Explanation Of Some Phenomena Using Kinetic Theory JSS2 Basic Science Lesson Note

 EXPLANATION OF SOME CHANGES IN STATES OF MATTER USING KINETIC THEORY

Melting: When a solid is heated, the particles acquire more kinetic energy and vibrate more vigorously. Eventually, at a certain temperature called the melting point of the solid, the force of the vibrations overcomes the binding forces and the solid structure collapses. The particles are no longer in fixed positions, but free to move. At this point, the solid is said to have melted or liquified.

Condensation and freezing: Condensation is a process whereby a vapour loses some of its kinetic energy when cooled and changes into a liquid state. When a liquid cools, it loses heat energy causing its temperature to drop. If cooling is allowed to continue, the temperature of the liquid keeps on dropping, until it reaches the freezing point of the liquid. The liquid changes into the solid state at this temperature. The freezing point of a substance is the same as its melting point.

Diffusion: This is defined as the movement of solute particles through a medium, from a region of higher concentration to a region of lower concentration, that is, movement from a more crowded region to a less crowded region. Once the solute particles become evenly distributed throughout the medium, there will be net diffusion in any particular direction. Diffusion is fastest in gases because gas particles have more kinetic energy than particles in liquids and solids. 

EVAPORATION AND BOILING

Evaporation

Particles in a liquid are attracted by other neighboring particles in all directions. 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 an increase in temperature. Evaporation increases 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 the boiling point.

Factors Affecting Evaporation

i. 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 occurs at all temperatures but the rate of evaporation increases with an 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 the temperature of the liquid body. Therefore evaporation results in a drop in temperature.

ii. 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.