Factors Affecting Vapour Pressure

The concept of vapour pressure is crucial to understand first to comprehend how several factors affect it, whether the nature of liquid or the effect of temperature. To understand vapour pressure, the best way is to consider a container with liquid filled in it at a high temperature. The high temperature leads to the increased motion of particles in the container. These high energy molecules move from the surface of the liquid. After leaving the surface, they occupy the empty space and transform themselves into vapours. These molecules then hit the container walls and the liquid’s surface. This collision results in the particles losing some of their kinetic energy. Now, the molecules might get captured by the liquid surface again. This is known as condensation. The process of evaporation and condensation continues until a dynamic equilibrium is reached. Dynamic equilibrium is achieved when both condensation and evaporation rates become equal. The pressure that vapours exert on the surface of a liquid at a particular temperature at the state of dynamic equilibrium is termed vapour pressure. 

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Factors affecting vapour pressure

More and more molecules of the vapour phase increase as time passes. Also, an increase in the condensation rate is observed. Eventually, the condensation rate reaches a stage where the evaporation rate is equal to it, which is known as dynamic equilibrium. The vapours formed inside the container exert a certain pressure on the container. This pressure is termed vapour pressure. 

Now, it might be clear that temperature is a primary factor that will affect the vapour pressure. But is the only temperature that affects it? No, there are some more factors. These factors are listed below to explain how they affect vapour pressure. 

1.Nature of liquid

The nature of liquid is one of the most critical factors that affect vapour pressure. The effect of the nature of the liquid can be explained based on the intermolecular forces. The vapour pressure decreases with an increase in the magnitude of intermolecular forces. When the magnitude of intermolecular forces decreases, the vapour pressure of the liquid increases. For example, at a particular temperature, the vapour pressure of glycerol is less than that of isopentane. This is because the magnitude of the intermolecular forces is higher in glycerol. 

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2.Liquid’s boiling point

The boiling point is also a crucial factor that affects the vapour pressure. The relationship between boiling point and vapour pressure is inversely proportional. It means that if the boiling point of a liquid is higher when compared to the boiling point of another liquid, the vapour pressure of the liquid with a higher boiling point will have a lower vapour pressure. For example, when the vapour pressures of ethyl alcohol and ether are compared, the vapour pressure of ether is higher because it has a lower boiling point. 

3.Effect of temperature on vapour pressure

Temperature is the most significant factor that affects vapour pressure. When a liquid is provided with a high temperature, the kinetic energy of its molecules increases. Molecules possessing higher kinetic energy tend to leave the liquid surface more quickly, and the vapour pressure will increase due to more molecules getting converted into vapour. Hence, we can conclude that temperature is directly proportional to the vapour pressure of a liquid. To be precise, the vapour pressure increases with an increase in temperature and vice-versa. 

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4.Concentration of solute

When a solute is added to a solvent, the vapour pressure of the liquid decreases. Comparing the vapour pressure of pure solvent and that of the solute added solvent will depict a higher vapour pressure of the pure solvent. Therefore, it can be said that the concentration of solute added also affects the vapour pressure of a liquid. 

Conclusion

The vapour pressure of a liquid is the pressure exerted by the molecule that leaves the liquid surface of the walls of the container. This vapour pressure is controlled or affected by certain factors. These factors include the nature of liquid, temperature, boiling point of the liquid, and solute concentration. The relationship of vapour pressure with each of these factors differs. The factors affecting the vapour pressure of a liquid are a significant concept to understand vapour pressure thoroughly.