Class 8 Science · Chapter 9

9. The Amazing World of Solutes, Solvents, and Solutions

Complete solutions to Keep the Curiosity Alive. CBSE / NCERT Class 8 Science (Curiosity).

Keep the Curiosity Alive

1State whether the statements are True [T] or False [F]. Correct the false statement(s).

(i) Oxygen gas is more soluble in hot water rather than in cold water. — F

Correct: oxygen gas is more soluble in cold water than in hot water.

(ii) A mixture of sand and water is a solution. — F

Correct: a mixture of sand and water is not a solution. Sand does not dissolve in water; it is a non-uniform mixture.

(iii) The amount of space occupied by any object is called its mass. — F

Correct: the amount of space occupied by an object is called its volume.

(iv) An unsaturated solution has more solute dissolved than a saturated solution. — F

Correct: an unsaturated solution has less solute dissolved than a saturated solution.

(v) The presence of different gases in the atmosphere is also a uniform mixture. — T

2Fill in the blanks.

(i) The volume of a solid can be measured by the method of displacement, where the solid is immersed in water and the rise in water level is measured.

(ii) The maximum amount of solute dissolved in 100 mL of solvent at a particular temperature is called solubility at that temperature.

(iii) Generally, the density decreases with increase in temperature.

(iv) The solution in which no more glucose can dissolve at a given temperature is called a saturated solution of glucose.

3You pour oil into a glass containing some water. The oil floats on top. What does this tell you?

Answer: (ii) Water is denser than oil

A less dense liquid floats on a denser liquid.

4A stone sculpture weighs 225 g and has a volume of 90 cm³. Calculate its density and predict whether it will float or sink in water.

Density = Mass ÷ Volume
= 225 g ÷ 90 cm³
= 2.5 g/cm³

The density of water is 1 g/cm³. The sculpture is denser than water, so it will sink.

5Which one is the most appropriate statement, and why are the others not appropriate?

Answer: (iii) No more solute can be dissolved into the saturated solution at that temperature.

(i) is wrong — a saturated solution cannot dissolve more solute at that temperature.
(ii) is wrong — it is the saturated solution, not the unsaturated one, that has the maximum solute dissolved.
(iv) is wrong — a saturated solution can form at any temperature.

6You have a bottle with a volume of 2 litres. You pour 500 mL of water into it. How much more water can the bottle hold?

2 L = 2000 mL
Water it can still hold = 2000 mL − 500 mL = 1500 mL

The bottle can hold 1500 mL (1.5 L) more water.

7An object has a mass of 400 g and a volume of 40 cm³. What is its density?

Density = Mass ÷ Volume = 400 g ÷ 40 cm³ = 10 g/cm³

8Analyse Fig. 9.25a and 9.25b. Why does the unpeeled orange float, while the peeled one sinks? Explain.

The orange peel has many tiny air pockets. These make the unpeeled orange, as a whole, less dense than water. So it floats.

When the peel is removed, the air pockets are removed too. The peeled orange becomes denser than water. So it sinks.

9Object A has a mass of 200 g and a volume of 40 cm³. Object B has a mass of 240 g and a volume of 60 cm³. Which object is denser?

Density of A = 200 g ÷ 40 cm³ = 5 g/cm³
Density of B = 240 g ÷ 60 cm³ = 4 g/cm³

Object A is denser, even though it has less mass.

10Reema moulds 120 g of modeling clay into a compact cube of volume 60 cm³. Later, she flattens it into a thin sheet. Predict what will happen to its density.

Density of the cube = 120 g ÷ 60 cm³ = 2 g/cm³

The density will remain the same (2 g/cm³). Changing the shape does not change the mass or the volume of the clay. So its density does not change.

11A block of iron has a mass of 600 g and a density of 7.9 g/cm³. What is its volume?

Volume = Mass ÷ Density
= 600 g ÷ 7.9 g/cm³
= 75.9 cm³ (about 76 cm³)

12A test tube filled with water and fitted with a glass tube is kept in a beaker of hot water (~70 °C). The water level in the glass tube rises. How does it affect the density?

On heating, the water expands and its volume increases. That is why the level rises in the glass tube.

The mass of the water stays the same. Since density = mass ÷ volume, a larger volume means lower density.

This shows that, generally, density decreases with an increase in temperature.

Discover, Design, and Debate

The “Discover, design, and debate” projects are hands-on tasks. Do them with your classmates under the guidance of your teacher, and write your own observations.

← 8. Nature of Matter: Elements, Compounds, and Mixtures 10. Light: Mirrors and Lenses →