Solids

Whereas particles in gases have the highest degree of disorder, the solid state has the most ordered system. Particles are fixed in a rather definite position and maintain a definite shape. Particles in solids do vibrate in position, however, and many even diffuse through the solid. (Example: Gold clamped to lead shows diffusion of some gold atoms into the lead over long periods of time.) Other solids do not show diffusion because of strong ionic or covalent bonds in network solids. (Examples: NaCl and diamond, respectively.)

When heated at certain pressures, some solids vaporize directly without passing through the liquid phase. This is called sublimation. Solids like solid carbon dioxide and solid iodine exhibit this property because of unusually high vapor pressure.

The temperature at which atomic or molecular vibrations of a solid become so great that the particles break free from fixed positions and begin to slide freely over each other in a liquid state is called the melting point. The amount of energy required at the melting point temperature to cause the change of phase to occur is called the heat of fusion. The amount of this energy depends on the nature of the solid and the type of bonds present.