Short Answer

Vapor pressure is affected mainly by temperature and the nature of the liquid. When temperature increases, more molecules gain energy to escape into vapor, so vapor pressure rises. Liquids with weak intermolecular forces also have higher vapor pressure because their molecules evaporate more easily.

Other factors like the presence of solutes and the strength of molecular attraction also influence vapor pressure. However, vapor pressure does not depend on the amount of liquid or the surface area once equilibrium is reached in a closed system.

Detailed Explanation

Factors Affecting Vapor Pressure

Vapor pressure is the pressure exerted by vapor molecules when a liquid and its vapor are in equilibrium in a closed container. It is an important property that helps explain evaporation, boiling, volatility, and behaviour of liquids at different temperatures. Vapor pressure changes depending on several conditions, but the most important factors are temperature and intermolecular forces.

Understanding these factors helps explain why some liquids evaporate quickly, why boiling points vary, and how weather and industrial processes are influenced by evaporation.

1. Temperature

Temperature is the most important factor affecting vapor pressure.

• When temperature increases, liquid molecules gain more kinetic energy.
• More molecules escape into the vapor phase.
• The number of vapor molecules and the pressure they create increases.

Therefore, vapor pressure increases with temperature.

For example:

• Water at 25°C has lower vapor pressure than water at 60°C, because more molecules evaporate at higher temperatures.

This is why clothes dry faster on a hot day and why water boils when its vapor pressure equals external pressure.

2. Nature of the Liquid (Intermolecular Forces)

The strength of intermolecular forces determines how easily molecules escape into the vapor phase.

• Weak intermolecular forces → High vapor pressure
  Liquids like alcohol, ether, petrol evaporate quickly.
• Strong intermolecular forces → Low vapor pressure
  Liquids like water, glycerin, honey evaporate slowly.

This is because stronger forces require more energy to break, so fewer molecules can escape, resulting in low vapor pressure.

3. Molecular Mass of the Liquid

Liquids with low molecular mass tend to have higher vapor pressure.

• Lighter molecules escape more easily.
• Heavier molecules require more energy to vaporize.

For example:

• Butane (lighter) has higher vapor pressure than octane (heavier).

Thus, molecular size and weight affect vapor pressure.

4. Presence of Solutes (Raoult’s Law)

Adding a solute (salt, sugar, etc.) to a liquid lowers its vapor pressure.

This happens because:

• Solute particles block surface area of the liquid
• Fewer solvent molecules can escape
• Evaporation rate decreases

For example:
Saltwater has lower vapor pressure than pure water.
This is why seawater boils at a slightly higher temperature.

5. Surface Area (Affects Rate, Not Final Vapor Pressure)

Surface area does not change the equilibrium vapor pressure, but it affects how quickly equilibrium is reached.

• Larger surface area → faster evaporation
• Faster evaporation → vapor pressure equilibrium is reached sooner

But the final vapor pressure remains the same, because it depends only on temperature and the nature of the liquid, not on how much of the surface is exposed.

6. External Pressure (Does Not Directly Change Vapor Pressure)

External pressure does not affect vapor pressure at equilibrium.
However, it affects boiling point, which is related to vapor pressure.

Example:
At higher altitude, external pressure is low, so water boils at a lower temperature.

7. Strength of Hydrogen Bonding

Liquids with strong hydrogen bonding have low vapor pressure.

Examples:

• Water
• Alcohols
• Formic acid

Hydrogen bonding makes molecules strongly attached, so fewer escape into vapor.

8. Impurities and Chemical Nature

Chemical composition also influences vapor pressure:

• Nonpolar liquids usually have higher vapor pressure
• Polar liquids have lower vapor pressure

Impurities reduce volatility by disrupting the surface and intermolecular interactions.

Practical Examples of Vapor Pressure Effects

Perfumes and Deodorants

They evaporate quickly because they have high vapor pressure.

Petrol and LPG

These fuel vapors are easily released because of high vapor pressure.

Weather and Humidity

Water vapor pressure affects evaporation rate, humidity, and rainfall.

Pressure Cookers

By increasing external pressure, water’s vapor pressure balance shifts, raising boiling point and cooking food faster.

Why Understanding Vapor Pressure Is Important

• Helps explain boiling and evaporation
• Important in designing distillation processes
• Determines volatility of fuels
• Useful in meteorology and climate studies
• Essential in pharmaceutical and food industries

Vapor pressure is a fundamental property that connects temperature, molecular structure, evaporation, and phase changes.

Conclusion

Vapor pressure is mainly affected by temperature and the nature of the liquid. As temperature rises or intermolecular forces weaken, vapor pressure increases. Other factors like molecular mass, presence of solutes, hydrogen bonding, and impurities also influence it. Understanding these factors helps explain evaporation, boiling, volatility, and many natural and industrial processes involving liquids.