Evaporation and Boiling

Evaporation

Molecules at the surface of a liquid may either escape and change state into the gaseous state or fall back into the liquid and remain in the liquid state. Only the faster molecules actually escape into the gaseous phase because they have enough energy to escape from the molecular forces of the liquid and become a gas.Evaporation results in cooling

Since the temperature of the liquid is indicated by the kinetic energies of its molecules, this escaping of molecules with high kinetic energy results in an overall cooling of the liquid as the molecules with the higher kinetic energies have left the liquid and ones with lower k.e are left behind.

The rate of evaporation of a liquid can be increased by

• increasing the temperature to the liquid. This increases the k.e of the molecules helping more molecules to escape

• increasing surface area - there would be a much larger surface from which particles can escape and cause evaporation

• a draught over the surface - this would cause molecules that have already escaped to be blown away and reduce the changes of them returning to the liquid state.

Boiling

Recap:

Molecules escape from a liquid by evaporation.

Molecules in the gas phase exert a pressure on the walls of the container that holds it

When a liquid is placed in a closed container, the space above it gets filled with vapour. The vapour molecules collide with the walls of the container, bounce off and cause a pressure. This pressure is called the vapour pressure.

When the number of molecules of the vapour in the space above the liquid has reached its maximum the space is said to be saturated. The pressure at this point is the saturated vapour

When a liquid is heated, the saturated vapour pressure increases. Boiling occurs at the point when the saturated vapour pressure equals the external atmospheric pressure. At this point the liquid molecules form bubbles inside the liquid without the atmospheric pressure being able to crush them. These bubble burst at the surface causing the boiling effect.

If the external atmospheric pressure if high, it’s harder for the liquid to boil

If the external atmospheric pressure if low, it’s easier for the liquid to boil

Water boils faster at higher altitudes because of the lower atmospheric pressure.

Differences with evaporation and boiling

Evaporation

occurs at the surface of the liquid

occurs at all temperatures

Boiling

bubbles from within the liquid

occurs at a definite temperature called the boiling point