Keep the Curiosity Alive — Chapter Exercises — "The Amazing World of Solutes, Solvents, and Solutions"

1. 15 marksCuriosity Grade 8, Chapter 9, page 149

   State whether each is True or False and correct the false ones. (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 presence of different gases in the atmosphere is also a uniform mixture.

   Hint. Four are false. Correct each rather than just marking it.

   	T/F	Correction
   (i) Oxygen is more soluble in hot water	False	Oxygen is more soluble in cold water — the solubility of gases generally decreases as temperature increases
   (ii) Sand and water is a solution	False	It is a non-uniform mixture. A solution is a uniform mixture, and the sand does not dissolve
   (iii) Space occupied is called mass	False	Space occupied is volume. Mass is the quantity of matter in the object
   (iv) Unsaturated has more solute than saturated	False	The saturated solution has more — it holds the maximum possible at that temperature
   (v) Gases in the atmosphere are a uniform mixture	True	The gases are evenly distributed and cannot be distinguished

   ✦ The mark is in the correction, not in the letter F. Writing 'false' identifies the error; writing what the statement should have said proves you know why. Since four of the five are false, this question is really four short-answer questions in disguise, and treating it as a tick-box exercise loses most of the marks available.

   Statement (iv) is the one to think about hardest. Saturated does not mean 'full of solute' in some absolute sense — it means no more can dissolve at that temperature. A saturated solution of a barely soluble substance contains very little solute and is still saturated.

2. 24 marksCuriosity Grade 8, Chapter 9, page 150

   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. (iii) Generally, the density ___ with increase in temperature. (iv) The solution in which glucose has completely dissolved and no more can dissolve at a given temperature is called a ___ solution.

   Hint. Each blank has a precise word from the chapter — not a paraphrase.

   (i) The solid is immersed (submerged) in water and the rise (increase) in water level is measured. — Activity 9.7: the level goes from 50 mL to 55 mL, and the 5 mL rise is the stone's volume.

   (ii) The maximum amount of solute dissolved in a fixed quantity of solvent (or solution) at a particular temperature is called solubility. — The Snapshot gives 100 mL as the fixed quantity.

   (iii) Generally, the density decreases with increase in temperature. — Because the volume increases while the mass stays the same.

   (iv) A saturated solution of glucose. — No more solute can dissolve at that temperature.

   ✦ Blank (ii) is the one where a vague answer costs a mark. 'Dissolved in water' is not enough: solubility is defined per fixed quantity of solvent and at a particular temperature, and both conditions are part of the definition. Since a maximum amount without a stated quantity of solvent would be meaningless, examiners look for the qualifier and not just the noun.

   Blank (iii) has a trap in the word 'generally'. Water between 0 °C and 4 °C behaves the other way, which is why ice floats — so the general rule is stated as a general rule.

3. 32 marksCuriosity Grade 8, Chapter 9, page 150

   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

   Hint. The one that floats is the less dense of the two.

   Answer: (ii) Water is denser than oil.

   Why. The less dense liquid sits on top. The chapter's own Think like a scientist box measures it: a packet of oil labelled 1 litre with a mass of 910 g gives a density of 910 ÷ 1000 = 0.91 g/mL, against water's 1 g/mL. So the oil is less dense, and it floats.

   Why the others fail:

   Option	Fault
   (i) Oil is denser	The denser liquid goes to the bottom; the oil is on top
   (iii) Same density	Then they would not separate into layers at all
   (iv) Oil dissolves in water	If it dissolved there would be a single uniform liquid and no layer — this is a non-uniform mixture

   ✦ Option (iv) is the interesting distractor, because it tests a different part of the chapter altogether. Floating is not dissolving. Sawdust floats and does not dissolve; salt dissolves and neither floats nor sinks, because it is no longer there as a solid at all. Since the oil forms a visible separate layer, it has plainly not dissolved.

4. 43 marksCuriosity Grade 8, Chapter 9, page 150

   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.

   Hint. Calculate first, then compare with water's density.

   Density.

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

   Prediction. The mass of 1 mL of water is close to 1 g at room temperature, so water is about 1 g/cm³. The sculpture at 2.5 g/cm³ is two and a half times as dense as water, so it will sink.

   Its relative density with respect to water is 2.5 — a number with no units.

   ✦ Always finish by comparing with water, since that is what the question actually asks. A density of 2.5 g/cm³ on its own predicts nothing; it predicts sinking only once you set it beside 1 g/cm³. Because the comparison is the reasoning, an answer that stops at the number has done the arithmetic and skipped the physics.

   And keep the chapter's caution in view. Density is not the only factor deciding whether something floats, so a careful answer says the sculpture is denser than water and is a solid lump with no trapped air — which is what makes the prediction safe here.

5. 54 marksCuriosity Grade 8, Chapter 9, page 150

   Which statement is most appropriate, and why are the others not? (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.

   Hint. Two of the wrong options are the two definitions swapped over.

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

   This is the chapter's definition: when the solute stops dissolving and begins to settle at the bottom, the solution is called a saturated solution at that particular temperature.

   Why the others fail:

   Option	Fault
   (i)	Describes an unsaturated solution — the solution in which more solute can be dissolved at a given temperature
   (ii)	Describes a saturated solution. Options (i) and (ii) are the two definitions swapped over
   (iv)	Saturation has nothing to do with high temperature. Activity 9.1 reaches a saturated salt solution at room temperature, and Activity 9.2 begins with one saturated at 20 °C

   ✦ Option (iv) contains a real misunderstanding worth naming. Higher temperature usually raises solubility, so a saturated solution at 70 °C holds more solute than one at 20 °C — but both are saturated. Since saturation is about reaching the limit and not about how high the limit is, a solution can be saturated at any temperature at all.

   Note also the words at that temperature in the correct option. Without them the statement would be wrong, since heating the solution would let more dissolve.

6. 62 marksCuriosity Grade 8, Chapter 9, page 150

   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?

   Hint. Get both quantities into the same unit first.

   Convert, then subtract.

   2 litres = 2 × 1000 mL = 2000 mL Remaining capacity = 2000 mL − 500 mL = 1500 mL

   That is 1.5 litres.

   ✦ The whole question is the conversion, and it is the step most often skipped. Subtracting 500 from 2 gives a meaningless answer, because you cannot take millilitres away from litres any more than you could take centimetres away from metres without converting. Bringing both quantities to the same unit before doing arithmetic is a habit worth having, and this question exists to build it.

   The relationships you need, all given on page 143: 1 L = 1000 mL, 1 L = 1 dm³, and 1 mL = 1 cm³. So the answer could equally be written as 1500 cm³, which is the same quantity again.

7. 72 marksCuriosity Grade 8, Chapter 9, page 150

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

   Hint. Straight substitution into the formula.

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

   Write the unit. Grams divided by cubic centimetres gives g/cm³, and an answer of '10' without the unit is incomplete — density is not a pure number.

   ✦ A useful check on any density answer: compare it with water. At 10 g/cm³ this object is ten times as dense as water, so it would sink readily. That is in the range of common metals, so the answer is at least plausible — a calculated density of 0.01 or 500 g/cm³ would be a signal to check the arithmetic.

   Compare with the chapter's worked examples to get a feel for the scale: aluminium 2.7 g/cm³, the stone in Activity 9.7 3.28 g/cm³, oil 0.91 g/mL, water about 1 g/cm³. Anything above 1 sinks in water; anything below floats.

8. 84 marksCuriosity Grade 8, Chapter 9, page 150

   Why does an unpeeled orange float while a peeled one sinks? Explain.

   Hint. Removing the peel changes both the mass and the volume — but not by the same proportion.

   Because the peel lowers the whole fruit's density, and taking it off raises it.

   The reasoning. The peel of an orange is thick and spongy, full of tiny air pockets. It therefore adds a great deal of volume for very little mass. With the peel on, the fruit's overall density — flesh plus peel plus trapped air — is less than that of water, so it floats.

   Peel it, and both the mass and the volume fall. But the volume falls proportionally more, because most of what you removed was air-filled space rather than matter. So the density of what remains is higher than water's, and the peeled orange sinks.

   ✦ The flesh of the orange has not changed at all, and that is the point of the question. The same material floats or sinks depending on what is attached to it — which is exactly the chapter's warning that the density of a substance is not the only factor that decides whether it will float or sink.

   The peel is doing for the orange what a hull does for a steel ship, and what the air in a raft of bamboo does: enclosing air lowers the average density of the whole object well below that of the material it is made of.

9. 93 marksCuriosity Grade 8, Chapter 9, page 150

   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?

   Hint. Neither the masses nor the volumes alone will tell you.

   Calculate both.

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

   Object A is denser, at 5 g/cm³ against B's 4 g/cm³.

   ✦ The question is built to punish comparing one column at a time. B has the greater mass — 240 g against 200 g — so a student who looks only at mass answers B and is wrong. B also has the greater volume, and looking only at volume gives no answer at all. Since density is mass per volume, both numbers have to enter the comparison together, and the only safe route is to calculate each density and then compare.

   This is the same reasoning as the concentration question on page 137 — 2 spoons in 100 mL against 4 spoons in 50 mL. Amount-per-fixed-amount quantities can never be compared by looking at either half on its own.

   Both objects would sink in water, since both densities exceed 1 g/cm³.

10. 103 marksCuriosity Grade 8, Chapter 9, page 150

    Reema's modelling clay weighs 120 g and is moulded into a compact cube of volume 60 cm³. She then flattens it into a thin sheet. Predict what happens to its density.

    Hint. Ask what actually changed when she flattened it.

    The density does not change. It stays at 2 g/cm³.

    As a cube: Density = 120 g ÷ 60 cm³ = 2 g/cm³

    Why nothing changes. Flattening rearranges the clay; it does not add or remove any. The mass is still 120 g. And the volume is still 60 cm³ — the sheet is much wider and much thinner, but it takes up exactly the same amount of space. Since neither number in the formula has changed, the density cannot change either.

    ✦ This is the chapter's statement on page 141 tested directly: the density of a substance is independent of its shape or size. It is independent of shape, which is why flattening does nothing; and independent of size, which is why cutting the sheet in half would not change it either — halving the mass and the volume together leaves the ratio untouched.

    A useful way to see it. Density describes the material, not the object. It is a fact about clay, and Reema has not changed what the clay is.

11. 113 marksCuriosity Grade 8, Chapter 9, page 150

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

    Hint. This time the unknown is the denominator; rearrange the formula.

    Rearrange the formula.

    Density = Mass ÷ Volume, so Volume = Mass ÷ Density

    Volume = 600 g ÷ 7.9 g/cm³ = 75.95 cm³ (about 76 cm³)

    Check the units. Grams divided by grams-per-cubic-centimetre leaves cubic centimetres — so the unit of the answer confirms the rearrangement was done the right way round. If you had multiplied instead, the units would have come out as g²/cm³, which is not a volume, and that alone would tell you something had gone wrong.

    ✦ The formula holds three quantities and any two give the third, so all three forms are worth knowing:

    To find	Use
    Density	Mass ÷ Volume
    Volume	Mass ÷ Density
    Mass	Density × Volume

    A sense check on the answer. 76 cm³ is a block a little over 4 cm on each side — a plausible size for 600 g of iron, which is a heavy but easily lifted lump.

12. 124 marksCuriosity Grade 8, Chapter 9, page 151

    In Fig. 9.26, a test tube of water is fitted with a glass tube. When the test tube is placed in a beaker of hot water at about 70 °C, the water level in the glass tube rises. How does this affect the density?

    Hint. Two of the three quantities in the formula are involved; work out which.

    The density of the water decreases.

    The reasoning, step by step.

    1. The hot water bath heats the water in the test tube.
    2. As temperature increases, the particles of a substance ... tend to move away and spread, so the water expands.
    3. The test tube is sealed except for the narrow glass tube, so the only place the extra volume can go is up the tube — which is why the level rises. The rise is the expansion made visible.
    4. No water has been added or lost, so the mass is unchanged.
    5. Density = Mass ÷ Volume, with the volume larger and the mass the same, so the density has fallen.

    ✦ The narrow glass tube is the whole design of the apparatus. Water expands only slightly on heating, and in the wide test tube alone the level would barely move. Forced into a narrow tube, that same small increase in volume becomes a large and obvious rise in height — the same reason measuring cylinders are made narrow and tall, used here to reveal an effect rather than to measure one.

    And this is exactly why hot air rises and hot air balloons work — the same expansion, the same unchanged mass, the same fall in density, in a different state of matter.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 9 (hecu109.pdf), Reprint 2026-27, pages 134-151. Questions are referenced from the NCERT textbook for identification.