3. 7 EVAPORATION AND DISTILLATION
What is the difference between evaporation and boiling? A liquid can evaporate at any
temperature, but boiling occurs only at one particular temperature called the boiling point.
Evaporation is slow at low temperatures, but fast at the boiling point.
What other things affect the rate at which evaporation occurs?
Factors that affect the rate of
evaporation. When you hang out your
clothes to dry, what can you do to help
the water evaporate quickly? There are
three things you can do:
(i) You can hang the clothes in a warm
place. As we just saw, temperature is
one factor that affects the rate of
evaporation.
The
higher
the
temperature, the faster the clothes dry.
(ii) You can spread the clothes out to
expose the biggest possible area to the
air. The area exposed is a factor that
affects the rate of evaporation. The
larger the area exposed, the faster the
clothes dry.
Three simple experiments
For each experiment, you will need two equally wet
pieces of paper. In each case (i) find out which one
dries first, and (ii) decide which factor made the water
evaporate faster.
1. Spread out one piece of paper in a warm place and
one in a cool place. Both places must be equally
windy.
2. Fold one piece of paper into quarters, but spread
out the other one. Place them side-by-side in the
same warm place.
3. Spread out one piece of paper in a calm place and
one in a windy place. Both places must be equally
warm.
(iii) You can hang the clothes in a place where the air is moving. The wind is the third factor that
affects the rate of evaporation. The stronger the wind, the faster the clothes dry.
Distillation. How can we obtain really pure water? If the water looks dirty we can filter it. But
even then, there may be something dissolved in the water. Remember, a solute passes through
the filter paper with the solvent. A good way to find out if a solute is present is to evaporate the
water. You can try this with the water you use at home or at school. Leave a clean plate full of
water out in the sun to evaporate. When the water has completely evaporated, look closely at the
plate. Has any solute been left on the plate?
In fact most of the water we use has some minerals from the earth dissolved in it. One way to
obtain really pure water is to
distil it. The impure water is
boiled in a flask. Only the water
evaporates into steam. Any
solute or suspension stays in the
flask. The steam passes through
a tube called a condenser which
has cold water flowing over it. As
it cools, the steam condenses
into pure water.
This process of evaporation and
condensation
is
called
distillation.
Even sea water,
which contains a lot of salt, can
be fully purified by distillation.
Pure water obtained in this way
is called distilled water.

1. What are the three factors which speed up
evaporation? What does the word factor mean?

3. What is distillation? How does a Liebig condenser
work?

2. In each of the experiments described above, why
must the two pieces of paper be equally wet?

4. A mixture of two miscible liquids with different
boiling points can be separated by distillation. Try to
explain how you think this works. Why must the
temperature be carefully controlled?
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