RI 1998 Sec 3 EOY P2 Q6
The figure below shows a gas jar full of air at atmospheric pressure. The jar is connected to a small reservoir containing a dark brown liquid which evaporates very easily.
The tap connecting the jar to the reservoir is then opened.
Nothing seems to happen for some time but gradually a brown gas is seen to move very slowly into the air in the jar.
Eventually, the whole jar is filled with the brown gas although the process began with only a small volume of liquid.
(a) Name the process described and explain the above observations in terms of simple kinetic molecular model of matter.
(b) State and explain how the pressure exerted by the contents of the gas jar changes as the process goes on, assuming that the temperature remains constant.
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Answer:
(a) The process is diffusion.
As the dark brown liquid evaporates, the brown liquid molecules become brown gas molecules. These brown gas molecules move randomly, colliding with air molecules. Slowly, the gas molecules move into the gas jar because of their random motion. After some time, there will be a uniform mixture of air molecules and brown gas molecules in the gas jar.
(b) As the process goes on, more and more brown gas molecules are present in the gas jar. Since there are more gas molecules, there is a higher frequency of collisions on the wall of the gas jar. This results in an increasing pressure.
