Difference between Saturated and Unsaturated vapor
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Saturated vapor |
Unsaturated vapor |
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When a space contains the maximum amount of vapor that it can hold at that temperature, then the vapor in the space is called saturated vapor. |
When a space contains less than the maximum amount of vapor that it can hold at that temperature, then vapor in the space is called unsaturated vapor. |
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Vapor in a closed space and in contact with liquid is called saturated vapor |
Vapor in closed space but not in contact with liquid can be unsaturated. |
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It doesn’t obey gas laws |
It obeys gas laws |
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It is independent of temperature |
It changes to saturated by decreasing volume or temperature |
In the figure, two barometer tubes X and Y, filled with mercury is inverted over mercury in a trough. Suppose that the tube X has Torricellian vacuum and tube Y has unsaturated water vapor. When the volume of unsaturated vapor is decreased by lowering tube Y in the mercury, the pressure increases with the decrease in volume and reaches point B from A. The difference in levels of mercury in tube X and Y shows the unsaturated vapor pressure. On further decreasing the volume, a point B is reached at which vapor is saturated and exerts maximum pressure. There is no change in pressure on further decrease in volume and vapor condenses into water molecules. A graph plotted between the pressure and volume of the vapor is shown in the figure.
Variation of Vapor pressure with temperature
Consider two barometric tubes X and Y as shown in the figure. X has a Torricellian vacuum and Y has unsaturated vapor. Both tubes are enclosed by a wider glass tube containing water which can be heated with a heating coil and temperature can be noted with the thermometer.
At the beginning, water is heated to a certain temperature of about 80 degrees Celsius and then heating is stopped. The vapor in Y at higher temperatures becomes unsaturated and the vapor pressure increases. When the temperature is decreased, the vapor pressure also decreases in Y. when a graph is plotted between the vapor pressure and temperature, we obtain the following graph.
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