2. Is Matter Around us Pure ? Class 9 Science Chapter 2 NCERT Solutions (CBSE)
CBSE Class 9 Science Chapter 2: Is Matter Around us Pure ? – Full NCERT Textbook Solutions
Chapter 2: Is Matter Around us Pure ?
Internal Questions:
1. What is meant by a substance?
Answer: A substance is a pure single form of matter. It can be an element or a compound, characterized by its unique set of physical and chemical properties.
2. List the points of differences between homogeneous and heterogeneous mixtures.
Answer: The differences between homogeneous and heterogeneous mixtures are :
|
Homogeneous Mixture |
Heterogeneous Mixture |
|
1. It has a uniform composition throughout the mixture. |
1. It has a non-uniform composition throughout the mixture. |
|
2. The components of the mixture are not visible to the naked eye. |
2. The components of the mixture are visible to the naked eye. |
|
3. Particles are evenly distributed. |
3. Particles are unevenly distributed. |
|
4. It does not show the Tyndall effect (particles are too small to scatter light). |
4. It may show the Tyndall effect (e.g., dust in air, milk). |
|
5. Examples: Sugar solution , Salt solution , Air (mixture of gases) , Alloys like brass ,.... |
5. Examples: Mixture of sand and sugar , Oil and water , Granite (different mineral grains visible) , Soil , ....... |
Internal Questions:
Example 2.1: A solution contains 40 g of common salt in 320 g of water. Calculate the concentration in terms of mass by mass percentage of the solution.
Solution: Mass of solute (salt) = 40 g and Mass of solvent (water) = 320 g
Mass of solution = Mass of solute + Mass of solvent = 40 g + 320 g = 360 g
Mass percentage of solution 
Internal Questions:
1. Differentiate between homogeneous and heterogeneous mixtures with examples.
Answer: The differences between homogeneous and heterogeneous mixtures are:
|
Homogeneous Mixture |
Heterogeneous Mixture |
|
1. It has a uniform composition throughout the mixture. |
1. It has a non-uniform composition throughout the mixture. |
|
2. The components of the mixture are not visible to the naked eye. |
2. The components of the mixture are visible to the naked eye. |
|
3. Particles are evenly distributed. |
3. Particles are unevenly distributed. |
|
4. It does not show the Tyndall effect (particles are too small to scatter light). |
4. It may show the Tyndall effect (e.g., dust in air, milk). |
|
5. Examples: Sugar solution , Salt solution , Air (mixture of gases) , Alloys like brass ,.... |
5. Examples: Mixture of sand and sugar , Oil and water , Granite (different mineral grains visible) , Soil , ....... |
2. How are sol, solution and suspension different from each other?
Answer: Sol, solution, and suspension are different types of mixtures:
Sol: A sol is a colloidal dispersion where solid particles are dispersed in a liquid. The particles are larger than those in a solution but smaller than in a suspension.
Solution: A solution is a homogeneous mixture of two or more substances where the solute is uniformly dissolved in the solvent at a molecular level.
Suspension: A suspension is a heterogeneous mixture where solid particles are dispersed in a liquid, but the particles are larger and not fully dissolved, resulting in a cloudy or opaque appearance.
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.
Solution: Given: Mass of sodium chloride (solute) = 36 g Mass of water (solvent) = 100 g
Mass of solution = Mass of solute + Mass of solvent
Mass of solution = 36 g + 100 g
Mass of solution = 136 g
The concentration of the solution is the mass of the solute divided by the mass of the solution, multiplied by 100 to express it as a percentage:
Concentration 
Therefore, the concentration of the saturated solution of sodium chloride at 293 K is 26.47%.
Internal Questions:
1. How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than 25°C), which are miscible with each other?
Answer: A mixture of kerosene and petrol can be separated by simple distillation. Since the difference in their boiling points is more than 25°C, the component with the lower boiling point (petrol) vaporises first. The vapour is cooled and condensed to collect petrol, while kerosene remains behind in the flask.
2. Name the technique to separate
(i) butter from curd, (ii) salt from sea-water, (iii) camphor from salt.
Answer: The techniques are :
(i) Butter from curd → Centrifugation (or churning)
(ii) Salt from sea-water → Evaporation
(iii) Camphor from salt → Sublimation
3. What type of mixtures are separated by the technique of crystallization ?
Answer: Crystallisation is used to separate homogeneous mixtures, especially solids dissolved in liquids. It is mainly used to obtain a pure solid from its solution, for example, getting pure salt or sugar from their impure solutions.
Internal Questions:
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.
Answer: The classification of the given processes as chemical or physical changes:
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
2. Try segregating the things around you as pure substances or mixtures.
Answer: Segregating the things around me as pure substances or mixtures:
Pure Substances:
Elements: Copper wire, iron nail, aluminium foil.
Compounds: Salt (NaCl), sugar, distilled water, baking soda.
Mixtures:
Homogeneous Mixtures: Air (inside the room), tap water, soft drink, vinegar, alloy (e.g., brass statue).
Heterogeneous Mixtures: Wooden table, soil, granite rock, a bowl of dal or vegetable soup, a mixture of sand and salt.
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.
Answer: The suitable separation techniques are :
(a) Sodium chloride from its solution → Evaporation
(b) Ammonium chloride from mixture → Sublimation
(c) Metal pieces from engine oil → Filtration
(d) Pigments from flower extract → Chromatography
(e) Butter from curd → Centrifugation (churning)
(f) Oil from water → Separating funnel
(g) Tea leaves from tea → Filtration
(h) Iron pins from sand → Magnetic separation
(i) Wheat grains from husk → Winnowing
(j) Fine mud particles in water → Centrifugation
2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
Answer: To make tea, first take water as the solvent and heat it. Add sugar, which is the solute; it dissolves in water as it is soluble, forming a solution. Then add tea leaves, which are insoluble. After boiling, filter the mixture. The liquid tea obtained is the filtrate, and the tea leaves left behind 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)
|
Substances Dissolved |
Temperature in K |
||||
|
283 |
293 |
313 |
333 |
353 |
|
|
Solubility |
|||||
|
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) Let 
grams of potassium nitrate are needed to produce a saturated solution in 50 grams of water at 313 K.
We have, 
grams
So, 31 grams of potassium nitrate would be needed to produce a saturated solution in 50 grams of water at 313 K.
(b) As the solution cools, the solubility of potassium chloride decreases. So, excess potassium chloride will come out of the solution in the form of crystals.
(c) At 293 K:
Potassium nitrate = 32 g
Sodium chloride = 36 g
Potassium chloride = 35 g
Ammonium chloride = 37 g
Highest solubility: Ammonium chloride (37 g).
(d) Generally, the solubility of most salts increases with increase in temperature and decreases when temperature is lowered.
4. Explain the following giving examples.
(a) saturated solution (b) pure substance (c) colloid (d) suspension
Answer: (a) Saturated solution : A solution in which no more solute can be dissolved at a given temperature is called a saturated solution. Example: Salt solution in water where no more salt dissolves.
(b) Pure substance : A substance made up of only one kind of particles with fixed composition and definite properties is called a pure substance. Example: Gold, distilled water.
(c) Colloid : Colloids are heterogeneous mixtures in which the particle size is too small to be seen with the naked eye, but is big enough to scatter light. Example: Milk, fog.
(d) Suspension : A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium . Example: Mud in 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.
Answer: The classification is:
Homogeneous mixtures: Soda water , Air , Vinegar and Filtered tea .
Heterogeneous mixtures: Wood and Soil .
6. How would you confirm that a colourless liquid given to you is pure water?
Answer: Measure its boiling point and freezing point. Pure water boils at 373 K (100°C) and freezes at 273 K (0°C) at normal pressure. If the given liquid shows these fixed temperatures, it confirms that it is pure water.
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.
Answer: The materials that fall in the category of a "pure substance" are: (a) Ice , (c) Iron , (d) Hydrochloric acid , (e) Calcium oxide and (f) Mercury
[ Not pure substances (mixtures): (b) Milk (g) Brick (h) Wood (i) Air ]
8. Identify the solutions among the following mixtures.
(a) Soil
(b) Sea water
(c) Air
(d) Coal
(e) Soda water.
Answer: The solutions among the following mixtures are: (b) Sea water , (c) Air and (e) Soda water
[ Not solutions: (a) Soil (d) Coal ]
9. Which of the following will show “Tyndall effect”?
(a) Salt solution
(b) Milk
(c) Copper sulphate solution
(d) Starch solution.
Answer: The substances that will show the Tyndall effect among the given options are: (b) Milk and (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
Answer: The classification is:
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
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.
Answer: (a) Growth of a plant → Chemical change (new cells and complex substances are formed through photosynthesis and other biochemical processes).
(b) Rusting of iron → Chemical change (iron reacts with oxygen and moisture to form a new substance, hydrated iron oxide/rust).
(c) Mixing of iron filings and sand → Physical change (no new substance is formed; they can be separated by physical means like a magnet).
(d) Cooking of food → Chemical change (raw food undergoes chemical reactions to form new substances with different properties).
(e) Digestion of food → Chemical change (complex food molecules are broken down into simpler ones by enzymes and acids in the body).
(f) Freezing of water → Physical change (water changes from liquid to solid state; the chemical composition H₂O remains the same).
(g) Burning of a candle → Chemical change (wax reacts with oxygen to produce carbon dioxide, water vapour, heat, and light; new substances are formed).
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