What is the effect of temperature on vapor pressure?

Short Answer

The vapor pressure of a liquid increases when the temperature increases. This happens because higher temperature gives more energy to the liquid molecules, helping them escape into the vapor phase more easily. As more molecules enter the vapor state, the vapor pressure becomes higher.

At lower temperatures, fewer molecules have enough energy to evaporate, so vapor pressure remains low. Thus, temperature and vapor pressure are directly related — when one increases, the other also increases.

Detailed Explanation

Effect of Temperature on Vapor Pressure

Temperature plays the most important role in determining the vapor pressure of a liquid. Vapor pressure is the pressure created by vapor molecules above the surface of a liquid when evaporation and condensation occur at the same rate. As temperature changes, the movement and energy of liquid molecules also change, which directly affects how many molecules escape from the liquid surface to form vapor.

The relationship is simple and clear: vapor pressure increases with an increase in temperature. This is because heat gives more kinetic energy to molecules, making it easier for them to overcome intermolecular forces and enter the vapor phase.

Why Vapor Pressure Increases with Temperature

When a liquid is heated:

  • Molecules gain kinetic energy
  • They move faster and vibrate more strongly
  • More molecules have enough energy to break free from the liquid surface
  • Evaporation becomes faster
  • The number of vapor molecules increases
  • These vapor molecules exert more pressure

This higher pressure is the increased vapor pressure.

In contrast, at lower temperatures:

  • Molecular motion is slow
  • Few molecules escape
  • Vapor pressure remains low

Thus, temperature directly controls how many molecules can escape into the air above the liquid.

Molecular Explanation

All liquids have intermolecular forces that keep their molecules close together. Temperature affects how easily these forces can be overcome.

  • At low temperature:
    Intermolecular forces hold most molecules together, so evaporation is slow.
  • At high temperature:
    Molecules gain enough energy to overcome these forces, leading to fast evaporation and high vapor pressure.

This explains why liquids evaporate faster on hot days and slowly on cold days.

Effect on Different Types of Liquids

Not all liquids respond the same way to temperature changes. Liquids with weak intermolecular forces show a big increase in vapor pressure even with small temperature rises.

Examples:

  • Ether
  • Alcohol
  • Petrol

Liquids with strong intermolecular forces need more heating to increase their vapor pressure.

Examples:

  • Water
  • Glycerin
  • Oil

However, for all liquids, vapor pressure always increases with temperature.

Temperature and Boiling Point Relation

Boiling occurs when vapor pressure equals external pressure.
Since vapor pressure rises with temperature:

  • As temperature increases, vapor pressure rises
  • When it matches atmospheric pressure, the liquid boils

So boiling point depends strongly on vapor pressure, which depends on temperature.

This also explains:

  • Water boils at lower temperature in mountains (low pressure)
  • Water boils at higher temperature in pressure cookers (high pressure)

Practical Examples of Temperature Effect on Vapor Pressure

  1. Drying of Clothes

Clothes dry faster on hot days because water’s vapor pressure is higher, so evaporation is faster.

  1. Perfume Spreading

Perfumes evaporate quickly in warm environments due to higher vapor pressure.

  1. Cooling Effect of Sweat

Sweat evaporates more on hot days because increased temperature raises vapor pressure of water on the skin.

  1. Fuel Evaporation

Petrol evaporates faster at high temperatures because its vapor pressure rises sharply.

  1. Cooking

Boiling water depends on temperature-driven vapor pressure increases.

Graphical Idea

Even without a graph, you can imagine the pattern:

  • Low temperature → low vapor pressure
  • Moderate temperature → medium vapor pressure
  • High temperature → high vapor pressure

This steady rise continues until the boiling point is reached.

Importance of This Relationship

Understanding how temperature affects vapor pressure helps in:

  • Predicting evaporation rates
  • Designing pressure cookers
  • Distillation and purification
  • Weather forecasting and humidity studies
  • Fuel storage and safety
  • Chemical reactions involving heat

Vapor pressure behaviour is essential in both science and daily life.

Conclusion

Temperature has a direct effect on vapor pressure. As temperature increases, molecules gain more energy, escape into the vapor phase more easily, and produce higher vapor pressure. At lower temperatures, evaporation is slow, and vapor pressure remains low. This relationship is important in explaining boiling, evaporation, cooking, weather patterns, and many industrial processes.