Molecules that exist in the solid state do not move around through the solid. If two solids come into contact, they can only react at the surface of contact and any reaction between solid compounds is generally very slow. Solids can react together more rapidly if the surface areas of the solids are increased by grinding up the solids into powders. This is still not very efficient for most reactions.

Molecules must be free to move around in order to interact with each other. The two states of matter that best meet these criteria are the gas state and the liquid state.

In the body, the chemical compounds that react are dissolved in an aqueous environment. A solution enables molecules that would normally exist in the solid state to easily come into contact with other molecules.