Boiling Point And Vapor Pressure: What, How, Relation And Facts

In this article we will discuss about the relationship between Boiling Point and Vapor Pressure.

Boiling point of a substance depends upon the pressure of the system and vapor pressure of a substance depends upon the temperature of the system.

In our primary school days our science textbook taught us that water boils at 100 degree Celsius. As we got to higher classes we learnt that water boils at 100 degree Celsius at atmospheric pressure at sea level and the boiling point decreases as we go higher in altitude.

This primarily explains the phenomenon of dependence of boiling point with pressure.

So, what is boiling? Physically it is the phenomenon of transformation of liquid into vapor. In order to understand boiling in a more geeky way, we must understand the other term that is vapor pressure. Vapor pressure is the pressure exerted by any liquid over its surface at a particular temperature.

This vapor pressure increases as the temperature of the liquid increases. The temperature at which vapor pressure and total pressure of the system are same is known as Boiling Point of a substance.

Is Vapor Pressure The Same as Boiling Point?

No, vapor pressure and boiling point are not same but vapor pressure is specific to a liquid and the same varies with temperature. Because of the same boiling point is also specific to a liquid.

Different liquids exert different vapor pressures at a particular temperature. The vapor pressure exerted by a liquid depends upon the intermolecular forces present between constituent molecules of the liquid.

At a certain temperature, for lower intermolecular forces in between the molecules of a liquid results higher vapor pressure exerted by the liquid. IF vapor pressure at a particular temperature is more, the boiling point of the liquid will be less. Hence, it can be concluded that although vapor pressure and boiling point are not same they are interlinked and are specific to a liquid.

What is Boiling Point and Vapor Pressure?

Boiling is characterized by vigorous evaporation of liquid to vapor, when the vapor pressure of the liquid reaches the pressure of the liquid above it, which is also called system pressure.

Under atmospheric conditions, the boiling point is also called normal boiling point.Once boiling starts, the temperature remains constant if the pressure above the liquid surface is kept constant until all the liquid is boiled off.

As shown in the picture below, the molecules in a liquid contained in a vessel are in constant motion. Some of the molecules escape the liquid surface to the atmosphere and some strike back from atmosphere to the liquid.

At a particular temperature, equilibrium is formed between the molecules escaping from the bulk liquid and those striking back to the bulk liquid.

The amount of molecules that remain in vapor space at a particular temperature depends upon the intermolecular forces of the liquid. These molecules that remain in vapor space over the liquid create the vapor pressure of the liquid.

boiling point and vapor pressure
Boiling Water; Image Credit: Wikimedia

Effect of Vapor Pressure on Boiling Point

Vapor pressure is inversely proportional to the boiling point of a liquid.

As the vapor pressure is lowered, the boiling point increases as more energy is required to vaporize the required amount of solvent to reach the system pressure. For a particular liquid, the vapor pressure is dependent on space available for solvent molecules to escape the bulk liquid at the liquid- vapor interface.

If more solid is dissolved in the liquid like in a sugar solution, less space would be available at the vapor-liquid interface for escape of solvent molecules to vapor space and hence lower vapor pressure is exerted.

This leads to requirement of higher energy to achieve the same vapor pressure as compared to pure solvent leading to higher normal boiling point.

Boiling Point Elevation

Relation between Boiling Point and Pressure

Higher system pressure results higher boiling point and lower system pressure result lower boiling point i.e. system pressure is directly proportional to boiling point.

If the system pressure is higher, more energy would be required to create vapor pressure equivalent to the system pressure. Similarly, if System pressure is lower, boiling point is achieved at lower temperature as vapor pressure requirement is lower.

This can also be explained by change in boiling point of water with elevation difference. As shown below, water boils at 100°C at sea level at 1 atmosphere pressure.

But as we go up to a hill top, the atmospheric pressure decreases and water boils at a lower temperature. Because of the same a person cooking rice in a open vessel would need more time to cook as the cooking is taking place at lower temperatures.

On the other hand, if the cooking is carried out in a pressured system like that in a pressure cooker, the cooking is much faster as boiling takes place at higher temperature.

Why are Boiling Point and Vapor Pressure Inversely Related?

Boiling point and vapor pressure are inversely related because higher energy will be required to vaporize enough liquid into gas phase and produce a vapor pressure equivalent to system pressure (atmospheric pressure if vessel is open to atmosphere) thus reaching the boiling point.

Vapor pressure will be lower for liquids which have higher intermolecular forces and vice versa. So liquid with lower vapor pressure at a particular temperature would require higher energy requirement to overcome the intermolecular forces in liquid form and escape to vapor compared to liquids with higher vapor pressures.

Similarly, if the vapor pressure of a solvent is lowered by adding solutes into it thus reducing the exposed surface area in the vapor- liquid interface the required energy to vaporize increases. This leads to elevation of boiling point of the solution thus formed.

What Affects Vapor Pressure?

Vapor pressure for a particular liquid is dependent upon its temperature

Vapor pressure of different liquids is different for a particular temperature. Vapor pressure of few liquids is shown below at 25°C.

substancevapor pressure at 25oC
diethyl ether70 kpa
bromine30 kpa
water3 kpa

The vapor pressure is different for different substances because these liquids have different intermolecular forces. In general, liquids of lower densities, which will have lower molecular size and hence lower intermolecular forces have higher vapor pressure at a particular temperature.

If temperature is increased in a system, the kinetic energy of the molecules increases, as a result the molecules move rapidly and more number of molecules escape to vapor form. In this way vapor pressure increases with increasing temperature.

Vapor pressure however is not impacted by shape or size of the vessel where the liquid is kept. Foe example, a liquid in a vertical vessel or a horizontal vessel shall have same vapor pressure.

Factors that affect Vapor Pressure

What Causes Vapor Pressure to Increase?

Vapor pressure increases with increase in temperature.

Vapor is always in equilibrium with the liquid surface below. The number of molecules in vapor form depends upon the kinetic energy of the individual molecules, which decides their probability of existing in vapor or liquid form.

For a particular liquid, if the temperature is increased the kinetic energy of the molecules in the liquid increases, making more of them eligible to overcome the intermolecular forces and exist in vapor form.

Higher number of molecules in vapor form increases the vapor pressure. A typical change of vapor pressure of water with temperature is shown in the image below.

Vapor Pressure Vs Temperature

Factors Affecting Boiling Point

The factors that affect boiling point are pressure, impurities in liquid and liquid intermolecular forces.

If the system pressure is higher, more heat would be required to increase the vapor pressure of the liquid to achieve the system pressure. Boiling point impact on external pressure for water is shown by the curve below.

Factors Affecting Boiling Point

If we consider, a pure solvent and same solute with impurities, the boiling point of the pure solvent is always less. It is because more surface area available for a pure solvent for vapor to escape the liquid state compared to solute with impurities, where some of the sites in vapor liquid interface are occupied.

A liquid having larger molecules with higher intermolecular forces will have higher boiling point. This is because less vapor can break free above the liquid surface by overcoming the intermolecular forces at a particular temperature compared to liquid with smaller constituent molecules having lower intermolecular forces.

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