollers

Question 48Gaps are left between successive rails on a railway track to allow for expansion in ______.

Correct Answer: summer

Question 49In riveting, hot rivets are used so that on ______, they grip the plates firmly.

Correct Answer: cooling

Question 50Electric wires are kept slightly ______ in summer to prevent breaking in winter.

Correct Answer: loose
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Short Answer Questions - Practice concise answers to these questions. Click "Show Answer" to reveal the response.
Question 1

What are the tiny particles that make up matter called?

Answer:
Molecules
Question 2

What is the approximate size of a molecule in meters?

Answer:
10^-10 m
Question 3

What form of energy do molecules possess due to their motion?

Answer:
Kinetic energy
Question 4

What form of energy do molecules possess due to forces of attraction?

Answer:
Potential energy
Question 5

What is the sum of internal kinetic and potential energy called?

Answer:
Internal energy
Question 6

What is the SI unit of heat?

Answer:
Joule
Question 7

Approximately how many joules make up one calorie?

Answer:
4.2 J
Question 8

What physical quantity is determined by the average kinetic energy of molecules?

Answer:
Temperature
Question 9

What happens to the temperature of a body when it absorbs heat?

Answer:
It rises
Question 10

What are the three states of matter?

Answer:
Solid, Liquid, Gas
Question 11

What is the process of a solid changing into a liquid called?

Answer:
Melting
Question 12

At what temperature does ice melt?

Answer:
0°C
Question 13

What is the reverse process of melting?

Answer:
Freezing
Question 14

What is the process of a liquid changing into a gas at a fixed temperature called?

Answer:
Boiling
Question 15

What is the reverse process of vaporization?

Answer:
Condensation
Question 16

At what temperature does water boil at standard pressure?

Answer:
100°C
Question 17

What is the process where a solid changes directly to vapour?

Answer:
Sublimation
Question 18

Name a substance that undergoes sublimation.

Answer:
Camphor
Question 19

What is the process of vapour changing directly to solid called?

Answer:
Solidification
Question 20

What is the change of liquid to gas at all temperatures called?

Answer:
Evaporation
Question 21

Is evaporation a surface phenomenon or a bulk phenomenon?

Answer:
Surface phenomenon
Question 22

Is boiling a slow or rapid process?

Answer:
Rapid
Question 23

What is the heat absorbed during a change of state called?

Answer:
Latent heat
Question 24

Does temperature change during the change of state?

Answer:
No
Question 25

What happens to the size of a body when heated?

Answer:
It expands
Question 26

What is the expansion of a body due to heating called?

Answer:
Thermal expansion
Question 27

Which state of matter expands the most on heating?

Answer:
Gas
Question 28

What is the temperature at which molecular motion completely ceases?

Answer:
Absolute zero
Question 29

Why does a liquid have a definite volume?

Answer:
Cohesive forces
Question 30

Do gas molecules have strong or negligible attraction forces?

Answer:
Negligible
Question 31

Does the rate of evaporation increase or decrease with temperature?

Answer:
Increase
Question 32

How does blowing air affect the rate of evaporation?

Answer:
Increases it
Question 33

How does increased surface area affect evaporation?

Answer:
Increases it
Question 34

How does humidity affect the rate of evaporation?

Answer:
Decreases it
Question 35

What sensation does evaporation produce on the skin?

Answer:
Cooling
Question 36

What do visible bubbles in boiling water consist of?

Answer:
Water vapour
Question 37

How does an increase in pressure affect the boiling point?

Answer:
Increases it
Question 38

Why does water boil below 100°C on mountains?

Answer:
Low atmospheric pressure
Question 39

What is the approximate temperature inside a pressure cooker?

Answer:
125°C
Question 40

Does boiling take place throughout the liquid or just on the surface?

Answer:
Throughout the liquid
Question 41

What is the increase in length of a solid called?

Answer:
Linear expansion
Question 42

What is the increase in the area of a solid called?

Answer:
Superficial expansion
Question 43

What is the increase in the volume of a solid called?

Answer:
Cubical expansion
Question 44

In the ball and ring experiment, why does the ball get stuck after heating?

Answer:
It expands
Question 45

Does a longer rod expand more or less than a shorter one for the same temperature rise?

Answer:
More
Question 46

Which expands more for the same heat: copper or iron?

Answer:
Copper
Question 47

What device uses a bimetallic strip to control temperature?

Answer:
Thermostat
Question 48

What symbol represents the coefficient of linear expansion?

Answer:
Alpha (α)
Question 49

What is the unit for the coefficient of linear expansion?

Answer:
Per °C
Question 50

What alloy is used for clock pendulums because it barely expands?

Answer:
Invar
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Long Answer Questions - Practice detailed answers to these comprehensive questions. Click "Show Answer" to reveal the response.
Question 1

Explain the concept of heat and temperature on the basis of the kinetic theory of matter.

Answer:
According to the kinetic theory of matter, matter is composed of tiny particles called molecules which are in continuous motion. Due to this motion, molecules possess kinetic energy. When a substance absorbs heat, the motion of its molecules becomes rapid, which increases their average kinetic energy. Conversely, when a substance is cooled, the motion of its molecules slows down, decreasing their kinetic energy. Heat is defined as the total internal energy of the substance, which is the sum of the internal kinetic energy and internal potential energy of all its molecules. Temperature, on the other hand, is the measure of the average kinetic energy of the molecules. A rise in average kinetic energy results in a rise in temperature, while a fall results in a decrease in temperature.
Question 2

Describe the three main effects of heat on a body.

Answer:
Heat mainly produces three effects on a body: 1. Change in temperature: When heat is supplied to a body, its temperature generally rises due to the increase in the average kinetic energy of its molecules. Conversely, removing heat causes the temperature to fall. 2. Change in state: Heat can cause a substance to change its physical state at a constant temperature. For example, a solid can change into a liquid (melting) or a liquid into a gas (boiling). 3. Change in size (Thermal Expansion): When a body is heated, it generally expands (increases in size). This is known as thermal expansion. Upon cooling, the body usually contracts.
Question 3

Differentiate between boiling and evaporation with at least four distinct points.

Answer:
1. Temperature: Boiling takes place at a fixed temperature known as the boiling point, whereas evaporation occurs at all temperatures. 2. Location: Boiling is a bulk phenomenon occurring throughout the entire mass of the liquid, while evaporation is a surface phenomenon occurring only at the free surface of the liquid. 3. Speed: Boiling is a rapid process, whereas evaporation is a slow and gradual process. 4. External Energy: Boiling requires a continuous external source of heat to maintain the temperature, whereas evaporation takes place by absorbing heat from the surroundings, often causing cooling.
Question 4

Explain the molecular mechanism of evaporation.

Answer:
Molecules in a liquid are in continuous motion and collide with each other. Through these collisions, some molecules acquire higher kinetic energy than others. The molecules at the surface of the liquid that possess sufficient kinetic energy can overcome the attractive forces (cohesive forces) exerted by other molecules inside the liquid. These high-energy molecules escape from the surface into the space above, forming vapour. This process of molecules escaping from the surface is called evaporation. Since the higher-energy molecules escape, the average kinetic energy of the remaining liquid decreases, which results in a drop in temperature.
Question 5

What are the factors that affect the rate of evaporation of a liquid? Explain how they affect it.

Answer:
The rate of evaporation depends on four main factors: 1. Temperature of the liquid: The rate of evaporation increases with an increase in temperature because more molecules acquire sufficient kinetic energy to escape the surface. 2. Area of the exposed surface: The rate of evaporation increases with an increase in surface area because a larger area allows more molecules to escape simultaneously. 3. Wind speed: Blowing air or wind increases the rate of evaporation by carrying away the vapour molecules near the surface, allowing more liquid molecules to escape. 4. Humidity: The presence of humidity (moisture) in the air decreases the rate of evaporation because the air is already saturated with vapour, leaving less space for new vapour molecules to enter.
Question 6

Why does evaporation produce cooling? Give a reason based on molecular theory.

Answer:
Evaporation produces cooling because it requires heat energy. During evaporation, the molecules at the surface of the liquid require energy to overcome the force of attraction from the molecules inside the liquid and escape into the vapour state. If there is no external heat source, the liquid draws this necessary heat energy from its own interior or from the immediate surroundings. As the heat is extracted from the liquid or surroundings, the temperature drops, resulting in a cooling effect.
Question 7

Describe an experiment to demonstrate the boiling of water and determine its boiling point.

Answer:
To demonstrate boiling, take a beaker filled with water and place it on a wire gauze over a tripod stand. Clamp a thermometer vertically such that its bulb is immersed in the water. Heat the beaker using a burner and record the temperature every minute. Initially, the temperature of the water will rise continuously. Eventually, bubbles will form throughout the liquid and rise to the surface, indicating that the water has started to boil. At this stage, the temperature will remain constant even though heat is still being supplied. This fixed constant temperature is the boiling point of water, which is 100 degrees Celsius under normal atmospheric pressure.
Question 8

How does pressure affect the boiling point of a liquid? Explain with examples of a pressure cooker and cooking at high altitudes.

Answer:
The boiling point of a liquid is directly proportional to the external pressure acting on its surface. When pressure increases, the boiling point increases; when pressure decreases, the boiling point decreases. In a pressure cooker, steam is trapped inside, which increases the pressure above the water. This raises the boiling point of water to about 125 degrees Celsius, allowing food to cook much faster. Conversely, at high altitudes (like mountains), atmospheric pressure is lower. Consequently, water boils at a temperature lower than 100 degrees Celsius, making it difficult to cook food properly as the temperature is insufficient for rapid cooking.
Question 9

Define thermal expansion and explain it on the basis of molecular motion.

Answer:
Thermal expansion is the increase in the size (length, area, or volume) of a substance when it is heated. On the basis of molecular motion, thermal expansion occurs because heating increases the average kinetic energy of the molecules. The molecules start vibrating more vigorously with larger amplitudes about their mean positions. This increased vibration pushes the molecules further apart, resulting in an increase in the average inter-molecular separation. As the average distance between molecules increases, the overall size of the substance increases, causing expansion.
Question 10

Describe the Ball and Ring experiment to demonstrate thermal expansion in solids.

Answer:
The Ball and Ring experiment involves a metal ball and a metal ring. At room temperature, the ball is sized to just pass through the ring. To demonstrate expansion: 1. Heat the metal ball using a burner for some time. 2. Try to pass the heated ball through the ring. It will not pass through because the heat has caused the ball to expand, increasing its diameter. 3. Allow the ball to cool down to room temperature. 4. Try passing it through the ring again. It will now slip through easily because it has contracted back to its original size upon cooling. This demonstrates that solids expand on heating and contract on cooling.
Question 11

Define Linear Expansion. State the three factors on which the increase in length of a rod depends.

Answer:
Linear expansion refers to the increase in the length of a solid body (like a rod or wire) when it is heated. The increase in length depends on three factors: 1. The original length of the rod: A longer rod will show a greater increase in length than a shorter one for the same temperature rise. 2. The increase in temperature: The greater the rise in temperature, the greater the expansion. 3. The material of the rod: Different metals expand by different amounts for the same temperature rise (e.g., copper expands more than iron).
Question 12

What is a bimetallic strip? Explain its working and one application.

Answer:
A bimetallic strip consists of two flat strips of different metals (usually brass and iron) of equal length riveted or bonded together. Since different metals have different coefficients of linear expansion, they expand by different amounts when heated. Brass expands more than iron. Therefore, when the strip is heated, the brass side elongates more than the iron side, forcing the strip to bend with the brass on the convex (outer) side and iron on the concave (inner) side. This property is used in thermostats, which are devices used to regulate temperature in electrical appliances like irons, refrigerators, and ovens by automatically making or breaking an electric circuit.
Question 13

Why is a small gap left between successive rails on a railway track?

Answer:
Railway tracks are made of steel, which expands when heated. During summer, the atmospheric temperature rises considerably, causing the rails to expand in length. If no gap were left between the joints of successive rails, the rails would have no space to expand. This would generate immense thermal stress, causing the rails to bend sideways or buckle, which could lead to train derailments. The small gaps provide the necessary space for this thermal expansion to occur safely.
Question 14

Explain how the process of riveting utilizes thermal expansion.

Answer:
Riveting is used to join two steel plates firmly. A rivet is a small steel rod with a head at one end. To join the plates: 1. Holes are drilled through the plates where they need to be joined. 2. The rivet is heated until it is red hot. 3. The hot rivet is inserted into the holes. 4. The other end of the rivet is hammered to form a new head, securing the plates. 5. As the rivet cools, it contracts. Since it is constrained by the heads, the contraction pulls the two plates tightly together, creating a very strong, often leak-proof joint.
Question 15

Why are telephone and electric wires kept slightly loose when hung between poles in summer?

Answer:
Telephone and electric wires are made of metal, which expands on heating and contracts on cooling. If the wires were strung tightly between poles during summer, they would contract when the temperature drops in winter. This contraction would create high tension in the wires, potentially causing them to snap or break. By keeping them slightly loose in summer (allowing for a sag), sufficient slack is provided to accommodate the contraction that occurs during winter without breaking the wires.
Question 16

Why is an iron rim heated before being fitted onto a wooden cart wheel?

Answer:
To ensure a firm grip, the iron rim is made slightly smaller in diameter than the wooden wheel. Before fitting, the iron rim is heated uniformly. Thermal expansion causes the rim's diameter to increase, allowing it to be slipped easily over the wooden wheel. Once in place, water is poured over it to cool it down. As the rim cools, it contracts and attempts to regain its original smaller size. This contraction exerts a large inward force on the wooden wheel, ensuring a very tight fit that holds the wheel together securely.
Question 17

Why does a thick glass tumbler crack when boiling water is poured into it?

Answer:
Glass is a poor conductor of heat. When boiling water is poured into a thick glass tumbler, the inner surface of the glass comes in contact with the hot water and expands rapidly. However, because glass conducts heat poorly, the heat does not immediately reach the outer surface, which remains at a lower temperature and does not expand at the same rate. This unequal expansion between the inner and outer layers of the thick glass creates stress within the material, causing the tumbler to crack. Thinner glass tumblers are less likely to crack because the heat is conducted to the outer surface more quickly, resulting in more uniform expansion.
Question 18

What is the Anomalous Expansion of water? Explain with reference to temperature ranges.

Answer:
Generally, liquids expand on heating and contract on cooling. However, water exhibits a unique behavior called anomalous expansion. When water is heated from 0 degrees Celsius, it contracts instead of expanding until it reaches 4 degrees Celsius. During this range, its volume decreases and its density increases. On heating beyond 4 degrees Celsius, water behaves normally and starts to expand. Therefore, water has its minimum volume and maximum density at 4 degrees Celsius. This behavior is contrary to that of most other substances.
Question 19

Describe an experiment to show that liquids expand on heating.

Answer:
Take an empty glass bottle and fit it with a cork having a hole. Pass a drinking straw or narrow glass tube through the hole. Fill the bottle completely with coloured water (to make the level visible) so that the water rises slightly into the straw. Mark the initial water level in the straw. Place the bottle in a vessel containing hot water or heat it gently. As the water in the bottle gets heated, you will observe the water level in the straw rising. This rise in level indicates that the volume of the water has increased due to heat, demonstrating that liquids expand on heating.
Question 20

Why is mercury used in thermometers? Explain with reference to thermal expansion.

Answer:
Mercury is used in thermometers primarily because it is a liquid metal that expands uniformly with a rise in temperature. This means that for equal increments of heat, the expansion is consistent, allowing for accurate calibration of the temperature scale. Additionally, mercury is opaque and shiny, making it easy to read the level in the capillary tube. It also has a high boiling point and does not stick to the glass walls of the thermometer.
Question 21

Describe an experiment to demonstrate that gases expand on heating.

Answer:
Take an empty glass bottle (which naturally contains air). Attach a deflated balloon securely to the neck of the bottle. Place the bottle in a water bath containing boiling water. As the air inside the bottle gets heated by the hot water, it expands. Since the bottle is rigid, the expanding air moves into the balloon, causing it to inflate. This inflation of the balloon demonstrates that the air (a gas) inside the bottle has expanded upon heating.
Question 22

Compare the expansivity of solids, liquids, and gases.

Answer:
Solids, liquids, and gases all expand on heating, but they do so to different extents. Solids have the strongest intermolecular forces and thus expand the least. Liquids have weaker intermolecular forces than solids and expand significantly more than solids for the same temperature rise (roughly 10 times more). Gases have negligible intermolecular forces and expand the most (roughly 100 times more than solids) for the same rise in temperature.
Question 23

What is the relation between the coefficients of linear (alpha), superficial (beta), and cubical (gamma) expansion?

Answer:
The coefficients of thermal expansion define how much a material expands per degree rise in temperature. The coefficient of superficial expansion (beta) is approximately two times the coefficient of linear expansion (alpha), so beta = 2 x alpha. The coefficient of cubical expansion (gamma) is approximately three times the coefficient of linear expansion (alpha), so gamma = 3 x alpha. Therefore, the ratio of the three coefficients is alpha : beta : gamma = 1 : 2 : 3.
Question 24

Explain why Pyrex glass is preferred for making laboratory and kitchen glassware.

Answer:
Pyrex glass is a special type of glass that has a very low coefficient of thermal expansion compared to ordinary glass. This means it expands and contracts very little when subjected to temperature changes. Consequently, when hot liquids are poured into Pyrex containers or when they are heated, the thermal stresses generated due to unequal expansion are minimal. This prevents the glass from cracking or shattering, making it safe and durable for kitchen and laboratory use involving heat.
Question 25

Explain the concept of latent heat and why it is called 'hidden' heat.

Answer:
Latent heat is the amount of heat energy absorbed or released by a substance during a change of state (e.g., solid to liquid or liquid to gas) without any change in its temperature. For example, when ice melts at 0 degrees Celsius, it absorbs heat but the temperature remains at 0 degrees Celsius until all the ice has turned to water. This heat is used to overcome the intermolecular forces of attraction and increase the potential energy of the molecules rather than their kinetic energy. It is called 'latent' (meaning hidden) because it does not manifest as a rise in temperature on a thermometer.
Question 26

Describe the Bar and Gauge experiment and what it proves.

Answer:
The Bar and Gauge experiment consists of a metal bar and a gauge with a gap that exactly fits the length of the bar (or a diameter gap for a rod) at room temperature. 1. Initially, show that the cold metal bar fits perfectly into the gap of the gauge. 2. Heat the metal bar strongly using a burner. 3. Try to fit the heated bar into the gauge again. It will not fit because the bar has expanded in length/size. 4. Allow the bar to cool. Once cooled, it will fit into the gauge again. This experiment proves that solids undergo expansion when heated and contraction when cooled.
Question 27

Explain the variation of density with temperature for most substances and specifically for water.

Answer:
For most substances (solids, liquids, and gases), heating causes an increase in volume (expansion) while the mass remains constant. Since density is mass divided by volume (Density = Mass / Volume), an increase in volume leads to a decrease in density. Therefore, generally, density decreases as temperature increases. However, water is an exception between 0 degrees Celsius and 4 degrees Celsius. In this range, water contracts on heating, meaning its volume decreases. Consequently, the density of water increases as it is heated from 0 to 4 degrees Celsius, reaching a maximum value of 1000 kg/m^3 at 4 degrees Celsius. Above 4 degrees Celsius, it behaves normally, and its density decreases.
Question 28

Why is one end of a steel girder in a bridge supported on rollers?

Answer:
Steel girders used in bridges are very long and are subject to expansion and contraction due to seasonal temperature changes (summer and winter). If both ends of the girder were fixed rigidly into the concrete pillars, the thermal expansion would generate massive stress, potentially damaging the pillars or causing the bridge to buckle. To prevent this, one end of the girder is fixed, while the other end is allowed to rest on rollers. These rollers allow the girder to expand and contract freely without exerting dangerous horizontal forces on the supporting pillar.
Question 29

Why does a glass stopper stuck in a bottle loosen when the neck is warmed?

Answer:
When a glass stopper is stuck, warming the neck of the bottle applies heat primarily to the outer neck region. Due to thermal expansion, the glass neck expands and its diameter increases slightly. Although the stopper may also warm up eventually, the direct heating of the neck causes it to expand away from the stopper, creating a small clearance or space. This loosens the grip of the neck on the stopper, allowing it to be removed easily.
Question 30

Distinguish between the three states of matter based on intermolecular spacing and force of attraction.

Answer:
1. Solids: The intermolecular spacing is very small, and the molecules are tightly packed. The intermolecular force of attraction is very strong, keeping molecules in fixed positions. 2. Liquids: The intermolecular spacing is larger than in solids but smaller than in gases. The intermolecular force is weaker than in solids, allowing molecules to move around within the liquid's volume. 3. Gases: The intermolecular spacing is very large. The intermolecular force of attraction is negligible, allowing molecules to move freely and independently in the entire available space.
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Quick Review Flashcards - Click to flip and test your knowledge!
Question
What is the total internal energy of a substance composed of?
Answer
It is the sum of the kinetic energy and potential energy of all its molecules.
Question
What is temperature a measure of in a substance?
Answer
Temperature is a measure of the average kinetic energy of the molecules of a substance.
Question
When two bodies at different temperatures are in contact, in which direction does heat flow?
Answer
Heat flows from the body at a higher temperature to the body at a lower temperature.
Question
What are the three main effects that heat can produce in a body?
Answer
Heat can cause a change in temperature, a change in state, and a change in the size of the body.
Question
The process of changing from a solid state to a liquid state at a fixed temperature is called _____.
Answer
melting
Question
The process of changing from a liquid state to a gaseous state at a fixed temperature is known as _____ or boiling.
Answer
vaporization
Question
What is the term for the heat absorbed or rejected during a change of state that does not cause a change in temperature?
Answer
Latent heat.
Question
What is the general effect on the size of a substance when it is heated?
Answer
The substance expands.
Question
The phenomenon of a substance changing size due to heating or cooling is called _____.
Answer
thermal expansion
Question
In which state of matter is the effect of thermal expansion the maximum?
Answer
Gases.
Question
The temperature at which molecular motion completely ceases is known as _____.
Answer
absolute zero
Question
What is the process of a liquid changing into its vapor at all temperatures from its surface called?
Answer
Evaporation.
Question
Is evaporation a surface phenomenon or a bulk phenomenon?
Answer
Evaporation is a surface phenomenon.
Question
Why does your palm feel cold when a drop of ether is placed on it?
Answer
The ether evaporates by drawing the necessary heat from your palm, causing a cooling sensation.
Question
How does an increase in the temperature of a liquid affect its rate of evaporation?
Answer
An increase in temperature increases the rate of evaporation.
Question
How does blowing air above the surface of a liquid affect its rate of evaporation?
Answer
Blowing air increases the rate of evaporation by carrying away the liquid molecules that have escaped.
Question
How does an increase in the surface area of a liquid affect its rate of evaporation?
Answer
Increasing the surface area increases the rate of evaporation.
Question
How does the presence of humidity in the air affect the rate of evaporation?
Answer
The presence of humidity decreases the rate of evaporation because the surrounding air is already saturated with vapor.
Question
Explain why evaporation produces a cooling effect.
Answer
During evaporation, the liquid draws the necessary heat from its surroundings to change into vapor, thus cooling the surroundings.
Question
What is the change from a liquid state to a gaseous state at a constant temperature called?
Answer
Boiling.
Question
What is the constant temperature at which a liquid boils called?
Answer
The boiling point.
Question
During boiling, what happens to the temperature of the liquid once it reaches its boiling point, even if heat is still being supplied?
Answer
The temperature does not rise any further; it remains constant.
Question
From where in the liquid do molecules start leaving during boiling?
Answer
Molecules leave from all parts of the liquid, not just the surface.
Question
How does an increase in pressure affect the boiling point of a liquid?
Answer
An increase in pressure increases the boiling point of a liquid.
Question
At what point does a liquid begin to boil in relation to atmospheric pressure?
Answer
A liquid boils when the pressure exerted by its vapor equals the surrounding atmospheric pressure.
Question
Why does water boil at a temperature below $100^\circ\text{C}$ at high altitudes?
Answer
At high altitudes, the atmospheric pressure is lower, so the vapor pressure needs to reach a lower value, which occurs at a lower temperature.
Question
How does a pressure cooker cook food faster?
Answer
It increases the pressure inside, which raises the boiling point of water (e.g., to $125^\circ\text{C}$), allowing food to cook at a higher temperature.
Question
What is a key difference between evaporation and boiling regarding the temperature at which they occur?
Answer
Evaporation takes place at all temperatures, while boiling occurs at a fixed temperature (the boiling point).
Question
Differentiate between evaporation and boiling based on where the process occurs within the liquid.
Answer
Evaporation occurs only from the surface of the liquid, while boiling takes place throughout the entire mass of the liquid.
Question
Compare the speed of evaporation and boiling.
Answer
Evaporation is a slow process, while boiling is a rapid process.
Question
Compare the source of heat for evaporation and boiling.
Answer
In evaporation, heat is absorbed from the surroundings, while in boiling, heat is supplied from an external source.
Question
What is the molecular explanation for the thermal expansion of solids?
Answer
On heating, the average kinetic energy of molecules increases, causing them to vibrate with larger amplitudes and increasing the average inter-molecular separation.
Question
In the ball and ring experiment, why does the heated metal ball not pass through the ring it could pass through at room temperature?
Answer
The ball expands on heating, and its diameter becomes larger than the diameter of the ring.
Question
The increase in the length of a solid (like a rod or wire) upon heating is known as _____ expansion.
Answer
linear
Question
The increase in the area of a solid (like a plate) upon heating is known as _____ expansion.
Answer
superficial
Question
The increase in the volume of a solid upon heating is known as _____ expansion.
Answer
cubical
Question
What are the three factors that the linear expansion of a metal rod depends on?
Answer
The original length of the rod, the increase in temperature, and the material of the rod.
Question
What is a bimetallic strip?
Answer
A strip made of two different metals, such as iron and brass, riveted together.
Question
Why does a bimetallic strip bend when heated?
Answer
It bends because the two metals expand by different amounts for the same temperature rise.
Question
What is the formula for the increase in length of a rod due to linear expansion?
Answer
The formula is $L_t - L_0 = L_0 \alpha t$, where $L_0$ is original length, $\alpha$ is the coefficient of linear expansion, and $t$ is the temperature change.
Question
What is the relationship between the coefficients of linear ($\alpha$), superficial ($\beta$), and cubical ($\gamma$) expansion?
Answer
The approximate relationship is $\alpha : \beta : \gamma = 1 : 2 : 3$.
Question
Why is one end of a steel girder in a bridge supported on rollers?
Answer
The rollers allow the girder to expand or contract with temperature changes without affecting the pillars or bridge structure.
Question
Why are small gaps left between successive lengths of railway tracks?
Answer
The gaps allow space for the rails to expand in summer due to high temperatures, preventing the track from bending sideways.
Question
Explain the process of riveting for joining steel plates.
Answer
Red-hot rivets are inserted and hammered. On cooling, they contract, pulling the plates tightly together to form a strong joint.
Question
Why are electric and telephone wires kept slightly loose when laid in summer?
Answer
They are kept loose so they do not contract too much and break in the winter.
Question
How is a steel rim fitted onto a wooden cart wheel?
Answer
The steel rim is heated to expand, slipped over the wheel, and then cooled to contract and form a tight fit.
Question
Why is glassware used in kitchens often made of Pyrex?
Answer
Pyrex glass has a very low coefficient of expansion, so it does not expand or crack significantly when heated.
Question
Why might a thick glass tumbler crack when hot liquid is poured into it?
Answer
The inner surface expands quickly while the outer surface remains cool, and this unequal expansion causes the glass to crack.
Question
What material, an alloy of iron and nickel, is used for the pendulum of a clock due to its negligible expansion?
Answer
Invar.
Question
How can a tight metal screw cap on a glass bottle be loosened using heat?
Answer
Warming the cap causes it to expand more than the glass neck, which loosens it.
Question
What type of thermal expansion do liquids primarily exhibit?
Answer
Liquids only have a definite volume, not a definite shape, so they exhibit only cubical expansion.
Question
Describe the anomalous behavior of water when heated from $0^\circ\text{C}$ to $4^\circ\text{C}$.
Answer
Water contracts when heated from $0^\circ\text{C}$ to $4^\circ\text{C}$.
Question
At what temperature does water have its maximum density?
Answer
Water has its maximum density at $4^\circ\text{C}$.
Question
What are the three factors that the cubical expansion of a liquid depends on?
Answer
The original volume of the liquid, the rise in temperature, and the nature of the liquid.
Question
Which expands more for the same temperature rise: solids or liquids?
Answer
Liquids expand much more than solids.
Question
What is the working principle of a mercury thermometer?
Answer
It works on the principle of thermal expansion of liquids; mercury expands uniformly with a rise in temperature.
Question
Which state of matter expands the most when heated?
Answer
Gases expand much more than liquids and solids.
Question
How does the density of most substances change when they are heated?
Answer
When a substance is heated, its volume increases while its mass remains the same, so its density decreases.
Question
What happens to the density of water as it is heated from $0^\circ\text{C}$ to $4^\circ\text{C}$?
Answer
The density of water increases as it is heated from $0^\circ\text{C}$ to $4^\circ\text{C}$.
Question
What happens to the density of water as it is heated further above $4^\circ\text{C}$?
Answer
The density of water decreases as it is heated above $4^\circ\text{C}$.