How is boiling point related to vapor pressure?

Short Answer

The boiling point of a liquid is directly related to its vapor pressure. A liquid boils when its vapor pressure becomes equal to the external pressure acting on it. At this point, vapor bubbles form inside the liquid and rise to the surface.

If a liquid has high vapor pressure at a lower temperature, it will boil easily and have a lower boiling point. If a liquid has low vapor pressure, it needs more heating, so its boiling point is higher. Thus, vapor pressure and boiling point are closely connected.

Detailed Explanation

Boiling Point and Vapor Pressure

The relationship between boiling point and vapor pressure is a fundamental concept in the study of liquids and phase changes. Vapor pressure is the pressure exerted by vapor molecules in equilibrium with a liquid. The boiling point is the temperature at which this vapor pressure becomes equal to the external pressure. When this happens, the liquid begins to boil, and bubbles of vapor form inside the liquid, not just at the surface.

Understanding this relationship helps explain why liquids boil at different temperatures, why boiling points change with altitude, and why some liquids evaporate or boil faster than others.

What Is Vapor Pressure

Every liquid has molecules that constantly move. Some of these molecules escape into the air above the liquid, forming vapor. The pressure exerted by this vapor is called vapor pressure. Vapor pressure increases with temperature because more molecules have enough energy to escape from the liquid surface.

What Is Boiling Point

The boiling point is the specific temperature at which a liquid changes into vapor rapidly. At the boiling point:

  • Vapor pressure = external pressure
  • Vapor bubbles form inside the liquid
  • Rapid evaporation takes place throughout the liquid

For water at sea level, the boiling point is 100°C because its vapor pressure becomes equal to the atmospheric pressure at that temperature.

How Vapor Pressure Determines Boiling Point

The key idea is:

A liquid boils when its vapor pressure equals the external pressure.

This means:

  • If vapor pressure reaches external pressure at a lower temperature → low boiling point
  • If vapor pressure reaches external pressure at a higher temperature → high boiling point

Thus, liquids that evaporate easily (high vapor pressure) boil at lower temperatures.
Liquids that evaporate slowly (low vapor pressure) boil at higher temperatures.

Nature of the Liquid and Intermolecular Forces

The strength of intermolecular forces affects vapor pressure:

  • Weak forces → high vapor pressure → low boiling point
    Examples: Ether, alcohol, petrol
  • Strong forces → low vapor pressure → high boiling point
    Examples: Water, glycerin, sulfuric acid

Liquids with strong forces need more heat to bring their vapor pressure up enough to match external pressure, so they boil at higher temperatures.

Effect of External Pressure on Boiling Point

Boiling point changes when external pressure changes.

  1. Higher External Pressure → Higher Boiling Point

In a pressure cooker:

  • Pressure is high
  • Water reaches higher vapor pressure
  • Boiling point increases above 100°C
  • Food cooks faster
  1. Lower External Pressure → Lower Boiling Point

At high altitude:

  • Atmospheric pressure is lower
  • Water vapor pressure equals external pressure at a lower temperature
  • Water boils below 100°C
  • Cooking takes longer

So, boiling point depends not only on the liquid but also on the surrounding pressure.

Vapor Pressure Curve

Each liquid has a vapor pressure curve showing how vapor pressure increases with temperature. When this curve meets the external pressure line, the boiling point is reached. This explains why boiling point is simply a temperature where vapor pressure becomes equal to the outside pressure.

Examples of Boiling Point Related to Vapor Pressure

Alcohol

Alcohol has high vapor pressure at low temperature.
Thus, it boils at around 78°C.

Water

Water has moderate vapor pressure.
It boils at 100°C at sea level.

Glycerin

Has very low vapor pressure due to strong hydrogen bonding.
It boils above 290°C.

These examples show the relationship between vapor pressure and boiling point clearly.

Importance of Understanding This Relationship

Knowing how vapor pressure relates to boiling point is important in:

  • Cooking and food industry
  • Distillation and purification
  • Chemical reactions
  • Designing pressure cookers
  • Weather and climate studies
  • Industrial processes involving heat

It helps predict how liquids behave when heated, stored, or transported.

Boiling and Energy Changes

At the boiling point, added heat does not increase temperature. Instead, all the supplied energy is used to overcome intermolecular forces and convert liquid into vapor. This is why temperature stays constant during boiling even though heat is supplied continuously.

Evaporation vs Boiling

Both depend on vapor pressure, but they occur differently:

  • Evaporation happens at all temperatures when some molecules escape.
  • Boiling happens only when vapor pressure equals external pressure.

Thus, vapor pressure controls both processes, but boiling requires a specific pressure balance.

Conclusion

Boiling point is directly related to vapor pressure. A liquid boils when its vapor pressure becomes equal to the external pressure. Liquids with high vapor pressure boil at lower temperatures, while those with low vapor pressure boil at higher temperatures. External pressure also affects boiling point, making vapor pressure a key concept in understanding phase changes, cooking, distillation, and many everyday and industrial processes.