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# Vapour Pressure

Vapour pressure is a property of liquid which related to evaporation. In other words, it is the pressure that is exerted by a vapour in equilibrium with its phases either as solid or liquid at a given temperature and in a closed system.

In simple terms, we can define vapour pressure as a measure of the tendency of a material to change its state to solid/liquid to gas or vapour when the temperature increases. It indicates the rate of evaporation of liquid. It is also closely related to the tendency of particles to escape from the liquid or solid-state.

## Relation Between Vapour Pressure And Temperature

The substance with a high vapour pressure at room temperature is called a volatile substance. The temperature at which the vapour pressure at the surface of a liquid is equal to the pressure exerted by surrounding is denoted as the boiling point of that substance.

As the temperature of a liquid substance increases, the molecules’ kinetic energy also increases. Thus, the number of molecules transitioning into a vapour also increases and thereby increasing the vapour pressure of the liquid.

Simply say, with an increase in the temperature, the vapour pressure becomes sufficient to overcome atmospheric pressure and lift the liquid to form vapour bubbles inside the bulk of a substance. Volatile components such as gases and solvents are characterised by high vapour pressure can be transported with winds and components with low vapour pressure are transported with soil, sediment etc.

The expression that establishes the relation between the vapour pressure and the temperature of pure liquid or solid substances is given by the Antoine equation.

$$\begin{array}{l}log P = A – \frac{B}{C+T}\end{array}$$

Or temperature-explicit form which is written as;

$$\begin{array}{l}T = \frac{B}{A – log P} – C\end{array}$$

Where:

P = absolute vapour pressure of a substance

T = temperature of the substance

A, B and C = substance-specific coefficients (i.e., constants or parameters)

Sometimes an equation with only two coefficients is also used:

$$\begin{array}{l}log\: P = A – \frac{B}{T}\end{array}$$

Or,

$$\begin{array}{l}T = \frac{B}{A – log P} \end{array}$$

## Pictorial Representation Of Concept Of Vapour Pressure The vapour pressure of any substance increases nonlinearly with temperature. As moving to height, atmospheric pressure decreases because air is less dense at higher altitude. Because the atmospheric pressure is low then the vapour pressure of liquid needs to be lower to reach the boiling point. Therefore, low heat is required to make vapour pressure equal to atmospheric pressure.

To explain the concept of vapour pressure, let us consider a closed container half of which is filled with water. Then the water in the container is allowed to escape ie, allow to evaporate by the heating process. Some of the molecules in the container will be converted to vapour with respect to time due to this evaporation. As a result, the amount of liquid will decrease and the amount of vapour will increase. This evaporated gas molecules will move randomly along the empty place in the container. Due to this random movement, some of these molecules will come in contact with the uppermost layer of the container and start to condense.

At the start, the rate of evaporation will not be equal to the rate of condensation. Only after some time, an equilibrium will be established. At that equilibrium, the rate of evaporation is equal to the rate of condensation.

As time changes the number of molecules in a gaseous state increases while the rate of condensation also increases. At last, it reaches a stage where the rate of evaporation becomes equal to the rate of condensation. This stage is known as the equilibrium stage. At this point, the pressure exerted by the molecules on the wall is known as the vapour pressure of the liquid.

### Water Vapour Pressure

When we talk about the vapour pressure of water, it is nothing but the pressure that the water vapour experiences when it is in thermodynamic equilibrium. It will be in its condensed state. Generally, when the pressure is high water condenses.

## Relative Lowering Of Vapour Pressure

For a given solution, the relative lowering of vapour pressure is given as the ratio of lowering vapour pressure of the solvent in a solution to the vapour pressure of the pure solvent.

## Vapour Pressure Units

Vapour pressure can only be calculated in a closed container. Vapour pressure is usually measured in standard units of pressure. The SI unit is the pascal (Pa) where one pascal is equal to one newton per square meter (N·m−2 or kg·m−1·s−2).

Very low vapour pressures of solids are also often measured using the Knudsen effusion cell method. Sometimes vapour pressure is also expressed in other units such as atmospheres (atm) or millimetres of mercury (mmHg) in a medical context.

## Factors Affecting Vapour Pressure

There are four factors on which vapour pressure depends. They are:

### Nature Of Liquid

Nature of liquid is explained on the basis of its intermolecular forces. That is, as the magnitude of the intermolecular forces increases vapour pressure will decrease.

### Effect Of Temperature

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 molecule increases and hence vapour pressure increases. So we can conclude that vapour pressure is directly proportional to temperature.

### Concentration Of Solute

The presence of solute in the liquid will decrease the vapour pressure. And this fall in vapour pressure also varies with the concentration of solute.

## Vapour Pressure Is Independent Of

### Humidity

Temperature is the only property that affects the vapour pressure for a certain amount of water vapour in the air. Humidity will affect only if all the other variables are constant. So don’t be confused between the effect of temperature and humidity.

### Volume

Vapour pressure will not be affected by the volume of the container. As we know that liquid in the container will be in equilibrium with the vapour. When the volume is changed say decreased, then some of the vapour in the container turns into a liquid state. And if the volume increases some of the liquid will change into its vapour state.

### Surface Area

Normally vapour pressure is independent of surface area. Change in surface area will not affect the vapour pressure of a substance.

## Important Questions

1. What do you mean by vapour pressure?

Vapour pressure can be defined as the measure of the tendency of the material to change into vapour state or gaseous state. The vapour pressure increases as the temperature increases. The boiling point is the temperature at which the vapour pressure of the surface of the liquid will be equal to the atmospheric pressure.

2. What is the property of a material which processes maximum vapour pressure?

The material having the lowest boiling point has the highest vapour pressure at room temperature The material having high boiling point will have a low vapour pressure.

3. What is the effect of boiling point in vapour pressure?

Boiling point is inversely proportional to vapour pressure. By heating, a liquid will allow more molecules to enter into the atmosphere so the vapour pressure increases. The boiling point of a liquid is the point at which vapour pressure will be equal to the atmospheric pressure.

4. Explain Raoult’s law?

Raoult’s law states that a solvent’s partial vapour pressure in a solution (or mixture) is equal or identical to the vapour pressure of the pure solvent multiplied by its mole fraction in the solution.

## Video Lesson – Vapour Pressure and Raoult’s Law Test your Knowledge on Vapour pressure