Intext Question & Answer Class 9 Science Exploration Chapter 5 Exploring Mixtures and their Separation

Think It Over (Page No. 72)

1. Why do suspended particles settle in muddy water over time but not in milk?

Answer: Muddy water is a suspension, where particles are large and heavy. Due to gravity, these particles settle down at the bottom when left undisturbed.

Milk, on the other hand, is a colloid. Its particles are very small and remain evenly dispersed, so they do not settle even after a long time.

2. How is evaporation different from boiling?

Answer:

Evaporation:

• Happens at any temperature
• Occurs only at the surface of the liquid
• It is a slow process

Boiling:

• Happens at a fixed temperature (boiling point)
• Occurs throughout the liquid
• It is a fast process

3. Why do you see bright rays of sunlight when it passes through small gaps between the leaves of a dense tree?

Answer: This happens due to the Tyndall Effect.

When sunlight passes through small gaps, it gets scattered by dust and tiny particles in the air, making the path of light visible as bright rays.

Pause and Ponder (Page No. 76)

1. A common talcum powder contains 4 % m/m zinc oxide, which acts as an antiseptic. How much zinc oxide is present in 300 g of the talcum powder?

Answer: Zinc oxide in talcum powder

We use mass by mass percentage:

$\%\,m/m = \frac{\text{mass of solute}}{\text{mass of solution}} \times 100$

%m/m=mass of solutionmass of solute​×100

Given:

• % m/m = 4%
• Total mass = 300 g

$$\text{Mass of zinc oxide} = \frac{4}{100} \times 300 = 12 \text{ g}$$

12 g of zinc oxide

2. Your mother gives you a bottle of orange juice concentrate to mix with water and serve it to your visiting friends. She asks you to mix two tablespoons of the concentrate with water in a glass tumbler. If each tablespoon measures 15 mL and you make 150 mL of juice per person, what is the % v/v of orange juice concentrate in the mixture you prepared?

Answer: % v/v of orange juice concentrate

First find volume of concentrate:

• 2 tablespoons × 15 mL = 30 mL

Total solution = 150 mL

Using % v/v:

$$\%\,v/v = \frac{30}{150} \times 100 = 20\%$$

20% (v/v)

3. Vinegar, used as a food preservative and additive, contains 5 % v/v acetic acid. Glacial acetic acid is a liquid, i.e., 100% acetic acid. If you want to make vinegar from glacial acetic acid, how would you proceed?

Answer: How to prepare vinegar (5% v/v) from glacial acetic acid

• Glacial acetic acid = 100% acetic acid
• Required = 5% v/v solution

This means:

• Take 5 mL acetic acid
• Add water to make total volume 100 mL

Procedure:

1. Measure 5 mL of glacial acetic acid.
2. Add water slowly.
3. Make total volume up to 100 mL.
4. Mix well.

Result: You get 5% v/v vinegar solution

Pause and Ponder (Page No. 79)

4. Refer to the solubility curves given in Activity 5.2. If equal masses of hot, saturated solutions of compounds ‘A’ and ‘B’ are cooled from 80 °C to 60 °C, which solution is likely to deposit more solid?

Answer: From the solubility curves (Activity 5.2):

• When cooling from 80 °C to 60 °C, the solubility of both A and B decreases.
• But compound B shows a larger decrease in solubility compared to A.

Greater decrease in solubility ⇒ more excess solute comes out as solid.

Answer: Compound B will deposit more solid.

5. Will there be any change in the size of common salt crystals if the rate of evaporation is increased or decreased? Explain.

Answer: Yes, the size of crystals does change with the rate of evaporation:

• Fast evaporation → particles don’t get enough time to arrange properly → small crystals form
• Slow evaporation → particles get enough time to arrange in an orderly pattern → large, well-shaped crystals form

Conclusion:

• Increase in evaporation rate → smaller crystals
• Decrease in evaporation rate → larger crystals

6. State whether the following statements are True or False. Also, correct the False statements.

(i) Salt can be separated from a salt solution by evaporation or distillation.

Answer: True

• Evaporation gives salt by removing water.
• Distillation can also separate it and even allows recovery of water.

(ii) Distillation can be used for separation of two liquids even when these have the same boiling point.

Answer: False
Correction: Distillation cannot separate liquids with the same boiling point. It works only when there is a difference in boiling points.

(iii) In paper chromatography, the solvent level should be above the sample spot at the beginning of the experiment.

Answer: False
Correction: The solvent level should be below the sample spot, otherwise the sample will dissolve directly instead of moving up the paper.

(iv) Evaporation and crystallization are the same processes.

Answer: False
Correction: Evaporation and crystallization are different processes.

• Evaporation removes the solvent.
• Crystallization forms pure solid crystals from a solution.

Pause and Ponder (Page No. 84)

7. Why do immiscible liquids form two separate layers in a separating funnel?

Answer: Immiscible liquids (like oil and water) do not mix with each other.
They form two layers mainly because:

• They have different densities
• The denser liquid settles at the bottom
• The lighter liquid stays on top

Also, there is no attraction between their molecules strong enough to mix, so they remain as separate layers.

8. Is sublimation different from evaporation? Justify.

Answer: Yes, sublimation and evaporation are different processes:

• Sublimation:
  • A solid directly changes into vapour without becoming liquid
  • Example: camphor, naphthalene
• Evaporation:
  • A liquid changes into vapour
  • Occurs at the surface of the liquid

Conclusion: Sublimation involves solid → vapour, while evaporation involves liquid → vapour, so they are different processes.

9. Clouds are made up of tiny water droplets or ice crystals floating in the air. Based on what you know about solutions, suspensions and colloids, what type of mixture do you think clouds are and why?

Answer: Clouds are a colloid.

• They consist of tiny water droplets or ice crystals dispersed in air
• The particles are small enough to remain suspended and do not settle easily
• This matches the properties of a colloid (particles remain uniformly distributed)

Clouds are colloids because tiny droplets remain suspended in air without settling.

10. Why do cities with a lot of smoke and dust in the air often look hazy?

Answer: Cities with lots of smoke and dust appear hazy due to the Tyndall Effect.

• Dust and smoke particles in air scatter light
• This scattering makes the air look cloudy or hazy
• It also reduces visibility because light does not travel in a straight path

Haze is caused by scattering of light by suspended particles in air.