Vapor Pressure

Evaporation When fastest particles in a sample of

liquid escape from the surface of the liquid

During evaporation the slower particles are left behind

What happens to the temperature of a liquid as it evaporates?

When first sealed the molecules gradually escape the surface of the liquid

The vapor above the liquid has a pressure

As the molecules build up above the liquid some condense back to a liquid.

Closed Container

Condensation Slower particles of gas feel effects of

intermolecular forces and become liquid

Leaves only the faster particles in the gas

What happens to the temperature of a the gas during condensation?

Rate of Vaporization = Rate of Condensation

Molecules are constantly changing phase “Dynamic”

The total amount of liquid and vapor remains constant “Equilibrium”

Dynamic equilibrium

The pressure of the gas at equilibrium is called the equilibrium vapor pressure

As the temperature goes up, more molecules vaporize

The equilibrium vapor pressure increases as temperature increases

Vapor Pressure Graph Vapor pressure can be plotted vs the

temperature The curve obtained is the equilibrium

vapor pressure Since different substances all

evaporate at different rates when heat is added, they all have different curves

Which of these substances vaporizes most easily? Which least?

liquid

gas

Boiling A liquid boils when the vapor

pressure = the external pressure Vapor pressure great enough to form

a bubble in the liquid Normal Boiling point is the

temperature a substance boils at 1 atm pressure.

The temperature of a liquid can never rise above it’s boiling point.

What is the boiling point of each liquid at 101.3 kPa?

Changing the Boiling Point Lower the pressure (going up into

the mountains). Lower external pressure requires

lower vapor pressure Lower vapor pressure means lower

boiling point. Food cooks slower.

On a mountain the pressure is 90 kPa, what is the boiling point of water there?

Raise the external pressure (Use a pressure cooker).

Raises the vapor pressure needed. Raises the boiling point. Food cooks faster.

Changing the Boiling Point