CHANGES IN STATES OF MATTER USING KINETIC THEORY

EVAPORATION AND BOILING

Evaporation

Particles in a liquid are attracted by other neighbouring particles in all direction. When particles with sufficient come near the surface of the liquid, they can break away from the attractive forces of the other nearby molecules and escape into the space above and become vapour or gas. This phenomenon is called evaporation. Evaporation occurs at any temperature, though the rate of evaporation increases with increase in temperature. Evaporation increase with wind, surface area and lower relative humidity.

Boiling

When a liquid is heated, the rate of evaporation increases. The vapour pressure of the liquid also increases until a temperature is reached, at which the vapour pressure equals the atmospheric pressure. When this happens, bubbles of vapour form freely in the liquid and rise to the surface. This phenomenon is called boiling. The temperature at which boiling takes place is known as boiling point.

Factors Affecting Evaporation

The temperature of the liquid

In a liquid, the particles are in motion. When water is heated, the motion of the particles will become more rapid than before. Each particle that collides with another one will change direction. As the heating continues, the particles will gain more energy. Some particles will gain sufficient energy to break through the surface tension of the liquid and escape as gas. Evaporation of liquids occur at all temperatures but the rate of evaporation increases with increase in temperature. Since evaporation results in the escape of energetic particles from the liquid body, the average kinetic energy of the liquid is lowered. This results in a drop in temperature of the liquid body. Therefore evaporation results in drop in temperature.

The nature of the liquid substance: Apart from the temperature, the rate of evaporation is also influenced by the type of liquid. For instance, the rate of evaporation of water is lower than that of petrol.

EVALUATION

1.The boiling point of pure water is ———-

a.0^oC b.100^oC c.250^oC d.1000^oC

2.The movement of solute particles through a medium, from a higher concentration to a lower concentration is called ————–

3.When a liquid is cooled, it ————

Condense b. Evaporates c. Freezes d. Melts e. Sublimes

SECTION B

Explain the following
Melting
Evaporation

iii Diffusion

2.State the factors affecting Evaporation.

EVAPORATION AND BOILING

Factors Affecting Evaporation

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