Heat and Energy - Questions & Answers

EXERCISE-6(A)

(A) MULTIPLE CHOICE TYPE :
(Choose the correct answer from the options given below).

1. The energy which flows from a hot body to a cold body is called :
(a) mechanical energy
(b) fuel energy
(c) solar energy
(d) heat energy
Answer: (d) heat energy

2. 1 Joule is equal to :
(a) 10⁶ erg
(b) 10⁵ erg
(c) 10⁷ erg
(d) 10⁹ erg
Answer: (c) 10⁷ erg

3. ............ determines the direction of flow of heat.
(a) Heat
(b) Temperature
(c) Coldness
(d) None of these
Answer: Temperature determines the direction of flow of heat. Option (b) is correct.

4. The amount of heat energy contained by a body depends on :
(a) the mass of the body
(b) the temperature of the body
(c) the nature of the material of the body
(d) all of the above
Answer: (d) all of the above

5. Celsius and Kelvin scales are related as :
(a) t K = 273 + t° C
(b) t K = 273 - t° C
(c) t K / t° C = 273
(d) 273 / t° C = Tk
Answer: (a) t K = 273 + t° C

6. The temperature of a body depends on the average ......... of the molecules.
(a) potential energy
(b) kinetic energy
(c) elastic energy
(d) atomic energy
Answer: The temperature of a body depends on the average kinetic energy of the molecules. Option (b) is correct.

7. The temperature at which pressure and volume of a gas becomes zero is :
(a) 0°C
(b) 0 K
(c) 0°F
(d) 273 K
Answer: (b) 0 K

8. The steam point on Fahrenheit scale is :
(a) 273°F
(b) 100°F
(c) 213°F
(d) 212°F
Answer: (d) 212°F

9. Celsius and Fahrenheit scales are related as :
(a) C/5 = (F-32)/9
(b) C/9 = (F+32)/5
(c) 5C = 9/(F+32)
(d) C/5 = 9(F+32)
Answer: (a) C/5 = (F-32)/9

10. The increase in the length of a solid on heating is called :
(a) contraction
(b) linear expansion
(c) cubical expansion
(d) superficial expansion
Answer: (b) linear expansion

11. On heating, liquids expand ............ than solids and gases expand ............ than liquids.
(a) more, less
(b) less, more
(c) more, much more
(d) less, less
Answer: On heating, liquids expand more than solids and gases expand much more than liquids. Option (c) is correct.

12. Water is cooled from 4°C to 0°C. It :
(a) contracts
(b) expands
(c) first contracts, then expands
(d) first expands, then contracts.
Answer: (b) expands

13. The change in volume of water when it is cooled from ......°C to ......°C is known as anomalous expansion of water.
(a) 0, 4
(b) 4, 0
(c) 1, 0
(d) 0, 1
Answer: The change in volume of water when it is cooled from 4°C to 0°C is known as anomalous expansion of water. Option (b) is correct.

14. The property of ice which prevents the flow of heat from water of a pond (or lake) to the atmosphere is :
(a) conduction
(b) insulation
(c) low specific heat
(d) none of the above
Answer: (b) insulation

15. Identify the correct graph showing the variation in the density of water with temperaure :
Answer: Graph (b), which correctly displays the density of water first increasing to reach its maximum at 4°C and then decreasing as it cools down to 0°C.

16. Some quantity of water at 0°C is heated to 20°C. Identify the correct graph showing the variation of volume of water with temperature.
Answer: Graph (d), which correctly displays the volume first decreasing as water is heated from 0°C to 4°C, hitting a minimum at 4°C, and steadily increasing thereafter.

(B) VERY SHORT ANSWER TYPE :

1. What is heat ? Write its S.I. unit.
Answer: Heat is the form of energy that is transferred between two bodies at two different temperatures when kept in contact. Its S.I. unit is the joule (J).

2. Two bodies at different temperatures are placed in contact. State the direction in which heat will flow.
Answer: Heat will naturally flow from the body at a higher temperature to the body at a lower temperature.

3. Name the S.I. unit of heat. How is it related to the unit calorie ?
Answer: The S.I. unit of heat is the joule (J). The relation is 1 calorie = 4.186 J (or nearly 4.2 J).

4. Define temperature and write its S.I. unit.
Answer: Temperature is a quantity which indicates the thermal state of a body (i.e., the degree of hotness or coldness) and determines the direction of flow of heat when two bodies at different temperatures are placed in contact. Its S.I. unit is the kelvin (K).

5. Why does a piece of ice when touched with hand, appear cool ? Explain.
Answer: It appears cool because heat energy flows from our warmer hand to the colder cube of ice.

6. Name two substances which expand on heating.
Answer: Brass and Iron.

7. Name two substances which contract on heating.
Answer: Water (from 0°C to 4°C) and silver iodide (from 80°C to 141°C).

8. What do you mean by anomalous expansion of water ?
Answer: The unusual expansion of water when it is cooled from 4°C to 0°C is known as the anomalous expansion of water.

9. At what temperature the density of water is maximum? State its value.
Answer: The density of water is maximum at 4°C. Its value is 1 g cm^-3 (or 1000 kg m^-3).

10. Deep pond of water has its top layer frozen during winter. State the expected temperature of water layer (i) just in contact with ice, (ii) at the bottom of pond.
Answer: (i) 0°C (ii) 4°C

(C) SHORT ANSWER TYPE :

1. Distinguish between heat and temperature.
Answer: Heat is a form of energy obtained due to random motion of molecules, with joule (J) as its S.I. unit, and its amount depends on mass, temperature, and material. Temperature, on the other hand, tells the thermal state of a body, has kelvin (K) as its S.I. unit, and depends on the average kinetic energy of the molecules. Heat is measured by the principle of calorimetry, while temperature is measured by a thermometer.

2. What do you understand by thermal expansion of a substance ?
Answer: The expansion of a substance on heating is called the thermal expansion of that substance.

3. Name the three kinds of thermal expansion. Out of solids, liquids and gases, which expands more ?
Answer: The three kinds are linear expansion, superficial expansion, and cubical expansion. Out of solids, liquids, and gases, gases expand the most on heating.

4. State the volume changes observed when a given mass of water is heated from 0°C to 10°C. Sketch a temperature-volume graph to show the behaviour.
Answer: When water is heated from 0°C, its volume first decreases until it reaches 4°C, making its volume minimum at this temperature. On heating further from 4°C to 10°C, its volume increases steadily.

5. Draw a graph to show the variation in density of water with temperature in the temperature range from 0°C to 10°C.
Answer: The required graph displays temperature on the X-axis and density on the Y-axis. The curve rises from 0°C to peak at its maximum point at exactly 4°C (1.000 g cm^-3), and then curves smoothly downwards as the temperature progresses toward 10°C.

6. A given mass of water is cooled from 10°C to 0°C. State the volume changes observed. Represent these changes on a temperature-volume graph.
Answer: As the water cools from 10°C to 4°C, its volume steadily decreases. It reaches its minimum volume at 4°C. Cooling it further from 4°C to 0°C causes the volume to expand.

7. Explain the following :
(a) Water pipes in colder countries often burst in winter.
(b) In winter, water tank (or ocean) starts freezing from the surface and not from the bottom.
(c) Fishes survive in ponds even when the atmospheric temperature is below 0°C.
(d) A hollow glass sphere which floats with its entire volume submerged in water at 4°C, sinks when water is heated above 4°C.
(e) A glass bottle completely filled with water and tightly closed at room temperature, is likely to burst when kept in the freezer of a refrigerator.
Answer:
(a) In winter, as atmospheric temperature falls below 4°C, the water inside the pipes expands due to anomalous expansion. This expansion exerts a large pressure on the pipes, causing them to burst.
(b) As water cools, it contracts and sinks until the entire body reaches 4°C. Upon further cooling below 4°C, it expands, becomes less dense, and stays at the surface where it ultimately freezes into ice.
(c) The anomalous expansion ensures that water at the bottom of the pond remains as a liquid at 4°C, even while the surface is frozen into ice, allowing fish and aquatic life to survive.
(d) At 4°C, water has its maximum density, allowing the sphere to float. As the water is heated above 4°C, it expands and its density decreases. As the water's density becomes less than that of the sphere, it sinks.
(e) As the water is cooled below 4°C in the freezer, it undergoes anomalous expansion. Because the bottle is completely full and tightly sealed, the expanding water creates immense internal pressure that bursts the glass.

(D) LONG ANSWER TYPE :

1. Describe an experiment to show that water has maximum density at 4°C. What important consequences follow from this peculiar property of water ? Discuss the importance of this phenomenon in nature.
Answer: Hope's experiment demonstrates this property. It uses a tall metallic cylinder with thermometers T₁ (top) and T₂ (bottom), surrounded at its center by a trough containing a freezing mixture of ice and salt. Initially, water is at room temperature. As the freezing mixture cools the water in the central portion, the water contracts, its density increases, and it sinks, causing T₂ to rapidly drop to 4°C while T₁ remains almost unchanged. Upon further cooling below 4°C, the water undergoes anomalous expansion, becomes less dense, and rises, causing T₁ to rapidly drop to 0°C while T₂ remains steady at 4°C. This proves that water has maximum density at 4°C. This property has important natural consequences: it helps in preserving aquatic life during very cold weather by allowing ice to form only on the surface of lakes, leaving water at 4°C below it. It is also responsible for the bursting of water pipelines and destruction of crops during cold nights.

2. Draw a diagram showing the temperature of various layers of water in an ice covered pond.
Answer: The diagram should depict the atmosphere below 0°C at the top. Directly underneath is a layer of solid ice at 0°C. Below the ice is a layer of liquid water at 0°C. Moving deeper downwards, the water temperature layers progressively increase to 1°C, 2°C, and 3°C, until the very bottom layer is shown maintaining a stable state at exactly 4°C.

EXERCISE-6(B)

(A) MULTIPLE CHOICE TYPE :
(Choose the correct answer from the options given below).

1. The gases which are good ........ of long wavelength infrared radiations are called greenhouse gases.
(a) emitters
(b) reflectors
(c) absorbers
(d) none of the above
Answer: The gases which are good absorbers of long wavelength infrared radiations are called greenhouse gases. Option (c) is correct.

2. Which of the following is a green house gas ?
(a) oxygen
(b) nitrogen
(c) chlorine
(d) carbon dioxide.
Answer: (d) carbon dioxide.

3. The increase of carbon dioxide gas in atmosphere will cause :
(a) decrease in temperature
(b) increase in temperature
(c) decrease in humidity
(d) increase in humidity.
Answer: (b) increase in temperature

4. Without green house effect, the average temperature of earth's surface would have been :
(a) -18°C
(b) 33°C
(c) 0°C
(d) 15°C
Answer: (a) -18°C

5. Global warming means ............ in average effective temperature near the earth's surface due to ....... in the amount of greenhouse gases in its atmosphere.
(a) decrease, increase
(b) decrease, decrease
(c) increase, decrease
(d) increase, increase
Answer: Global warming means increase in average effective temperature near the earth's surface due to increase in the amount of greenhouse gases in its atmosphere. Option (d) is correct.

6. The global warming has resulted in : (LS)
(a) the increase in yield of crops
(b) the decrease in sea levels
(c) the decrease in human deaths
(d) the increase in sea levels.
Answer: (d) the increase in sea levels.

(B) VERY SHORT ANSWER TYPE :

1. The conversion of part of energy into an unuseful form of energy is called ............ .
Answer: The conversion of part of energy into an unuseful form of energy is called degradation of energy.

2. What do you mean by greenhouse effect ?
Answer: Greenhouse effect is the process of warming of the earth's surface and its lower atmosphere by the absorption of infrared radiations of long wavelength emitted out from the surface of the earth by greenhouse gases.

3. Name three greenhouse gases.
Answer: Carbon dioxide, methane, and nitrous oxide (or chlorofluorocarbons).

4. Which of the following solar radiations pass through the atmosphere of the earth ? X-rays, ultraviolet rays, visible light rays, infrared radiation.
Answer: Visible light rays and infrared radiation of short wavelength pass through.

5. Name the radiations which are absorbed by the greenhouse gases.
Answer: Infrared radiations of long wavelength.

6. What would have been the temperature of earth's atmosphere in absence of greenhouse gases in it ?
Answer: The average temperature on earth would be -18°C (or 0°F).

(C) SHORT ANSWER TYPE :

1. What results in the increase of carbon dioxide contents of earth's atmosphere ?
Answer: Human activities such as the burning of fuels, deforestation, and industrial production are responsible for the increase.

2. State the effect of greenhouse gases on the temperature of earth's atmosphere.
Answer: They have an average warming effect, trapping heat and causing the average temperature of the earth's atmosphere to rise.

3. What do you mean by global warming ?
Answer: Global warming means the increase in average effective temperature near the earth's surface due to an increase in the amount of greenhouse gases in its atmosphere.

4. What causes the rise in atmospheric temperature?
Answer: An increase in the proportion of greenhouse gases present in the earth's atmosphere traps more heat radiated from the earth's surface, causing the rise in temperature.

5. State the cause of increase of greenhouse effect.
Answer: The increase in concentration of greenhouse gases in the atmosphere due to human activities like burning fossil fuels, deforestation, agricultural sources, and natural gas exploration.

6. What will be the effect of global warming at the poles? (HOTS)
Answer: The area of sea ice around both the poles is shrinking. It is expected that in the next 50 years, the ice at both the poles will melt completely and therefore the sea level will rise.

7. State the effect of global warming in coastal regions.
Answer: Buildings and roads in the coastal areas will get flooded due to rising sea levels, and they could suffer damage from hurricanes and tropical storms.

8. How will global warming affect the sea levels ?
Answer: Global warming leads to the melting of glaciers and polar ice, resulting in a steady increase in sea levels.

9. How will global warming affect agriculture ? (HOTS)
Answer: Warming will result in poor crop yield in farms in low latitude regions and will increase the rise of malnutrition. Farmers will have to shift their farming region from low latitudes to high latitudes.

10. State two ways to minimise the impact of global warming. (LS)
Answer: (1) Technological measures like using renewable sources of energy instead of fossil fuels. (2) Economic measures like reforestation and sustainable use of land.

11. What is carbon tax ? Who will pay it ?
Answer: Carbon tax is a tax calculated on the basis of carbon emission from the industry, number of employee hours, and turn over of the industry. It will be paid by industries that emit carbon dioxide to a great extent.

(D) LONG ANSWER TYPE :

1. State, in detail, the causes of global warming.
Answer: The cause of global warming is the increase in concentration of greenhouse gases present in the atmosphere of the earth due to human activities. The concentration of carbon dioxide has increased up to 25% due to industrial growth, combustion of fossil fuels, and clearing of forests. The concentration of methane has doubled due to agricultural sources like rice cultivation, animal husbandry, burning of biomass, and leakage in natural gas pipelines. The concentration of chlorofluorocarbons has also increased at a rate of 5% per year. These increases enhance the greenhouse effect and cause global warming.

2. What technological measures are necessary to minimise the impact of global warming ? (SDGs)
Answer: The necessary technological measures include: (a) Use of renewable sources of energy (like wind, solar, and tidal energy) to generate electricity instead of electricity from fossil fuel-based power plants. (b) Use of battery-operated vehicles such as electric cars, bikes, and scooters to drastically reduce carbon dioxide emissions. (c) Use of bio-char stoves for cooking in developing countries, which burn biomass without air to release smokeless combustible gases.

3. What policy measures should the government enforce in order to minimise global warming ?
Answer: To check global warming, the government must take two policy measures: (a) Educating children to live a sustainable lifestyle, making full use of what we have rather than consuming more in a competitive society. (b) Controlling population through family planning, welfare reforms, and the empowerment of women, ensuring free and easy access to family planning and promoting smaller families.

4. What do you mean by degradation of energy ? Explain it by taking two examples of your daily life.
Answer: Degradation of energy is the gradual decrease of useful energy due to radiation loss, friction, etc. When transforming energy from one form to another, the entire energy does not change into the desired form; a part of it changes into an undesirable form (usually heat) which is of no use. Examples: (i) When we light a bulb using electricity, less than 25% of electrical energy converts into light energy. The rest is wasted as heat in the filament. (ii) When we run a vehicle, only a part of the energy from fuel runs the vehicle, while a major part is wasted in heating the moving parts of the machine and working against friction between the ground and its tyres.

Competency Focussed Questions

(E) ASSERTION – REASON TYPE QUESTIONS :
(Choose the answer based on the codes given below.)
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are True and R is not the correct explanation of A
(c) Assertion is false but reason is true
(d) Assertion is true but reason is false

(i) Assertion (A) : Fishes survive in ponds even when the atmospheric temperature is below 0°C. (HOTS)
Reason (R) : The anomalous expansion of water helps in preserving aquatic life.
Answer: (a) Both A and R are true and R is the correct explanation of A.

(F) CASE STUDY BASED QUESTIONS :

1. A scientific research team records temperature data of a freshwater lake in the Himalayas during winter. Three thermometers A, B, and C are used, where A is calibrated in the Celsius scale, B in the Fahrenheit scale, and C in the Kelvin scale.
On a particular night, when the surface water of the lake freezes, the recorded temperatures are:
A -> 0°C, B -> 32°F, C -> 273 K
Next morning, the air temperature increases slightly, but the ice on the surface of the lake does not melt immediately. (SI)

(a) What happens to the fish present in the lake when the surface of the lake freezes?
Answer: The fish remain alive because the water below the ice layer remains at a higher temperature of up to 4°C.

(b) If the temperature of water at the bottom of the lake is measured, what would be the readings on thermometers A, B, and C?
Answer: The water at the bottom of the lake is at 4°C. Therefore, reading A = 4°C, reading B = 39.2°F, reading C = 277 K.

(c) Why does the ice formed on the surface of the lake not sink?
Answer: Ice does not sink because its density is less than the density of liquid water.

(d) Why does the ice on the lake take time to melt even after the air temperature rises above 0°C?
Answer: Ice takes time to melt because it is a poor conductor of heat (an insulator), which prevents the rapid transfer of heat from the warmer atmosphere into the frozen surface.

(e) Why is the Kelvin scale preferred in scientific calculations related to heat transfer?
Answer: The Kelvin scale is preferred because its zero point (0 K) is absolute zero, meaning negative temperatures are not possible, simplifying mathematical equations related to heat and gases.

2. Two cities, P and Q, have identical population and climate. City P uses coal-based thermal power and diesel vehicles, whereas City Q uses solar and wind energy along with electric vehicles. (SI)

(a) Which city is likely to show a higher average temperature rise after 10 years?
Answer: City P.

(b) Identify the main greenhouse gases responsible for the higher temperature rise in the city mentioned above by you in (a).
Answer: Carbon dioxide.

(c) "Energy is conserved, so using fossil fuels or renewable sources makes no difference." Is this statement scientifically correct? Give reason.
Answer: It is scientifically incorrect. While the total energy is conserved, burning fossil fuels leads to the degradation of energy into unusable forms like unwanted heat and causes a dangerous increase in greenhouse gases, whereas renewable sources do not cause such warming.

(d) Give physics-based reasons to explain which city will have long-term energy sustainability.
Answer: City Q will have long-term energy sustainability because it uses non-polluting renewable energy sources (solar and wind), avoiding the severe degradation of energy and rapid emission of greenhouse gases associated with the fossil fuels used by City P.