# Saturated, Unsaturated Vapor | Variation of Vapor Pressure with Volume & Temperature | Physics Grade XI

### Hygrometry

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# Saturated, Unsaturated Vapor | Variation of Vapor Pressure with Volume & Temperature

Hygrometry
The branch of physics which deals with the dampness of air is called hygrometry.

Saturated and Unsaturated vapor
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.

Difference between Saturated and Unsaturated vapor

 Saturated vapor Unsaturated vapor 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. 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. It doesn’t obey gas laws It obeys gas laws It is independent of temperature It changes to saturated by decreasing volume or temperature

Variation of Vapor Pressure with Volume
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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