Physical Changes II

Melting

Heating a solid increases the kinetic energy within the particles causing them to vibrate more vigorously. Melting occurs when the particles in a solid are able to overcome the strong forces of attraction between them, they begin to move further apart forming a liquid. The temperature remains constant while a solid is melting; this constant temperature is called the melting point.

Boiling

When a liquid is continually heated its temperature eventually reaches a certain point where it begins to boil. At this point the liquid particles have gained enough kinetic energy and have started to move fast enough to change to a gas both within the liquid and at the surface of the liquid.

◦The temperature remains constant while the liquid is boiling; this constant temperature is called the boiling point.

Boiling differs from evaporation in two ways:

-Boiling occurs at a specific temperature, whereas evaporation can take place at any time.

Boling takes place throughout the liquid whereas, evaporation only occurs at the surface of the liquid.

Evaporation

When a liquid is heated, the particles gain kinetic energy and move faster. When these particles have gained enough kinetic energy to overcome the forces of attraction between them the escape from the surface of the liquid in the form of a gas (vapour). These particles are said to evaporate. The particles that evaporates takes a lot of energy with them and this leads to cooling of the liquid.

Condensation

When the temperature of a gas is lowered, its particles loose kinetic energy and begin to move more slowly. The forces of attraction become stronger causing the particles to move closer together forming a liquid.

Freezing

When the temperature of a liquid is lowered, its particles loose kinetic energy and they begin to move more slowly. The forces of attraction between the particles become stronger causing the particles to move closer together forming a solid. The temperature at which this occurs is called the freezing point.

The freezing point of a pure substance has the same value of the melting point of that substance. E.g. water has a melting point and freezing point of 0 C.

Sublimation

When the forces of attraction in a solid are weak, if a little heat is applied it turns from a solid to a gas without going through the liquid state. When a substance turns directly from a solid to a gas or a gas to a solid it is said to ‘sublime’.

Substances which undergo sublimation include: Carbon dioxide ‘dry ice’, napthalene (moth balls), iodine and ammonia chloride.