### BOILING (Vaporization)

Evaporation takes place at the surface of a liquid and at any temperature. As a liquid is heated, its temperature rises. It will reach a point at which the liquid molecules have enough energy to overcome the forces holding them together. They can then move around freely in the gaseous state, and gas bubbles rise and break through the surface. This is called boiling. The vapour pressure inside the bubbles is equal to the atmospheric pressure. Boiling is the rapid evaporation taking place throughout the liquid rather than just at the surface.

The temperature at which boiling starts is called the boiling point of the liquid. No matter how much heat is given to a boiling liquid its temperature does not change. It is used to overcome the forces which holds the molecules together.

The heat needed to change 1 g of liquid at its boiling point to 1 g of gas without changing its temperature is called, the Latent Heat of Vaporization

Each liquid has its own boiling point and its own L. H of vaporization.

#### Q= M x LH(v)

• M : mass
• LH(v): Latent heat of vaporization

### THE EFFECT OF PRESSURE ON BOILING POINT

When the pressure on a liquid is reduced the boiling point is lowered.

People who live at high places have difficulty cooking foods by boiling because of the low atmospheric pressure. Pressure cookers are used to overcome this difficulty.

Pressure cookers cook food at a high temperature by raising the boiling point with increased pressure.

### THE EFFECT OF IMPURITIES ON BOILING POINT

When anything is dissolved in a liquid the boiling point of the solution is higher than that of the original liquid.
For example, the boiling point of a salt solution is higher than that of the water.

### CONDENSATION

Condensation is to change from a gaseous state to a liquid state. It is the reverse process of vaporization. The heat gained in vaporization is lost in condensation.