IS MATTER AROUND US PURE? - Q&A
Questions Page 15
1. What is meant by a substance?
A substance is a pure single form of matter that cannot be separated into other kinds of matter by any physical process. It consists of a single type of particles (atoms or molecules) which are the same in their chemical nature.
2. List the points of differences between homogeneous and heterogeneous mixtures.
The differences between homogeneous and heterogeneous mixtures are:
| Homogeneous Mixture | Heterogeneous Mixture |
|---|---|
| It has a uniform composition throughout. | It has a non-uniform composition throughout. |
| The particles are not visible to the naked eye. | The particles are often visible to the naked eye. |
| There are no visible boundaries of separation between the constituents. | There are visible boundaries of separation between the constituents. |
| Examples: Sugar in water, salt in water, air. | Examples: Sand and salt, oil and water, mixture of iron filings and sulphur. |
Questions Page 18
1. Differentiate between homogeneous and heterogeneous mixtures with examples.
Homogeneous Mixtures: Mixtures which have a uniform composition throughout are called homogeneous mixtures. The components are thoroughly mixed and cannot be distinguished from one another.
Examples: Salt solution, sugar solution, lemonade, air.
Heterogeneous Mixtures: Mixtures which contain physically distinct parts and have non-uniform compositions are called heterogeneous mixtures. The components can be seen separately.
Examples: Mixture of sodium chloride and iron filings, oil and water, chalk powder in water.
2. How are sol, solution and suspension different from each other?
| Property | Solution (True Solution) | Sol (Colloid) | Suspension |
|---|---|---|---|
| Type of Mixture | Homogeneous | Heterogeneous (appears homogeneous) | Heterogeneous |
| Particle Size | Very small (< 1 nm or 10-9 m) | Intermediate (1 nm to 1000 nm) | Large (> 1000 nm) |
| Tyndall Effect | Does not scatter light | Scatters beam of light (Tyndall effect) | Scatters beam of light |
| Stability | Stable (particles do not settle) | Stable (particles do not settle) | Unstable (particles settle down) |
| Filtration | Cannot be separated by filtration | Cannot be separated by filtration | Can be separated by filtration |
| Appearance | Transparent | Translucent | Opaque |
3. To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293 K. Find its concentration at this temperature.
Given:
Mass of solute (sodium chloride) = 36 g
Mass of solvent (water) = 100 g
Step 1: Calculate the mass of the solution.
Mass of solution = Mass of solute + Mass of solvent
Mass of solution = 36 g + 100 g = 136 g
Step 2: Calculate the concentration (mass by mass percentage).
Concentration = (Mass of solute / Mass of solution) × 100
Concentration = (36 / 136) × 100
Concentration = 0.2647 × 100
Concentration = 26.47%
Questions Page 19
1. Classify the following as chemical or physical changes:
• cutting of trees,
• melting of butter in a pan,
• rusting of almirah,
• boiling of water to form steam,
• passing of electric current through water and the water breaking down into hydrogen and oxygen gases,
• dissolving common salt in water,
• making a fruit salad with raw fruits, and
• burning of paper and wood.
• Cutting of trees: Physical change
• Melting of butter in a pan: Physical change
• Rusting of almirah: Chemical change
• Boiling of water to form steam: Physical change
• Passing of electric current through water...: Chemical change
• Dissolving common salt in water: Physical change
• Making a fruit salad with raw fruits: Physical change
• Burning of paper and wood: Chemical change
2. Try segregating the things around you as pure substances or mixtures.
Pure Substances: Water, Sugar, Salt, Iron nail, Copper wire, Aluminum foil.
Mixtures: Air, Soil, Milk, Tea, Lemonade, Wood, Steel, Paper.
Exercises
1. Which separation techniques will you apply for the separation of the following?
(a) Sodium chloride from its solution in water.
(b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride.
(c) Small pieces of metal in the engine oil of a car.
(d) Different pigments from an extract of flower petals.
(e) Butter from curd.
(f) Oil from water.
(g) Tea leaves from tea.
(h) Iron pins from sand.
(i) Wheat grains from husk.
(j) Fine mud particles suspended in water.
(a) Evaporation
(b) Sublimation
(c) Filtration
(d) Chromatography
(e) Centrifugation
(f) Separating funnel
(g) Filtration
(h) Magnetic separation
(i) Winnowing
(j) Centrifugation (or Loading followed by Filtration)
2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
1. Take water in a vessel and boil it. Water acts as a solvent.
2. Add tea leaves to the boiling water. The tea leaves are insoluble in water.
3. Add sugar and milk. Sugar is a solute and is soluble in water.
4. The sugar and the extracts from tea leaves dissolve in the water to form a solution.
5. Filter the mixture using a strainer.
6. The liquid that passes through the strainer is the tea, which is the filtrate.
7. The tea leaves that remain on the strainer are the residue.
3. Pragya tested the solubility of three different substances at different temperatures and collected the data as given below (results are given in the following table, as grams of substance dissolved in 100 grams of water to form a saturated solution).
(a) What mass of potassium nitrate would be needed to produce a saturated solution of potassium nitrate in 50 grams of water at 313 K?
From the table, solubility of Potassium nitrate at 313 K is 62 g in 100 g of water.
So, for 50 g of water (which is half of 100 g), the mass required will be half.
Mass needed = 62 / 2 = 31 g.
(b) Pragya makes a saturated solution of potassium chloride in water at 353 K and leaves the solution to cool at room temperature. What would she observe as the solution cools? Explain.
She would observe that crystals of potassium chloride separate out (precipitate) from the solution. This happens because the solubility of solids in liquids generally decreases as the temperature decreases. The excess solute that can no longer stay dissolved at the lower temperature forms solid crystals.
(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?
Solubilities at 293 K are:
- Potassium nitrate: 32 g
- Sodium chloride: 36 g
- Potassium chloride: 35 g
- Ammonium chloride: 37 g
The salt with the highest solubility at 293 K is Ammonium chloride (37 g).
(d) What is the effect of change of temperature on the solubility of a salt?
The solubility of a salt (solid) in water typically increases with an increase in temperature and decreases with a decrease in temperature.
4. Explain the following giving examples.
(a) Saturated solution
(b) Pure substance
(c) Colloid
(d) Suspension
(a) Saturated solution: A solution in which no more solute can be dissolved at a given temperature is called a saturated solution. Example: Adding salt to water until it stops dissolving.
(b) Pure substance: A substance consisting of a single type of particles (atoms or molecules) with fixed chemical properties is called a pure substance. Example: Gold, Water, Sugar.
(c) Colloid: A heterogeneous mixture where the particle size is intermediate between a true solution and a suspension (1 nm to 1000 nm). The particles are small enough not to settle but large enough to scatter light. Example: Milk, fog, smoke.
(d) Suspension: A heterogeneous mixture containing solid particles large enough for sedimentation (greater than 1000 nm). The particles are visible to the naked eye. Example: Muddy water, chalk powder in water.
5. Classify each of the following as a homogeneous or heterogeneous mixture.
soda water, wood, air, soil, vinegar, filtered tea.
Homogeneous mixtures: Soda water, air, vinegar, filtered tea.
Heterogeneous mixtures: Wood, soil.
6. How would you confirm that a colourless liquid given to you is pure water?
We can confirm if the liquid is pure water by determining its boiling point. Pure water boils exactly at 100°C (373 K) at atmospheric pressure (1 atm). If the liquid boils at this precise temperature without variation, it is pure water. If it boils at a higher or variable temperature, it contains impurities.
7. Which of the following materials fall in the category of a "pure substance"?
(a) Ice (b) Milk (c) Iron (d) Hydrochloric acid (e) Calcium oxide (f) Mercury (g) Brick (h) Wood (i) Air
The pure substances are:
(a) Ice
(c) Iron
(d) Hydrochloric acid (as a compound)
(e) Calcium oxide
(f) Mercury
8. Identify the solutions among the following mixtures.
(a) Soil (b) Sea water (c) Air (d) Coal (e) Soda water
The solutions are:
(b) Sea water
(c) Air
(e) Soda water
9. Which of the following will show "Tyndall effect"?
(a) Salt solution (b) Milk (c) Copper sulphate solution (d) Starch solution
The Tyndall effect is shown by colloids. The correct answers are:
(b) Milk
(d) Starch solution
10. Classify the following into elements, compounds and mixtures.
(a) Sodium (b) Soil (c) Sugar solution (d) Silver (e) Calcium carbonate (f) Tin (g) Silicon (h) Coal (i) Air (j) Soap (k) Methane (l) Carbon dioxide (m) Blood
Elements: Sodium, Silver, Tin, Silicon.
Compounds: Calcium carbonate, Soap, Methane, Carbon dioxide.
Mixtures: Soil, Sugar solution, Coal, Air, Blood.
11. Which of the following are chemical changes?
(a) Growth of a plant
(b) Rusting of iron
(c) Mixing of iron filings and sand
(d) Cooking of food
(e) Digestion of food
(f) Freezing of water
(g) Burning of a candle
The chemical changes are:
(a) Growth of a plant
(b) Rusting of iron
(d) Cooking of food
(e) Digestion of food
(g) Burning of a candle