Everything Science Grade 12

CHAPTER 3. REACTION RATES 3.6

Observations:

You should notice that inthe beaker that is uncovered, the water level drops

quickly because of evaporation. In the beaker that is covered, there is aninitial

drop in the water level,but after a while evaporation appears to stop and the

water level in this beaker is higher than that in the one that is open. Note that

the diagram below shows the situation at time= 0.

= evaporation

= condensation

bell jar

Discussion:

In the first beaker, liquidwater becomes water vapour as a result of evapo-

ration and the water level drops. In the secondbeaker, evaporation alsotakes

place. However, in thiscase, the vapour comesinto contact with the surface of

the bell jar and it cools and condenses to form liquid water again. This water is

returned to the beaker.Once condensation hasbegun, the rate at whichwater

is lost from the beaker will start to decrease. Atsome point, the rate ofevap-

oration will be equal tothe rate of condensationabove the beaker, and there

will be no change in thewater level in the beaker. This can be represented as

follows:

liquid� vapour

In this example, the reaction (in this case, a change in the phase of water)

can proceed in either direction. In one direction there is a change inphase

from liquid to gas. But the reverse can also takeplace, when gas condenses to

form liquid again.

In a closed system it is possible for reactions to be reversible, suchas in the demon-
stration above. In a closed system, it is also possible for a chemical reaction to reach
equilibrium. We will discuss theseconcepts in more detail.