Class 9th Science Chapter-2 Exercises

Chapter 2 – Is Matter Around Us Pure?
Exercise – Questions and Answers

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Question 1:
Which separation techniques will you apply for the separation of the following?

(a) Sodium chloride from its solution in water

Technique: Evaporation
Reason: Water evaporates on heating, leaving behind solid sodium chloride.

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(b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride

Technique: Sublimation
Reason: Ammonium chloride sublimes on heating, while sodium chloride does not.

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(c) Small pieces of metal in the engine oil of a car

Technique: Filtration
Reason: Metal particles are insoluble solids and can be separated using a filter.

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(d) Different pigments from an extract of flower petals

Technique: Chromatography
Reason: Different pigments move at different speeds on the chromatographic paper.

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(e) Butter from curd

Technique: Centrifugation
Reason: Butter (lighter) separates from curd (heavier) on spinning.

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(f) Oil from water

Technique: Separating funnel
Reason: Oil and water are immiscible liquids with different densities.

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(g) Tea leaves from tea

Technique: Filtration
Reason: Tea leaves are insoluble solids present in liquid tea.

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(h) Iron pins from sand

Technique: Magnetic separation
Reason: Iron is attracted by a magnet, sand is not.

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(i) Wheat grains from husk

Technique: Winnowing
Reason: Husk is lighter and gets blown away by wind, grains fall down.

(j) Fine mud particles suspended in water

Technique: Sedimentation and decantation (or centrifugation)
Reason: Mud particles settle at the bottom due to gravity.

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Question 2:
Write the steps you would use for making tea. Use the words solution, solvent, solute, dissolve, soluble, insoluble, filtrate and residue.

Answer: Steps for making tea

1. Take water in a pan. Water acts as the solvent.

2. Heat the water and add tea leaves. Some substances from the tea leaves are soluble in water and dissolve in it, forming a solution.

3. Add sugar, which is the solute, and it dissolves completely in the water as it is soluble.

4. Add milk and continue heating the mixture.

5. The tea leaves are insoluble in water and do not dissolve.

6. Filter the tea using a strainer. The liquid tea obtained is the filtrate.

7. The tea leaves left on the strainer are the residue.

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Question 3:
Pragya tested the solubility of three different substances at different temperatures and collected the data as given in the table (grams of substance dissolved in 100 g of water to form a saturated solution).

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(a) What mass of potassium nitrate would be needed to produce a saturated solution in 50 g of water at 313 K?

At 313 K, solubility of potassium nitrate = 62 g per 100 g of water

For 50 g of water:

$$\text{Required mass}=\frac{62}{100}\times 50=31\text{ g}$$

Answer: 31 g of potassium nitrate

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(b) Pragya makes a saturated solution of potassium chloride in water at 353 K and leaves it to cool at room temperature. What would she observe? Explain.

• At 353 K, solubility of potassium chloride = 54 g per 100 g water

• At room temperature (≈293 K), solubility = 35 g per 100 g water

On cooling, the excess potassium chloride crystallises out of the solution.

Answer: Crystals of potassium chloride will separate out because solubility decreases on cooling.

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(c) Find the solubility of each salt at 293 K. Which salt has the highest solubility at this temperature?

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 per 100 g water)

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(d) What is the effect of change of temperature on the solubility of a salt?

• For most salts, solubility increases with increase in temperature.

• Some salts like sodium chloride show very little change in solubility with temperature.

Answer: Generally, the solubility of salts increases with increase in temperature.

Question: 4

Explain the following giving examples.

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(a) Saturated solution

A saturated solution is a solution in which no more solute can dissolve in the solvent at a given temperature.

Example:
A solution of common salt in water in which extra salt remains undissolved at the bottom at room temperature.

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(b) Pure substance

A pure substance is a substance that contains only one kind of particles and has a fixed composition and uniform properties throughout.

Example:
Distilled water, oxygen, iron.

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(c) Colloid

A colloid is a heterogeneous mixture in which very fine particles of one substance are uniformly dispersed in another substance. The particles are not visible to the naked eye and do not settle down on standing.

Example:
Milk, fog, smoke.

(d) Suspension

A suspension is a heterogeneous mixture in which insoluble particles are large enough to be seen and settle down on standing. These particles can be separated by filtration.

Example:
Muddy water, sand in water.

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Question: 5
Classify each of the following as a homogeneous or heterogeneous mixture.

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Answer:

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Question 6

How would you confirm that a colourless liquid given to you is pure water?

Answer:

To confirm that the given colourless liquid is pure water, we can check its physical properties:

1. Boiling point test:
   Pure water boils at 100°C (373 K) at normal atmospheric pressure.

2. Freezing point test:
   Pure water freezes at 0°C (273 K).

If the given liquid shows both these fixed temperatures, it can be confirmed that the liquid is pure water.

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Question 7

Which of the following materials fall in the category of a “pure substance”?

Answer:

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Question 8

Identify the solutions among the following mixtures.

Answer:

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Question 9

Which of the following will show “Tyndall effect”?

Answer:

The Tyndall effect is shown by colloidal solutions, not by true solutions.

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Question 10

Classify the following into elements, compounds and mixtures.

Answer:

Elements

• (a) Sodium

• (d) Silver

• (f) Tin

• (g) Silicon

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Compounds

• (e) Calcium carbonate

• (k) Methane

• (l) Carbon dioxide

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Mixtures

• (b) Soil

• (c) Sugar solution

• (h) Coal

• (i) Air

• (j) Soap

• (m) Blood

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Quick revision tip (for exams):

• Elements → single type of atom

• Compounds → fixed chemical formula

• Mixtures → variable composition

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Question 11

Question:
Which of the following are chemical changes?

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Answer with reasons:

Chemical changes are:

(a), (b), (d), (e), (g)