Liquids

As we heat the solid its molecules gain energy until they are able to break out of the lattice and the solid melts. The molecules have not yet enough energy to move very far apart, when they are free to move. When they do so, they often collide with the sides of the container they are in, the liquid taking up the shape of the container. If not in a container the liquid spreads out on the surface - that is the substance has lost its definite shape. In a container, a liquid still has one surface open to the atmosphere as vapour. The molecules in a liquid as a whole have more energy than those in the crystal lattice, the force of attraction between the molecules being already less strong. It is probable that more of them will escape and thus liquids more than solids give off smell, e.g. perfumes, methylated spirit, eucalyptus oil, vinegar, etc. A liquid being heated is provided still more energy and the molecules begin to move about with greater vigour. Thus, many molecules having more energy reach the surface of the liquid, they overcome the force of attraction and leave the liquid as vapour.

When eventually all the molecules begin to behave in this manner the liquid boils and the temperature at which this process takes place is the boiling point of the liquid. The opposite process occurs if hot vapour is cooled. After having lost their energy the molecules begin to move shorter distances and more slowly, the attraction between them growing stronger until a point is reached where they can no longer remain free. They move forming small groups and drops of liquid are formed. Condensation has taken place.