Analyse Fig. 9.25a and 9.25b. Why does the unpeeled orange float, while the peeled one sinks? Explain. (a) (b) Fig. 9.25 (a) and (b), page 150 — redrawn sketch: (a) an unpeeled orange in a glass of water, (b) a peeled orange in a glass of water.

The peel is full of tiny air pockets . It adds a great deal of volume to the orange while adding very little mass, so the whole unpeeled fruit has an average density below 1 g/cm³ and floats (Fig. 9.25a). Remove the peel and you throw away that light, air-filled layer; what is left is dense juicy pulp with a density a little above 1 g/cm³, so it sinks (Fig. 9.25b). Unpeeled orange (Fig. 9.25a) Peeled orange (Fig. 9.25b) Mass A little more (peel included) A little less Volume Much more — the thick spongy peel Much less Average density = Mass/Volume Less than 1 g/cm³ More than 1 g/cm³ Result in water Floats Sinks Why it happens: This looks like a paradox — removing mass makes the orange sink — but density is a ratio . Peeling removes a little mass and a lot of volume, and taking away more from the bottom of the fraction than from the top makes the ratio bigger. The air trapped in the peel is doing exactly what the hollow cells do for sawdust and what a hollow bamboo pole does for a raft. Try this: Do it yourself with two oranges of the same size, one peeled and one not. Then remember the chapter's caution on page 140 — density is not the only factor that decides floating, but in this pair it is the deciding one.

NCERT Solutions for Class 8th Science Chapter 9 End-of-chapter question set (pages 149–151) — Keep the curiosity alive

Book page 149 Updated on2026-09-19

Q1.

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 presence of different gases in the atmosphere is also a uniform mixture.

Answer

Statement	T / F	Corrected statement
(i) Oxygen gas is more soluble in hot water rather than in cold water.	False	Oxygen gas is more soluble in cold water than in hot water.
(ii) A mixture of sand and water is a solution.	False	A mixture of sand and water is a non-uniform mixture, not a solution.
(iii) The amount of space occupied by any object is called its mass.	False	The amount of space occupied by any object is called its volume.
(iv) An unsaturated solution has more solute dissolved than a saturated solution.	False	An unsaturated solution has less solute dissolved than a saturated solution at the same temperature.
(v) The presence of different gases in the atmosphere is also a uniform mixture.	True	—

Why each one is what it is:

(i) A dissolved gas particle is loosely held between the water particles. Heating gives it enough energy to escape, so warm water holds less oxygen — which is why fish struggle in warm shallow water.
(ii) Sand does not dissolve. Its grains stay visible and settle at the bottom, so the components are not evenly distributed.
(iii) Mass is the quantity of matter (grams, kilograms); volume is the space occupied (cm³, mL, m³). Two different quantities — and their ratio is density.
(iv) Saturated means the maximum possible amount has already dissolved. Unsaturated means the solution is still below that maximum, so it must hold less.
(v) Air is a solution of gases — nitrogen, oxygen, carbon dioxide and others spread evenly, with no component visible separately.

Q2.

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.

Answer

Blank	Answer
(i) the solid is _____ in water and the _____ in water level is measured	immersed (dipped) … rise (increase)
(ii) The maximum amount of _____ dissolved in _____ at a particular temperature	solute … a fixed quantity (100 mL) of solvent
(iii) Generally, the density _____ with increase in temperature	decreases
(iv) … is called a _____ solution of glucose	saturated

Why these are the answers:

(i) The submerged solid pushes aside its own volume of water, and that water has nowhere to go but upward — so the rise in level equals the volume of the solid.
(ii) Solubility is a maximum, it is a property of a solute in a particular solvent, and it is quoted for a fixed quantity of solvent (100 mL) and a stated temperature. Leave any one of these out and the number means nothing.
(iii) On heating, the particles move apart and the volume increases while the mass stays the same. Density = Mass/Volume, so the density falls.
(iv) "Completely dissolved and no more can dissolve at that temperature" is the definition of a saturated solution.

Q3.

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

Answer

(ii) Water is denser than oil.

Density of water ≈ 1 g/mL
Density of cooking oil ≈ 0.91 g/mL (from the 1 litre / 910 g packet on page 141)

0.91 g/mL < 1 g/mL, so oil rises above water

Why the other options fail:

(i) If oil were denser it would sink and settle below the water, which is not what happens.
(iii) If the densities were equal there would be no reason for one to rise above the other; they would simply stay wherever they were put.
(iv) Oil does not dissolve in water at all — that is why you can still see two separate layers. If it dissolved you would get one clear uniform liquid.

Why it happens: When two liquids that do not mix are put together, the one that packs less mass into each millilitre ends up on top. Water particles attract one another strongly and pull together, squeezing the oil layer above them.

Q4.

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.

Answer

Mass = 225 g, Volume = 90 cm³

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

Density of water = 1 g/cm³
2.5 g/cm³ > 1 g/cm³ → the sculpture will sink

Its relative density with respect to water is 2.5 ÷ 1 = 2.5, a number with no unit — the sculpture is two and a half times as dense as water.

Why it sinks: Take 90 cm³ of water and it weighs 90 g; the same 90 cm³ of this stone weighs 225 g. For the space it occupies the stone is far too heavy, so it cannot stay up in the water.

Q5.

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.

Answer

The most appropriate statement is (iii) No more solute can be dissolved into the saturated solution at that temperature. That is exactly the definition of saturation.

Statement	Why it is not appropriate
(i) A saturated solution can still dissolve more solute	This describes an unsaturated solution. A saturated one has already reached its limit; extra solute settles at the bottom.
(ii) An unsaturated solution has dissolved the maximum amount possible	The two words have been swapped. Holding the maximum is what makes a solution saturated; unsaturated means it is still below the maximum.
(iv) A saturated solution forms only at high temperatures	A solution can be saturated at any temperature. Cold water saturates with less solute, hot water with more — Activity 9.1 makes a saturated salt solution at room temperature.

Why (iii) is worded carefully: Notice the phrase "at that temperature". Saturation is not a permanent state of a solution — heat it and the very same liquid becomes unsaturated, because its limit has moved up. Any statement about saturation that leaves out the temperature is incomplete.

Q6.

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?

Answer

Capacity of the bottle = 2 L
1 L = 1000 mL, so 2 L = 2 × 1000 = 2000 mL

Water already poured in = 500 mL

Space still empty = 2000 mL − 500 mL
Space still empty = 1500 mL = 1.5 L

Why you must convert first: You cannot subtract 500 from 2 — the two numbers are in different units. Bring both to the same unit (millilitres here) and the subtraction becomes meaningful. This is the first thing to check in any volume problem.

Tip: 1500 mL is also 1500 cm³, since 1 mL = 1 cm³. And since water has a density close to 1 g/mL, that much water would weigh about 1500 g, or 1.5 kg.

Q7.

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

Answer

Mass = 400 g, Volume = 40 cm³

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

Its relative density with respect to water is 10 ÷ 1 = 10. The object is ten times as dense as water, so it will sink in water.

What the number means: Every single cubic centimetre of this object carries 10 g of matter, while 1 cm³ of water carries only about 1 g. Densities as high as this belong to heavy metals — lead is about 11.3 g/cm³ and silver about 10.5 g/cm³.

Q8.

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

Fig. 9.25 (a) and (b), page 150 — redrawn sketch: (a) an unpeeled orange in a glass of water, (b) a peeled orange in a glass of water.

Answer

The peel is full of tiny air pockets. It adds a great deal of volume to the orange while adding very little mass, so the whole unpeeled fruit has an average density below 1 g/cm³ and floats (Fig. 9.25a). Remove the peel and you throw away that light, air-filled layer; what is left is dense juicy pulp with a density a little above 1 g/cm³, so it sinks (Fig. 9.25b).

	Unpeeled orange (Fig. 9.25a)	Peeled orange (Fig. 9.25b)
Mass	A little more (peel included)	A little less
Volume	Much more — the thick spongy peel	Much less
Average density = Mass/Volume	Less than 1 g/cm³	More than 1 g/cm³
Result in water	Floats	Sinks

Why it happens: This looks like a paradox — removing mass makes the orange sink — but density is a ratio. Peeling removes a little mass and a lot of volume, and taking away more from the bottom of the fraction than from the top makes the ratio bigger. The air trapped in the peel is doing exactly what the hollow cells do for sawdust and what a hollow bamboo pole does for a raft.

Try this: Do it yourself with two oranges of the same size, one peeled and one not. Then remember the chapter's caution on page 140 — density is not the only factor that decides floating, but in this pair it is the deciding one.

Q9.

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?

Answer

Object A: Density = 200 g ÷ 40 cm³ = 5 g/cm³
Object B: Density = 240 g ÷ 60 cm³ = 4 g/cm³

5 g/cm³ > 4 g/cm³ → Object A is denser

Why the heavier object is not the denser one: B has the greater mass — 240 g against 200 g — yet it is the less dense of the two, because that mass is spread through 60 cm³ instead of 40 cm³. Density asks a different question from mass: not "how much matter is there?" but "how much matter is packed into each unit of space?" To compare fairly you must always divide by the volume.

Check it yourself: Bring both to the same volume. In 120 cm³ you would have 600 g of A but only 480 g of B — the same conclusion, reached without dividing.

Q10.

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.

Answer

Nothing at all — the density stays exactly the same, 2 g/cm³.

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

Flattened into a sheet:
Mass = still 120 g (no clay has been added or removed)
Volume = still 60 cm³ (the clay has only been rearranged, not squeezed or stretched)
Density = 120 g ÷ 60 cm³ = 2 g/cm³

Why it happens: Flattening changes the shape, not the amount of matter and not the space it occupies. As the chapter states, the density of a substance is independent of its shape or size; it depends only on temperature and pressure. Since neither of those has changed, neither does the density.

Tip: A thin sheet of clay may well float on water while the cube sinks — but that is not a change of density. It is a change of shape, and it is exactly the reason a steel ship floats though a steel nail does not.

Q11.

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

Answer

Rearrange the density formula to make volume the subject.

Density = Mass / Volume → Volume = Mass / Density

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

Check it the other way round: 75.95 cm³ × 7.9 g/cm³ = 600.0 g. ✔

Why the answer is so small: 600 g of water would fill 600 cm³ — nearly half a large water bottle. The same 600 g of iron fills less than 76 cm³, a lump roughly 4 cm on a side, because iron packs 7.9 g into every cubic centimetre. That is what a high density means in practice.

Tip: Watch the units as you divide: g ÷ (g/cm³) = cm³. If your units do not come out as a volume, you have used the formula upside down.

Q12.

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?

Fig. 9.26 (a) and (b), page 151 — redrawn sketch of the setup: (a) a corked test tube of water with a glass tube through the cork, standing in a test-tube stand; (b) the same test tube clamped to a stand and lowered into a beaker of hot water.

Answer

The density of the water decreases. The rise in the glass tube is the visible sign that the water has expanded.

Mass of water in the test tube: unchanged — the tube is closed by the cork, nothing gets in or out
Volume of that water: increases — which is why the level climbs up the glass tube

Density = Mass / Volume
Same mass ÷ larger volume = smaller density

Why it happens: Heating gives the water particles more energy, so they move faster and push a little further apart from one another. The same number of particles — the same mass — now needs more room, and the narrow glass tube makes even a small expansion easy to see as a tall rise in level. This is the general rule of Section 9.5.2: the density of a substance decreases on heating and increases on cooling.

Did you know? The same effect explains why hot air rises and how a hot-air balloon lifts off (Fig. 9.22), and it is also the working principle of the liquid thermometer you used in Activity 9.2 — the liquid in the bulb expands up a narrow bore exactly as the water does here.

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