Solids are substances whose particles are held by forces of attraction strong enough to keep them together under the specific conditions being considered. The state (solid, liquid or gas) depends on the temperature and pressure. Even solids such as iron can be changed to liquids and gases if the temperature is high enough.

All particles have mass, so the total mass of a solid is simply the sum of the masses of its particles.

When dealing with reacting masses of substances, the number of particles is important. The mass of a solid is easily found by weighing on a balance and this may be related to the number of particles present using the mole concept.