# NCERT Solutions Class 9 Science Chapter 2 Is Matter Around Us Pure

### NCERT In-Text Questions Solutions

#### In-Text Questions Page 15

1. What is meant by a pure substance?
Ans:- A pure substance consists of a single type of particles that cannot be separated into other kinds of matter by any physical process.
2. List the points of differences between homogeneous and heterogeneous mixtures.
Ans:- Differences between homogeneous and heterogeneous mixture:

#### In-Text Questions Page 18

1. Differentiate between homogeneous and heterogeneous mixtures with examples.
Ans:-

2. How are sol, solution and suspension different from each other?
Ans:-

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.
Ans:- Mass of solute (NaCl) = 36 g
Mass of solvent (H2O) = 100 g
Mass of solution (NaCl + H2O) = 136 g

Concentration =
Mass of solute
Mass of solution
x100 =
36
136
x100=26.47%

Hence, the concentration of the solution is 26.47%.

#### In-Text Questions Page 24

1. How will you separate a mixture containing kerosene and petrol (difference in their boiling points is more than 25ºC), which are miscible with each other?
Ans:- Kerosene and petrol are miscible liquids and also the difference between their boiling point is more than 25ºC. So, they can be separated by the method of Distillation.
2. Name the technique to separate:
(i) butter from curd,
(ii) salt from sea-water,
(iii) camphor from salt.

Ans:- (i) Centrifugation
(ii) Evaporation
(iii) Sublimation
3. What type of mixtures are separated by the technique of crystallisation?
Ans:- The technique of crystallization is used to separate solids from a liquid solution in crystals form. Ex:- Crystallisation is used for the purification of salt that we get from sea water.

#### In-Text Questions Page 24

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.

Ans:-

2. Try segregating the things around you as pure substances or mixtures.
Ans:- Listed below are the classifications based on pure substances and mixtures:

#### Chapter End 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.

Ans:- (a) Evaporation
(b) Sublimation
(c) Filtration
(d) Chromatography
(e) Centrifugation
(f) Separating funnel
(g) Filtration
(h) Magnetic separation
(i) Winnowing/ sedimentation
(j) Decantation and filtration

2. Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.
Ans:- 1. Take a cup of water in a container as solvent and heat it.
2. Add sugar in it which is solute. Heat it till all sugar dissolves.
3. You get a solution of water and sugar.
4. Sugar is soluble in water completely.
5. Add half a tea-spoon of tea-leaves, it is insoluble in water.
6. Boil the content, add milk which is also soluble in water, boil again.
7. Filter the tea with the help of strainer, the tea collected in cup is filtrate and the tea leaves collected on the strainer is 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?
(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?
Ans:- (a) Given:
Mass of potassium nitrate required to produce a saturated solution in 100 g of water at 313 K = 50g
Mass of potassium nitrate required to produce a saturated solution in 50 g of water = 62 x 50/100 = 31 g
Hence 31 g of potassium nitrate is required.

(b) The solubility of potassium chloride in water is decreased when a saturated solution of potassium chloride cools and loses heat. Consequently, Pragya would observe some amount of potassium chloride in the form of crystals.

(c) At 293 K,
Solubility of Potassium nitrate = 32/100
Solubility of Sodium chloride = 36/100
Solubility of Potassium chloride = 35/100
Solubility of Ammonium chloride = 37/100
It is observed that the ammonium chloride salt has the highest amount of solubility when compared to any other salt at 293 K.

(d) Effect of change of temperature on the solubility of salts:
(i) On increasing the temperature, the solubility of a salt increases.
(ii) On decreasing the temperature, the solubility of a salt decreases.

4. Explain the following giving examples.
(a) saturated solution
(b) pure substance
(c) colloid
(d) suspension

Ans:- (a) At any particular temperature, a solution that has dissolved as much solute as it is capable of dissolving is called a saturated solution.
For example continue to dissolve salt in water until it will no longer dissolve creates a saturated solution.
(b) A pure substance consists of a single type of particles that cannot be separated into other kinds of matter by any physical process.
For example water, sugar, salt etc.
(c) A colloid is a heterogeneous mixture whose particles are uniformly spread throughout the mixture.
For example milk, smoke, fog, foam etc.
(d) A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium.
For example muddy water, mixture of chalk in water etc.

5. Classify each of the following as a homogeneous or heterogeneous mixture.
soda water, wood, air, soil, vinegar, filtered tea.
Ans:- The following is the classification of the given substances into homogenous and heterogenous mixture:

6. How would you confirm that a colourless liquid given to you is pure water?
Ans:- We can confirm if a colourless liquid is pure by setting it to boil. If it boils at 100°C it is said to be pure. Because pure substances have fixed melting and boiling point.

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

Ans:- Following substances from the above-mentioned list are pure substances:
(i) Iron
(ii) Ice
(iii) Hydrochloric acid
(iv) Calcium oxide
(v) Mercury

8. Identify the solutions among the following mixtures.
(a) Soil
(b) Sea water
(c) Air
(d) Coal
(e) Soda water

Ans:- The following are the solutions from the above-mentioned list of mixture:
(i) Sea water
(ii) Air
(iii) Soda water

9. Which of the following will show “Tyndall effect”?
(a) Salt solution
(b) Milk
(c) Copper sulphate solution
(d) Starch solution.

Ans:- Tyndall effect is exhibited by only milk and starch solution from the above-mentioned list of solutions.

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

Ans:-

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

Ans:- Chemical changes are:
(a) Growth of plant
(b) Rusting of iron
(c) Cooking of food
(d) Digestion of food
(e) Burning of a candle