All liquids exhibit a tendency for evaporation. The process of evaporation occurs at the surface of the liquid. If the kinetic energy of liquid molecules overcomes the intermolecular force of attraction in the liquid state, then the molecules from the surface of the liquid escape into space above the surface. The process is called 'evaporation.' If evaporation is carried out in a closed container system, then the vapours of liquid remain in contact with the liquid's surface. Like gas molecules, condensation of molecules also executes the continuous random motion. During these motions, molecules collide with each other and, even with the walls of the container, they lose their energy and return to the liquid state. This process is called 'condensation.'
Evaporation and condensation are continuous processes. Hence, after some time, an equilibrium is established at a constant temperature between evaporation and condensation. At the equilibrium number of molecules in the vapour, the state remains stable at a constant temperature.
The liquid's vapour pressure depends on the nature of the fluid and temperature, with an increase of intermolecular force of attraction Vapour pressure of liquid decreases, and with a rising temperature vapour pressure of liquid increases. Mercury manometer may be used to determine the vapour pressure of a liquid.
There are various factors on which vapour pressure depends. They are:
The nature of the liquid is explained based on its intermolecular forces. That is to say, as the magnitude of the intermolecular forces rise up, the vapour pressure will dwindle down
As the temperature of the liquid increases, the kinetic energy associated with the liquid also increases. And due to this increase in kinetic energy, the escaping tendency of the molecule increases; hence vapour pressure increases. So we can draw the inference that vapour pressure is directly proportional to temperature.
The existence of a solute in the liquid will significantly reduce the vapour pressure. And this fall in vapour pressure also differs with respect to the concentration of solute.
Temperature is the only property that affects the vapour pressure for a certain amount of water vapour in the air. Humidity will act only if all the other variables are constant. So don't get any sort of confusion between the effect of temperature and humidity.
Vapour pressure does not tend to get affected by the volume of the container. As we know, that liquid in the box will be in equilibrium with the vapour. When the work is changed, say decreased, then some of the container's vapour turns into a liquid state. And if the volume rises up, some of the liquid is bound to change into its vapour state.
Usually, vapour pressure is independent of surface area.
However, the following factors affect the vapour pressure of a liquid at equilibrium.
Intermolecular Forces of Attraction
The forces that mediate an interaction between atoms, including powers of attraction or repulsion are called the Intermolecular forces (IMF). For example, the covalent bond, involving sharing electron pairs between atoms, is much stronger than the parties present between neighbouring molecules.
The Volume of the Liquid Present Does Not Affect the Vapour Pressure of a Liquid at Equilibrium.
We can change the volume of a liquid (keeping temperature constant), but the vapour pressure of a fluid at equilibrium will remain the same.
The Temperature of the Liquid.
Weaker are the intermolecular forces of attraction, or higher is the temperature of the liquid, higher is the vapour pressure of a fluid at equilibrium.
The most common unit for vapour pressure is the torr. One torr = 1 mm Hg (one millimetre of mercury).
Most materials have external vapour pressures. For example, water has a vapour pressure of approximately 20 torrs at room temperature (22 °C = 72 °F). Note that the vapour pressure increases with the temperature; water will have a vapour pressure of 760 torr = 1 atm at its boiling point of 100 oC (212 oF).
The boiling point refers to the temperature of any substance at which the vapour pressure of a liquid becomes equivalent to the pressure encompassing the liquid, and the fluid converts into a vapour.
The higher the vapour pressure of a liquid at a given temperature, the lower the standard boiling point (i.e., the boiling point at atmospheric pressure) of the liquid.
The vapour pressure chart to the right has graphs of the vapour pressures versus temperatures for a variety of liquids. As can be seen in the map, the liquids with the highest vapour pressures have the lowest standard boiling points.
For example, at any given temperature, methyl chloride has the highest vapour pressure of any of the liquids in the chart. It also has the lowest standard boiling point (−24.2 °C), which is where the vapour pressure curve of methyl chloride (the blue line) intersects the horizontal pressure line of one atmosphere (atm) of absolute vapour pressure.