NCERT Solutions for Class 8 Science Curiosity Chapter 9 The Amazing World of Solutes, Solvents and Solutions

1. State whether the statements given below 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.
(ii) A mixture of sand and water is a solution.
(iii) The amount of space occupied by any object is called its mass.
(iv) An unsaturated solution has more solute dissolved than a saturated solution.
(v) The mixture of different gases in the atmosphere is also a solution.
See Answer(i) [F] False. The correct statement is: Oxygen gas is more soluble in cold water rather than in hot water. The solubility of most gases decreases as the temperature increases.
(ii) [F] False. The correct statement is: A mixture of sand and water is a non-uniform mixture (a suspension), not a solution. In a solution, the components must be evenly distributed.
(iii) [F] False. The correct statement is: The amount of space occupied by any object is called its volume. Mass is the amount of matter in an object.
(iv) [F] False. The correct statement is: A saturated solution has more (or equal) solute dissolved than an unsaturated solution. An unsaturated solution can still dissolve more solute.
(v) [T] True. Air is a perfect example of a gaseous solution, with nitrogen as the solvent and other gases like oxygen as the solutes.

2. Fill in the blanks.
(i) The volume of a solid can be measured by the method of displacement, where the solid is ______ in water and the ______ in water level is measured.
(ii) The maximum amount of ______ dissolved in ______ at a particular temperature is called solubility at that temperature.
(iii) Generally, the density ______ with increase in temperature.
(iv) The solution in which glucose has completely dissolved in water, and no more glucose can dissolve at a given temperature, is called a ______ solution of glucose.
See Answer(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 a fixed quantity of a solvent (or solution) at a particular temperature is called solubility at that temperature.
(iii) Generally, the density decreases with increase in temperature.
(iv) The solution in which glucose has completely dissolved in water, and no more glucose can dissolve at a given temperature, is called a saturated solution of glucose.

3. You pour oil into a glass containing some water. The oil floats on top. What does this tell you?
(i) Oil is denser than water
(ii) Water is denser than oil
(iii) Oil and water have the same density
(iv) Oil dissolves in water
See AnswerThe correct answer is (ii) Water is denser than oil. When two liquids that don’t mix are combined, the less dense liquid will float on top of the more dense liquid. Since oil floats on water, water must be the denser of the two.

4. A 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.
See AnswerUsing the formula:
Density = Mass / Volume
Density = 225 g / 90 cm³
= 2.5 g/cm³
The density of water is approximately 1 g/cm³.
Since the sculpture’s density (2.5 g/cm³) is greater than the density of water, it will sink.

5. Which one of the following is the most appropriate statement and why are the other statements not appropriate?
(i) A saturated solution can still dissolve more solute at a given temperature.
(ii) An unsaturated solution has dissolved the maximum amount of solute possible at a given temperature.
(iii) No more solute can be dissolved into the saturated solution at that temperature.
(iv) A saturated solution forms only at high temperatures.
See AnswerThe most appropriate statement is (iii) No more solute can be dissolved into the saturated solution at that temperature. This is the very definition of saturation.
Here’s why the others are incorrect:
(i) This describes an unsaturated solution, not a saturated one.
(ii) This describes a saturated solution, not an unsaturated one.
(iv) Saturated solutions can be formed at any temperature; the amount of solute needed to reach saturation simply changes with temperature.

6. You 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?
See AnswerFirst, we have to convert litres to millilitres:
2 litres = 2 × 1000 mL = 2000 mL.
The bottle can hold 2000 mL in total.
We have already poured 500 mL.
Amount of water the bottle can still hold
= Total Volume – Poured Volume
= 2000 mL – 500 mL
= 1500 mL.
The bottle can hold 1500 mL more water (or 1.5 litres).

7. An object has a mass of 400 g and a volume of 40 cm³. What is its density?
See AnswerWe’ll use density formula:
Density = Mass / Volume
Density = 400 g / 40 cm³ = 10 g/cm³
The density of the object is 10 g/cm³.

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

See AnswerThe unpeeled orange floats (in Fig. 9.25a) because its peel is full of tiny air pockets. This trapped air increases the orange’s total volume significantly without adding much mass. This makes the overall density (mass divided by volume) of the unpeeled orange less than the density of water, causing it to float.
When you peel the orange (Fig. 9.25b), we remove the peel and all the trapped air. The remaining orange flesh is denser than water, so it sinks.

9. Object 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?
See AnswerTo find out, we need to calculate the density of each object.
Density of Object A = Mass / Volume = 200 g / 40 cm³ = 5 g/cm³
Density of Object B = Mass / Volume = 240 g / 60 cm³ = 4 g/cm³
Comparing the two, Object A (5 g/cm³) is denser than Object B (4 g/cm³).

10. Reema has a piece of modeling clay that weighs 120 g. She first moulds it into a compact cube that has a volume of 60 cm³. Later, she flattens it into a thin sheet. Predict what will happen to its density.
See AnswerThe density of the modeling clay will not change. Density is an intrinsic property of a substance. When Reema flattens the clay, she changes its shape, but she doesn’t change the amount of matter (its mass, 120 g) or the space it takes up (its volume, 60 cm³). Since both the mass and the volume remain the same, its density (120 g / 60 cm³ = 2 g/cm³) also remains exactly the same.

11. A block of iron has a mass of 600 g and a density of 7.9 g/cm³. What is its volume?
See AnswerTo find the volume, we can rearrange the density formula:
Volume = Mass / Density
Volume = 600 g / 7.9 g/cm³ ≈ 75.95 cm³
The volume of the iron block is approximately 75.95 cm³.

12. You are provided with an experimental setup as shown in Fig. 9.26a and 9.26b. On keeping the test tube (Fig 9.26b) in a beaker containing hot water (~70°C), the water level in the glass tube rises. How does it affect the density?

See AnswerThis experiment shows the effect of temperature on density. When the water in the test tube is heated, its particles gain energy, move faster and spread farther apart. This causes the water to expand, so its total volume increases (which is why the level rises in the narrow glass tube).
Since Density = Mass / Volume and the mass of the water hasn’t changed but its volume has increased, the density of the water has decreased. So, heating the water causes its density to decrease.