Chapter 2: Is Matter Around Us Pure - NCERT Solutions
In-Text Questions (Page No- 15)
Q1. What is meant by a substance?
Answer - A substance is a kind of matter that has a fixed composition and definite properties. It cannot be separated into other kinds of matter by physical methods. Examples include pure water, iron, and oxygen.
Q2. List the points of differences between homogeneous and heterogeneous mixtures.
Answer -
| Homogeneous Mixture | Heterogeneous Mixture |
|---|---|
| Has a uniform composition throughout. | Does not have a uniform composition. |
| Components are not visible separately. | Components are easily visible. |
| Example: Salt dissolved in water. | Example: Sand and iron filings. |
In-Text Questions (Page No- 18)
Q1. Differentiate between homogeneous and heterogeneous mixtures with examples.
Answer -
| Homogeneous Mixture | Heterogeneous Mixture | |
|---|---|---|
| Composition | Uniform throughout | Not uniform |
| Visibility of components | Components not visible separately | Components visible separately |
| Example | Sugar dissolved in water | Oil and water mixture |
Q2. How are sol, solution, and suspension different from each other?
Answer -
| Property | Solution | Sol | Suspension |
|---|---|---|---|
| Type of mixture | Homogeneous | Heterogeneous (colloid) | Heterogeneous |
| Particle size | Very small (< 1 nm) | Intermediate (1–1000 nm) | Large (> 1000 nm) |
| Visibility | Particles not visible | Particles not visible to naked eye | Particles visible |
| Stability | Stable | Stable | Unstable |
| Tyndall effect | Does not show | Shows | May show if particles are large enough |
Q3. 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.
Answer - Mass of solute (sodium chloride) = 36 g
Mass of solvent (water) = 100 g
Total mass of solution = 36 g + 100 g = 136 g
Concentration of solution = (Mass of solute / Mass of solution) × 100
= (36 / 136) × 100
= 26.47%
Therefore, the concentration of the solution is 26.47%.
Mass of solvent (water) = 100 g
Total mass of solution = 36 g + 100 g = 136 g
Concentration of solution = (Mass of solute / Mass of solution) × 100
= (36 / 136) × 100
= 26.47%
Therefore, the concentration of the solution is 26.47%.
In-Text Questions (Page No- 19)
Q1. 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, 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.
Answer -
| Change | Type |
|---|---|
| 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 and the water breaking down into hydrogen and oxygen gases | 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 |
Q2. Try segregating the things around you as pure substances or mixtures.
Answer -
| Pure Substances | Mixtures |
|---|---|
| Distilled water | Salt water |
| Iron | Air |
| Oxygen gas | Milk |
| Carbon dioxide | Soil |
| Gold | Sand and sugar mixture |
Exercises
Q1. 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) 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
Answer - (a) Evaporation or crystallization
(b) Sublimation (Ammonium chloride sublimes)
(c) Filtration (using a filter paper)
(d) Chromatography (for separating pigments)
(e) Centrifugation or churning
(f) Separation using a separating funnel
(g) Filtration
(h) Magnetic separation (using a magnet)
(i) Winnowing (using wind)
(j) Sedimentation followed by decantation or filtration
(b) Sublimation (Ammonium chloride sublimes)
(c) Filtration (using a filter paper)
(d) Chromatography (for separating pigments)
(e) Centrifugation or churning
(f) Separation using a separating funnel
(g) Filtration
(h) Magnetic separation (using a magnet)
(i) Winnowing (using wind)
(j) Sedimentation followed by decantation or filtration
Q2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
Answer - Steps to make tea:
1. Boil water (solvent) in a vessel.
2. Add tea leaves (solute) into the boiling water.
3. Stir to dissolve the soluble components of tea leaves.
4. Let the mixture steep to form a solution.
5. Use a strainer to separate the insoluble tea leaves (residue) from the liquid (filtrate).
6. Add sugar (solute) which dissolves in the tea (solvent) to form a sweet solution.
7. Serve the tea.
1. Boil water (solvent) in a vessel.
2. Add tea leaves (solute) into the boiling water.
3. Stir to dissolve the soluble components of tea leaves.
4. Let the mixture steep to form a solution.
5. Use a strainer to separate the insoluble tea leaves (residue) from the liquid (filtrate).
6. Add sugar (solute) which dissolves in the tea (solvent) to form a sweet solution.
7. Serve the tea.
Q3. The following table gives the solubility of various salts at different temperatures (solubility refers to the amount in grams of the salt 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?
(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.
(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?
(d) What is the effect of change of temperature on the solubility of a salt?
| Substance | 283 K | 293 K | 313 K | 333 K | 353 K |
|---|---|---|---|---|---|
| Potassium nitrate | 21 | 32 | 62 | 106 | 167 |
| Sodium chloride | 36 | 36 | 36 | 37 | 37 |
| Potassium chloride | 35 | 35 | 40 | 46 | 54 |
| Ammonium chloride | 24 | 37 | 41 | 55 | 66 |
(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?
(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.
(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?
(d) What is the effect of change of temperature on the solubility of a salt?
Answer - (a) At 313 K, the solubility of potassium nitrate is 62 g per 100 g of water. So, for 50 g of water:
Required mass = (62 × 50) / 100 = 31 g
(b) As the solution cools, the solubility of potassium chloride decreases. The excess dissolved salt will start to crystallize out. So, crystals of potassium chloride will be seen settling at the bottom.
(c) Solubility at 293 K:
Potassium nitrate: 32 g
Sodium chloride: 36 g
Potassium chloride: 35 g
Ammonium chloride: 37 g
Ammonium chloride has the highest solubility at this temperature.
(d) In general, the solubility of salts increases with increase in temperature. This means more solute can dissolve in a given amount of solvent at higher temperatures.
Required mass = (62 × 50) / 100 = 31 g
(b) As the solution cools, the solubility of potassium chloride decreases. The excess dissolved salt will start to crystallize out. So, crystals of potassium chloride will be seen settling at the bottom.
(c) Solubility at 293 K:
Potassium nitrate: 32 g
Sodium chloride: 36 g
Potassium chloride: 35 g
Ammonium chloride: 37 g
Ammonium chloride has the highest solubility at this temperature.
(d) In general, the solubility of salts increases with increase in temperature. This means more solute can dissolve in a given amount of solvent at higher temperatures.
Q4. Explain the following giving examples.
(a) Saturated solution
(b) Pure substance
(c) Colloid
(d) Suspension
(a) Saturated solution
(b) Pure substance
(c) Colloid
(d) Suspension
Answer - (a) Saturated solution: A solution in which no more solute can dissolve at a given temperature. Example: Sugar dissolved in water until no more dissolves.
(b) Pure substance: Matter that has a fixed composition and definite properties. Examples include pure water, oxygen.
(c) Colloid: A mixture where particles are dispersed but not settled out. Example: Milk.
(d) Suspension: A heterogeneous mixture where particles settle out over time. Example: Muddy water.
(b) Pure substance: Matter that has a fixed composition and definite properties. Examples include pure water, oxygen.
(c) Colloid: A mixture where particles are dispersed but not settled out. Example: Milk.
(d) Suspension: A heterogeneous mixture where particles settle out over time. Example: Muddy water.
Q5. Classify each of the following as a homogeneous or heterogeneous mixture.
soda water, wood, air, soil, vinegar, filtered tea.
soda water, wood, air, soil, vinegar, filtered tea.
Answer - Homogeneous mixtures: soda water, air, vinegar, filtered tea
Heterogeneous mixtures: wood, soil
Heterogeneous mixtures: wood, soil
Q6. How would you confirm that a colourless liquid given to you is pure water?
Answer - To confirm if the liquid is pure water:
1. Check if it boils at 100°C and freezes at 0°C.
2. Pure water does not leave any residue upon evaporation.
3. It is tasteless and odourless.
4. Using chemical tests or purity analysis in labs can confirm purity.
1. Check if it boils at 100°C and freezes at 0°C.
2. Pure water does not leave any residue upon evaporation.
3. It is tasteless and odourless.
4. Using chemical tests or purity analysis in labs can confirm purity.
Q7. 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) Soil
(j) Air
(a) Ice
(b) Milk
(c) Iron
(d) Hydrochloric acid
(e) Calcium oxide
(f) Mercury
(g) Brick
(h) Wood
(i) Soil
(j) Air
Answer - Pure substances: (a) Ice, (c) Iron, (d) Hydrochloric acid, (e) Calcium oxide, (f) Mercury
Impure mixtures or others: (b) Milk, (g) Brick, (h) Wood, (i) Soil, (j) Air
Impure mixtures or others: (b) Milk, (g) Brick, (h) Wood, (i) Soil, (j) Air
Q8. Identify the solutions among the following mixtures.
(a) Soil
(b) Sea water
(c) Air
(d) Coal
(e) Soda water
(a) Soil
(b) Sea water
(c) Air
(d) Coal
(e) Soda water
Answer - Solutions: (b) Sea water, (c) Air, (e) Soda water
Non-solutions: (a) Soil, (d) Coal
Non-solutions: (a) Soil, (d) Coal
Q9. Which of the following will show “Tyndall effect”?
(a) Salt solution
(b) Milk
(c) Copper sulphate solution
(d) Starch solution
(a) Salt solution
(b) Milk
(c) Copper sulphate solution
(d) Starch solution
Answer - Tyndall effect is shown by colloids and suspensions:
(b) Milk
(d) Starch solution
Salt solution and copper sulphate solution do not show Tyndall effect as they are true solutions.
(b) Milk
(d) Starch solution
Salt solution and copper sulphate solution do not show Tyndall effect as they are true solutions.
Q10. 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
(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
Answer - Elements: (a) Sodium, (d) Silver, (f) Tin, (g) Silicon
Compounds: (e) Calcium carbonate, (k) Methane, (l) Carbon dioxide
Mixtures: (b) Soil, (c) Sugar solution, (h) Coal, (i) Air, (j) Soap, (m) Blood
Compounds: (e) Calcium carbonate, (k) Methane, (l) Carbon dioxide
Mixtures: (b) Soil, (c) Sugar solution, (h) Coal, (i) Air, (j) Soap, (m) Blood
Q11. 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
(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
Answer - Chemical changes:
(a) Growth of a plant
(b) Rusting of iron
(d) Cooking of food
(e) Digestion of food
(g) Burning of a candle
These involve formation of new substances and are irreversible.
Physical changes:
(c) Mixing of iron filings and sand
(f) Freezing of water
These do not involve any chemical transformation.
(a) Growth of a plant
(b) Rusting of iron
(d) Cooking of food
(e) Digestion of food
(g) Burning of a candle
These involve formation of new substances and are irreversible.
Physical changes:
(c) Mixing of iron filings and sand
(f) Freezing of water
These do not involve any chemical transformation.