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Class 8 Science: Solutes, Solvents, and Solutions, 8 practice questions with answers

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These are 8 practice questions on the ninth chapter of the new NCERT Class 8 Science textbook, Curiosity, called The Amazing World of Solutes, Solvents, and Solutions. Each has a short answer and the textbook page it comes from. They cover naming the solute and the solvent, solubility at different temperatures, gases dissolved in water, density and relative density, choosing a measuring cylinder, the mass of a plank, warm air rising, and how pressure changes density. For the particle picture behind this chapter, see our Class 8 Particulate Nature of Matter guide. Try each question before you read the answer.

These questions are our own, drafted with AI help. Every fact was checked against the page it cites by a test script, but no practising teacher has reviewed them yet. If you find a mistake, please tell us.

What the chapter says you need

Facts from the NCERT textbook (Curiosity, Grade 8, Chapter 9, printed pages 134 to 151):

Solutions

Question 1. Name the solute and the solvent in each: (a) jaggery dissolved in water to make a sweet drink; (b) 15 mL of lemon juice stirred into 250 mL of water; (c) 30 mL of glycerine mixed with 70 mL of water.

Answer: (a) solute jaggery, solvent water; (b) solute lemon juice, solvent water; (c) solute glycerine, solvent water. (a) When a solid dissolves in a liquid, the solid is the solute and the liquid is the solvent. (b) and (c) are two liquids mixed together. Then the liquid present in the smaller amount is called the solute and the one in the larger amount the solvent: 15 mL is less than 250 mL, and 30 mL is less than 70 mL, so water is the solvent both times. (Solute, solvent and solution, page 135.)

Question 2. These are about the most sugar that dissolves in 100 mL of water at three temperatures: 200 g at 20 °C, 240 g at 40 °C and 290 g at 60 °C. (a) At which of these temperatures can the most sugar dissolve, and what does that show? (b) At 40 °C, 220 g of sugar is dissolved in 100 mL of water. Is the solution saturated or unsaturated? (c) The same solution is cooled to 20 °C. About how much of the sugar can no longer stay dissolved?

Answer: (a) 60 °C: solubility rises with temperature; (b) unsaturated; (c) about 20 g. (a) The most solute that a fixed amount of solvent can dissolve is its solubility. Here it is highest at 60 °C, which fits the chapter's finding that for most substances solubility increases as the temperature rises. (b) At 40 °C up to 240 g can dissolve, and only 220 g has, so more sugar could still dissolve: the solution is unsaturated. (c) At 20 °C only about 200 g can stay dissolved, so about 220 − 200 = 20 g cannot. (That this extra sugar separates out of the solution is our reasoning from the definition, not a statement of the chapter.) (Saturated and unsaturated solutions and solubility, page 137; temperature and solubility, page 138.)

Question 3. On a hot summer afternoon the water of a small pond warms up, and its fish may come to the surface. Using the chapter, explain why warm water is harder on fish than cool water.

Answer: less oxygen can dissolve in warm water. Fish and other water animals and plants live on the oxygen dissolved in water, even though only a little dissolves. The solubility of a gas in water generally decreases as the temperature rises. So cool water holds more oxygen, while warm water holds less, and there is less oxygen for the fish. (Solubility of gases, page 139.)

Density

Question 4. A 500 mL bottle of honey holds 710 g of honey, and 500 mL of milk has a mass of 515 g. (a) Find the density of each in g/mL. (b) What is the relative density of each? (c) Write the density of the honey in kg/m³.

Answer: (a) honey 1.42 g/mL, milk 1.03 g/mL; (b) 1.42 and 1.03; (c) 1420 kg/m³. (a) Density = mass ÷ volume: honey 710 g ÷ 500 mL = 1.42 g/mL; milk 515 g ÷ 500 mL = 1.03 g/mL. (b) Relative density = density of the substance ÷ density of water. One millilitre of water has a mass of about one gram, so water's density is about 1 g/mL, and the relative densities are 1.42 and 1.03, numbers with no unit. (c) 1 kg/m³ = 1 g/1000 mL, so 1 g/mL = 1000 kg/m³ and 1.42 g/mL = 1420 kg/m³. (Density, its units and relative density, pages 140 and 141.)

Question 5. You need 13 mL of a medicine, and you have measuring cylinders of 10 mL, 25 mL, 100 mL and 250 mL. (a) Which one is the best choice? (b) Why not the 10 mL one? (c) Why not the 250 mL one?

Answer: (a) the 25 mL cylinder; (b) 13 mL would need two steps; (c) it is much less accurate. (a) The 25 mL cylinder holds 13 mL in one go and reads small amounts finely. (b) 13 mL is more than 10 mL, so it would have to be measured in two steps (10 mL, then 3 mL), which is not convenient. (c) The smallest volume a cylinder can read depends on its capacity: usually 0.1 mL for a 10 mL or 25 mL cylinder, but 2 mL for a 250 mL one, so a small amount would be measured far less accurately. (Choosing a measuring cylinder, pages 143 and 144.)

Question 6. A wooden plank is 100 cm long, 20 cm wide and 2 cm thick, and the wood has a density of 0.7 g/cm³. (a) What is the volume of the plank? (b) What is its mass, in grams and in kilograms?

Answer: (a) 4000 cm³; (b) 2800 g, which is 2.8 kg. (a) For a block with a regular shape, volume = length × width × height = 100 cm × 20 cm × 2 cm = 4000 cm³. (b) Density = mass ÷ volume, so mass = density × volume = 0.7 g/cm³ × 4000 cm³ = 2800 g, and 2800 g ÷ 1000 = 2.8 kg. (Volume of a regular solid, page 145; density, page 140.)

Temperature, pressure and density

Question 7. A heater is switched on near the floor of a closed room. After a while the air near the ceiling is warmer than the air near the floor. (a) What happens to the density of air when it is heated, and why? (b) Why does the warm air end up near the ceiling?

Answer: (a) it decreases, because the air spreads out while its mass stays the same; (b) warm air is less dense than the cooler air around it, so it moves up. (a) On heating, the particles of a substance move apart and spread, so its volume increases while its mass does not change. Since density = mass ÷ volume, the density decreases. (b) The heated air near the floor is now less dense than the cooler air around it, so it rises, as the hot air in a hot air balloon does, and collects near the ceiling. (Effect of temperature on density, page 147.)

Question 8. A syringe is filled with air, its nozzle is closed with a finger, and the plunger is pushed in. (a) What happens to the air's volume, mass and density? (b) If the syringe were filled with water instead, what would happen?

Answer: (a) volume decreases, mass stays the same, density increases; (b) the water would hardly be squeezed, so its density would barely change. (a) Pressing on a gas pushes its particles closer together. The volume falls while the mass stays the same, so the density (mass ÷ volume) increases. (b) Liquids are nearly incompressible, so pressure has only a small effect on their density. (Effect of pressure on density, page 148.)

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Sources

All opened and read on 2 November 2026. Page numbers are the printed page numbers of the textbook; printed page = PDF page + 133 for this chapter.

The facts are from the textbook pages above. The questions, answers and checks are our own. This guide is general information; follow your teacher and the textbook.