NCERT Solutions for Class 9th Science Chapter 5 .3.2 Distillation — In-text Questions

Book page 805 Updated on2026-09-08

Q1.

However, we may also want to recover the solvent. What should we do then?

Answer

Use distillation instead of plain evaporation.

Evaporation: solution → solute recovered, solvent lost to the air
Distillation: solution → solute left in the flask and solvent condensed and collected

Why it works: Both methods do the same first step — the solvent is boiled off as vapour. The difference is what happens next. In evaporation the vapour simply escapes into the room. In distillation the vapour is led into a condenser, where circulating cold water takes away its latent heat of vaporisation, so it turns back into a liquid and drips into a receiving flask. Nothing is thrown away, so you end up with both components in pure form.

Did you know? This is how a camp still gives you drinking water from salty or muddy water: the water is boiled off and condensed, while the salt and dirt stay behind in the boiling vessel.

Q2.

How can we separate the two miscible liquids?

Answer

By distillation, provided their boiling points differ by at least about 25 °C.

Distillation set-up. The lower-boiling liquid vaporises first, the condenser turns the vapour back into liquid, and the distillate collects in the conical flask.

Boiling point of acetone = 56 °C
Boiling point of water = 100 °C
Difference = 100 °C − 56 °C = 44 °C, which is more than 25 °C → distillation will work

Heat the mixture in a distillation flask. Acetone, the lower-boiling liquid, boils first at about 56 °C; the thermometer bulb at the mouth of the side arm holds steady near 56 °C while it distils over. Its vapour passes through the water condenser, is cooled and collected in the conical flask as pure acetone. Water, needing 100 °C, stays behind in the distillation flask.

Why the 25 °C rule: No liquid waits politely for its turn — even below its boiling point some of it evaporates. If the two boiling points are close, the vapour rising from the flask is a mixture of both liquids and the distillate is impure. A gap of about 25 °C or more means that when the first liquid is boiling, the second contributes only a small amount of vapour, so the distillate is essentially pure. For smaller gaps, fractional distillation is used instead.

Q3.

Is it possible to separate the mixture of two miscible liquids by evaporation and obtain both the liquids?

Answer

No. Evaporation can give you back only one of the two — and even that one is not obtained pure.

The liquid that evaporates escapes into the air and is lost; there is no arrangement to catch it. So one component is gone for good.
Both liquids evaporate at the same time, because evaporation goes on at all temperatures. The liquid left behind therefore still contains some of the other liquid — it is not pure either.

Evaporation is useful only when the solute is a non-volatile solid, such as salt in water, and you do not care about recovering the solvent.

Why distillation succeeds where evaporation fails: Distillation adds two things — a controlled temperature that makes the lower-boiling liquid vaporise first, and a condenser that catches that vapour and turns it back into liquid. Because the vapour is captured instead of released, both components are recovered.

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