EN हिं

NCERT Solutions Curiosity Chapter 9 .3 Solubility of Gases — In-text Questions

Book page 1399 Updated on2026-09-05

Q1.
Do gases also dissolve in water?
Answer

Yes. Many gases, including oxygen, dissolve in water — though oxygen dissolves only to a small extent.

Small as that amount is, everything living in a pond, a river or the sea depends on it. Fishes, aquatic plants and other water organisms take their oxygen from the water itself, not from the air above it, and it is this dissolved oxygen that keeps them alive.

Why it happens: Gas particles at the surface of the water are constantly striking it, and some of them slip in between the water particles and are held there. Because gas particles are already far apart and are not tightly bound to one another, they need no tearing apart the way a salt crystal does — but water can hold only a small number of them, so the solubility of a gas is low.
Did you know? The fizz in a cold drink is carbon dioxide dissolved in water under pressure. Open the bottle, the pressure drops, and the gas comes out of solution as bubbles.
Q2.
Is the mixture of gases in water a uniform or non-uniform mixture?
Answer

It is a uniform mixture — that is, a solution. The gases dissolve evenly through the water; you cannot see them, they do not float up as visible bubbles, and a sample taken from any depth has the same dissolved gas in it.

Why it happens: A dissolved gas is present as separate particles fitted into the gaps between water particles, exactly as dissolved salt is. Particles that small and that well spread cannot be seen even under a magnifying device, which is the test for a uniform mixture.
Tip: The bubbles you sometimes see clinging to the inside of a glass of water left standing are gas that has come out of solution as the water warmed up — they are not the dissolved gas itself.
Q3.
Does temperature affect the solubility of gases in liquids also? If so, how?
Answer

Yes, and in the opposite way to solids: the solubility of gases generally decreases as the temperature increases. Cold water holds more dissolved oxygen; as water warms up, the dissolved oxygen escapes.

SoluteEffect of raising the temperatureExample from the chapter
Most solidsSolubility increasesBaking soda in water, 20 °C → 70 °C
GasesSolubility decreasesOxygen in pond water in summer
Why it happens: A dissolved gas particle is only loosely held in the gaps between water particles. Heating gives it more energy — enough to break free and escape into the air above. For a solid the heat is used to tear particles off a crystal, which helps dissolving; for a gas there is nothing to tear apart, so the extra energy only helps it leave.
Did you know? This is why fish suffer in a shallow pond during a summer afternoon, and why warm water discharged from a factory into a river can harm aquatic life even if it carries no pollutant at all.
Q4.
I observed that in some non-uniform mixtures, such as sawdust in water, the sawdust floats, whereas in the mixture of sand and water, the sand sinks. I wonder why that happens?
Answer

Because of density — the mass packed into unit volume. Sawdust is less dense than water, so it floats; sand is much denser than water, so it sinks.

Density = Mass / Volume
Water ≈ 1 g/cm³    Wood and sawdust: less than 1 g/cm³    Sand: about 2.6 g/cm³
Why it happens: Wood is not solid all through — it is built of tiny hollow cells full of air, so a piece of wood packs very little mass into its volume. Sand is grains of rock with no air trapped inside, so the same volume of sand carries far more mass. Neither one dissolves, so both stay as visible particles and the mixture is non-uniform in each case; what differs is only whether the particles sit on top or at the bottom.
Tip: The book adds an important caution on page 140 — density is not the only factor that decides whether something floats. A steel ship floats although steel is far denser than water, because of the shape it is given and the air it encloses.
Was this helpful? Report an error