SCIENCE CLASS- 8
CHAPTER-9 (The Amazing World of Solutes, Solvents, and Solutions)
Question 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 than in cold water.
Answer: False
Correct Statement:
Oxygen gas is more soluble in cold water than in hot water.
Explanation: The solubility of gases generally decreases with an increase in temperature.
(ii) A mixture of sand and water is a solution.
Answer: False
Correct Statement:
A mixture of sand and water is not a solution; it is a heterogeneous mixture.
Explanation: Sand does not dissolve in water and can be seen separately.
(iii) The amount of space occupied by any object is called its mass.
Answer: False
Correct Statement:
The amount of space occupied by any object is called its volume.
Explanation: Mass is the amount of matter contained in an object, whereas volume is the space occupied by it.
(iv) An unsaturated solution has more solute dissolved than a saturated solution.
Answer: False
Correct Statement:
An unsaturated solution has less solute dissolved than a saturated solution at the same temperature.
Explanation: A saturated solution contains the maximum amount of solute that can dissolve at a given temperature.
(v) The presence of different gases in the atmosphere is also a uniform mixture.
Answer: True
Explanation: Air is a homogeneous (uniform) mixture because its components are evenly distributed throughout.
Question 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.
Answer:
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 __________ dissolved in __________ at a particular temperature is called solubility at that temperature.
Answer:
The maximum amount of solute dissolved in solvent at a particular temperature is called solubility at that temperature.
(iii) Generally, the density __________ with increase in temperature.
Answer: decreases
Explanation: Heating causes particles to move farther apart, increasing volume and decreasing density.
(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.
Answer: saturated
Question 3. You 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.
Explanation:
A substance with lower density floats on a substance with higher density.
Since oil floats on water, oil is less dense and water is more dense.
Question 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.
Given:
Mass = 225 g
Volume = 90 cm3
Formula:
Density = Mass ÷ Volume
Density = 225 ÷ 90
Density = 2.5 g/cm3
Answer:
Density of sculpture = 2.5 g/cm3
Density of water = 1 g/cm3
Since 2.5 g/cm3 is greater than 1 g/cm3, the sculpture will sink in water.
Question 5. Which one of the following is the most appropriate statement, and why are the other statements not appropriate?
Answer:
Correct Statement: (iii) No more solute can be dissolved into the saturated solution at that temperature.
Reason:
This is the correct definition of a saturated solution.
Why others are incorrect:
- (i) Incorrect because a saturated solution cannot dissolve more solute at that temperature.
- (ii) Incorrect because an unsaturated solution does not contain the maximum amount of solute.
- (iv) Incorrect because a saturated solution can form at any temperature depending on the amount of solute dissolved.
Question 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?
Given:
Total volume = 2 litres = 2000 mL
Water already present = 500 mL
Calculation:
Remaining capacity = 2000 − 500
= 1500 mL
Answer:
The bottle can hold 1500 mL more water.
Question 7. An object has a mass of 400 g and a volume of 40 cm3. What is its density?
Given:
Mass = 400 g
Volume = 40 cm3
Formula:
Density = Mass ÷ Volume
Density = 400 ÷ 40
Density = 10 g/cm3
Answer:
Density of the object = 10 g/cm3
Question 8. Analyse Fig. 9.25(a) and 9.25(b). Why does the unpeeled orange float, while the peeled one sinks? Explain.
Answer:
The peel of an orange contains many tiny air spaces.
These air spaces reduce the average density of the unpeeled orange.
Therefore, its density becomes less than that of water and it floats.
When the peel is removed, the air spaces are removed too.
The density of the orange increases and becomes greater than that of water.
Therefore, the peeled orange sinks.
Question 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?
For Object A:
Density = 200 ÷ 40
= 5 g/cm3
For Object B:
Density = 240 ÷ 60
= 4 g/cm3
Comparison:
Density of A = 5 g/cm3
Density of B = 4 g/cm3
Answer:
Object A is denser than Object B.
Important Formulae
Density = Mass ÷ Volume
Volume = Mass ÷ Density
Mass = Density × Volume
Key Concepts Learned
- Density is the mass per unit volume of a substance.
- Objects denser than water sink, while less dense objects float.
- Air is a homogeneous mixture of gases.
- Solubility depends on temperature.
- Gases are generally more soluble in cold water.
- A saturated solution contains the maximum amount of dissolved solute.
- Volume of irregular solids can be measured by water displacement method.
- Temperature usually decreases the density of substances.