CBSE Important Questions Class 8 Science Chapter 9 – The Amazing World of Solutes, Solvents and Solutions

Class 8 Science Chapter 9 The Amazing World of Solutes, Solvents and Solutions is part of the new NCERT Curiosity textbook. It explains what a solution is and what we mean by a solute and a solvent. Students learn about saturated and unsaturated solutions, how temperature and pressure change solubility, how to find the density of an object by water displacement, and why dissolved oxygen matters so much to the life in our rivers and ponds.

These CBSE important questions for Class 8 Science Chapter 9 cover identifying saturated and unsaturated solutions from a diagram, calculating and comparing densities, why gas bubbles form when a cold drink bottle is opened, the effect of temperature on solubility, air as a gaseous solution, how to dissolve sugar faster, and why hot industrial water harms aquatic life. Every question has a clear step-by-step answer in simple English with figures, so students can practise on their own, parents can check the working, and teachers can use them for class tests.

Q1. Look at the three beakers in the figure. Which of the following correctly identifies all three beakers?
(a) A – Non-uniform mixture; B – Unsaturated solution; C – Saturated solution
(b) A – Solution; B – Saturated solution; C – Unsaturated solution
(c) A – Non-uniform mixture; B – Solution; C – Non-uniform mixture
(d) A – Saturated solution; B – Unsaturated solution; C – Non-uniform mixture

Three beakers A, B and C showing different mixtures and solutions

Answer: (a) A – Non-uniform mixture; B – Unsaturated solution; C – Saturated solution
Beaker A is cloudy and not the same throughout, so the substance has not dissolved; it is a non-uniform (heterogeneous) mixture. Beaker B is completely clear, so all the solute has dissolved and more could still be dissolved, which makes it unsaturated. Beaker C is clear but has extra solute settled at the bottom, which shows it has dissolved as much as it can; it is a saturated solution.

Q2. Three objects are placed on digital weighing balances and then lowered into a measuring cylinder of water to find their volumes. The volumes are: metal cube 25 mL, wooden block 30 mL, plastic toy 40 mL. Calculate and compare their densities, arrange them in increasing order of density, and say which would sink fastest.

A metal cube, a wooden block and a plastic toy on digital weighing balances

Answer: Density = Mass ÷ Volume
Metal cube: 189.0 g ÷ 25 cm³ = 7.56 g/cm³
Wooden block: 24.0 g ÷ 30 cm³ = 0.80 g/cm³
Plastic toy: 42.0 g ÷ 40 cm³ = 1.05 g/cm³
Increasing order of density: Wooden block < Plastic toy < Metal cube
The metal cube will sink the fastest, because its density is about 7.56 times that of water, so the downward pull on it is far greater than the upward buoyant force. The wooden block (0.80 g/cm³) is lighter than water, so it floats.

Q3. Observe the diagram showing two beakers with different solutions. Identify which beaker represents (a) an unsaturated solution and (b) a saturated solution. (c) What can you do to make beaker B like beaker A?

Two beakers, one with all solute dissolved and one with solute settled at the bottom

Answer: (a) Beaker A is unsaturated. The solution is completely clear, which means all the solute has dissolved and more could still dissolve.
(b) Beaker B is saturated. Some solute has settled at the bottom, which shows that the solvent cannot take in any more at that temperature.
(c) To make beaker B like beaker A, either heat the solution, because most solids dissolve better at a higher temperature, or add more water (solvent), so that the extra solute can dissolve.

Q4. What does the formation of gas bubbles on opening a soft drink bottle indicate?
(a) Gas solubility increases with temperature
(b) Gas solubility is independent of pressure
(c) Gas dissolves better at lower pressure
(d) Gas solubility decreases as pressure decreases

Answer: (d) Gas solubility decreases as pressure decreases
A soft drink is bottled with carbon dioxide dissolved in it under high pressure. The moment the cap is opened, the pressure inside falls to that of the atmosphere. The gas can no longer stay dissolved, so it escapes as bubbles. This shows that the solubility of a gas in a liquid increases with pressure and falls when the pressure drops.

Q5. A student added baking soda to water at 20 °C, 50 °C and 70 °C until no more could dissolve. Evaluate these conclusions:
Statement I: The solubility of baking soda increases with temperature.
Statement II: A saturated solution at a lower temperature becomes unsaturated at a higher temperature.
Statement III: Temperature has no effect on the amount of solute that can dissolve.
Statement IV: The concentration of a saturated solution remains constant regardless of temperature.
Which statements are scientifically valid?
(a) Only I, II and IV
(b) Only I and II
(c) Only II and III
(d) Only I and IV

Answer: (b) Only I and II
Statement I is correct: most solids, including baking soda, dissolve more at a higher temperature. Statement II follows from this: a solution that was saturated at 20 °C can hold more solute at 70 °C, so it becomes unsaturated. Statement III is wrong, because temperature clearly does matter, and Statement IV is wrong, because the concentration of a saturated solution changes when the temperature changes.

Q6. Match the following solutions with their appropriate classification:

Column A Column B
1. ORS (Oral Rehydration Solution) P. Gaseous solution
2. Air Q. Liquid solution with solid solute
3. Sugar syrup R. Saturated solution
4. Water with undissolved salt at bottom S. Concentrated liquid solution

(a) 1-Q, 2-R, 3-P, 4-S
(b) 1-S, 2-Q, 3-P, 4-R
(c) 1-P, 2-Q, 3-R, 4-S
(d) 1-Q, 2-P, 3-S, 4-R

Answer: (d) 1-Q, 2-P, 3-S, 4-R
ORS is solid salt and sugar dissolved in water, so it is a liquid solution with a solid solute (Q). Air is a mixture of gases, so it is a gaseous solution (P). Sugar syrup has a large amount of sugar in a little water, so it is a concentrated liquid solution (S). Water with extra salt lying at the bottom cannot dissolve any more, so it is a saturated solution (R).

Q7. Consider the following statements about saturated and unsaturated solutions:
Statement I: A saturated solution at 20 °C can dissolve more solute if the temperature is increased to 50 °C.
Statement II: An unsaturated solution is one in which more solute can be dissolved at a given temperature.
Statement III: In a saturated solution, the undissolved solute always floats on top.
Which of the above statements is/are correct?
(a) Only I and III
(b) Only II and III
(c) Only I and II
(d) All three statements

Answer: (c) Only I and II
Statement I is correct, because solubility usually rises with temperature. Statement II is the correct definition of an unsaturated solution. Statement III is wrong: the undissolved solute is denser than the solution, so it settles at the bottom, not on top.

Q8. Read the Assertion (A) and Reason (R) and choose the correct option.
Assertion (A): Air is considered a gaseous solution.
Reason (R): In air, nitrogen is present in the largest amount and acts as the solvent, while oxygen and other gases act as solutes.
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

Answer: (a) Both A and R are true, and R is the correct explanation of A
Air is a uniform mixture of gases, so it is a gaseous solution. Nitrogen makes up about 78 per cent of it, the largest share, so it acts as the solvent, while oxygen, carbon dioxide and the other gases are present in smaller amounts and act as solutes. That is exactly why air is called a solution, so R explains A.

Q9. Define the terms 'solute' and 'solvent' with one example each.

Answer: Solute: the substance that is present in the smaller amount and dissolves in the solvent. For example, the sugar in a sugar solution.
Solvent: the substance that is present in the larger amount and in which the solute dissolves. For example, the water in a sugar solution.
Together, the solute and the solvent make a solution.

Q10. Explain with an example why air is considered a solution even though it is a mixture of gases.

Answer: Air is considered a solution because it is a homogeneous mixture: the gases are mixed so evenly that we cannot see or separate them by looking at it, and every part of a sample of air has the same composition.
In air, nitrogen is present in the largest amount, about 78 per cent, so it acts as the solvent. Oxygen (about 21 per cent), carbon dioxide, argon and water vapour are present in smaller amounts, so they act as solutes.
Since a solution simply means a uniform mixture of a solute in a solvent, air fits the definition exactly; it is just that here both solute and solvent are gases.

Q11. A chef wants to make sugar syrup quickly. She has sugar cubes and powdered sugar, and can use cold water or hot water. What choices should she make to prepare the syrup fastest? Give two reasons.

Answer: She should use powdered sugar and hot water.
1. Powdered sugar has a much larger surface area than a cube of the same mass, so more of the sugar is in contact with the water at the same time and it dissolves faster.
2. In hot water the particles move faster and the solubility is higher, so the sugar dissolves both more quickly and in greater amount.
Stirring the mixture would make it dissolve faster still, because it keeps bringing fresh water into contact with the sugar.

Q12. Why is it dangerous for aquatic life when industrial waste heats up river water? Explain in terms of solubility of gases.

Answer: The solubility of a gas in water decreases as the temperature rises.
When hot industrial waste water is let into a river, the river water becomes warmer and can no longer hold as much dissolved oxygen. The oxygen escapes into the air, so the amount of dissolved oxygen in the water falls.
Fish and other water animals breathe this dissolved oxygen through their gills. With less of it available, they suffocate and may die. This kind of damage is called thermal pollution.

Related Study Material on Extramarks

Q.1 Solid sodium chloride does not conduct electricity while molten sodium chloride conducts. Explain why?

Marks:2
Ans

Solid sodium chloride does not conduct electricity because the particles (ions) are held together by strong forces of electrostatic attraction. In molten state the ions become free and move to conduct electricity, since on heating the bonds between ions become weak.

Q.2 Why is an acid or an ionic salt added to water in the electrolysis of water?

Marks:2
Ans

Pure water or distilled water is a bad conductor of electricity whereas acids and bases are good conductors of electricity. When an acid or ionic salts are dissolved in distilled water then the resulting solution conducts electricity.

Q.3 Define electrolysis.

Marks:1
Ans

The chemical reaction (decomposition) of an electrolyte (conducting liquid) into its components when electricity is passed through it is called electrolysis.

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