The pressure exerted by vapours into the environment is known as vapour pressure. Vapour pressure is said to be constant when the pressure exerted by the vapours present in a closed environment has achieved equilibrium, i.e., the condition in which a substance can exist in both the gas phase and the liquid phase. In other words, vapour pressure is the measure of the tendency of a substance to get transformed into a gaseous or vapour state. Vapour pressure is directly proportional to temperature. This means that when we increase the temperature, a significant and proportionate increase in the formation of vapours can be observed, which further increases the vapour pressure.
Examples of Vapour Pressure
There are a number of daily life examples that employ the concept of vapour pressure. Some of them are listed below:
1. Tea Kettle
A tea kettle demonstrates vapour pressure in the best possible manner. When the water poured inside the kettle starts boiling, the number of water vapours inside the kettle begins to increase. The water vapours exert pressure on the lid of the kettle, thereby pushing it upwards.
2. Humid Air
One of the best examples of vapour pressure in real life is the relative humidity experienced by a person in a particular area. The relative humidity can be defined as the ratio of the actual vapour pressure of water in the air to the saturated vapour pressure of water in the air. The value of saturated vapour pressure of water in the air is fixed for any particular temperature value. This means that the actual vapour pressure of water in the air is the main criterion that determines the relative humidity in the environment.
3. Smell of Brandy or Whisky
One can easily smell the brandy, whisky, or other pungent drinks easily from a distance. This is because such drinks are volatile in nature and a part of such drinks gets converted to vapour form easily. The vapours exert a significant amount of vapour pressure into the surroundings, which can be sensed easily from a distance.
4. LPG Cylinders
In absence of vapour pressure, an LPG cylinder is incapable of supplying the fuel to the burner. When the burner knob is twisted, the valve opens and the vapour pressure increases. This allows the vapours of the fuel to flow in the direction of the burner. Similarly, twisting the burner in opposite direction causes the valve to close and decrease the vapour pressure. This causes the flow of fuel vapours to cut off.
5. Pressure Cooker
The entire operation of a pressure cooker is based on the concept of vapour pressure. When heat is supplied to the contents that are poured into the pressure cooker, water vapours begin to form. This tends to increase the value of vapour pressure in the utensil. The vapour pressure continues to increase gradually and triggers the whistle mechanism of the pressure cooker. The vapour pressure decreases only when the source of heat is terminated as the absence of heat would terminate any further building of water vapours and causes the existing vapours to condense back to liquid form.
6. Boiling of Liquids
The vapour pressure of a liquid substance is strongly related to its boiling point. To boil a fluid, it is required to be heated. The increase in the temperature of fluid causes a significant increase in the magnitude of vapour pressure. The boiling point of the fluid and the vapour pressure both quantities are inversely related to each other. This means that the two entities are linked to each other in such a way that the higher is the vapour pressure, the lower will be the boiling point and if the vapour pressure is low, the boiling point will be high.
7. Drying Clothes
The drying of clothes is an important phenomenon that makes use of the concept of vapour pressure. Mostly to dry clothes at a faster rate, they are subjected to an environment that has a high temperature. For instance, traditionally the clothes are dried by exposing them to the sunlight. The rays of the sunlight falling on the damp clothes tend to increase the temperature, which further increases the vapour pressure. The water vapours tend to move from the region of low pressure to the high-pressure region, i.e., in this case, from damp clothes to the surroundings. As a result, the clothes get dry.
8. Water Present in Lakes
The reason why the water present in large water bodies do not get evaporated easily is that the low vapour pressure of such water bodies helps in the development of high surface tension of water, which binds the water molecules together and prohibits them from evaporating at once. Also, the water molecules present in huge water bodies such as lakes, ponds, seas, oceans, etc. tend to keep evaporating and condensing simultaneously, thereby allowing it to maintain a constant volume of water.
9. Skin Cracks
The skin cracks that are observed usually in winters is a result of low vapour pressure. The air in the winters lack moisture and is dry in nature. The less number of water vapours present in the air during winters correspond to a less value of vapour pressure. The moisture present in the skin cells creates a vapour pressure that is significantly higher than the vapour pressure present in the environment; therefore, the vapour molecules tend to move from a region of high pressure to a region of low pressure. As a result, the skin is unable to retain moisture and appears chapped or cracked.
10. Frost Formation in Freezers
The reason why you are frequently required to perform the defrosting of freezers can be explained easily with the help of the concept of vapour pressure. The temperature of a freezer is maintained in such a way that the surface temperature has a lower value than the temperature inside the freezer. This is because the heat is required to be evacuated through the surface of the freezer. A corresponding amount of saturated vapour pressure gets developed relative to the temperature value on the surface of the freezer and inside the freezer. The value of saturated vapour pressure is less at the surface and is comparatively high inside the freezer. Suppose an ice tray filled with water is placed inside the freezer. The water present in the ice tray begins to evaporate and the water vapours begin to form. These water vapours tend to move from the region of high pressure, i.e., from the centre of the freezer towards the region of low pressure, i.e., towards the surface of the freezer. Since the temperature at the surface of the freezer is lower than the inner region, the water vapour molecules begin to undergo condensation and get deposited on the edges of the freezer in the form of frost.
11. Cooling of Dry Air
The cooling of dry air is yet another example of vapour pressure and is able to provide a reasonable explanation for the fact that the places that contain dry air have relatively cool nights. Suppose that you have a closed container that is partially filled with liquid and contains water vapours in the empty space, i.e., the equilibrium state of the container is maintained. The system is required to be cooled by extracting heat, thereby causing some of the water vapours to condense and get transformed into liquid form. As a result, the water level in the container rises and the number of water vapours falls. This means that if there exists a system that has a high moisture content in the air or a high value of vapour pressure, then the heat capacity of the system would be significantly high. Similarly, if the amount of vapour pressure in the environment is low or when the air is drier, the change in temperature can be effectively reduced and the environment is comparitively cold.