Collision theory is a special type of kinetic theory that states that for a chemical reaction to take place there must be a collision between reactants. In addition, to cause a reaction the colliding reactant molecules must surpass a minimum energy termed the activation energy and, in many cases, the reactant molecules must have a certain spatial orientation with respect to each other.

The collision theory provides an explanation for the experimental observation that, in a chemical reaction, if two types of molecules in the gas phase are combined, not all of the molecules instantly enter the reaction.

Reactions between molecules can occur in the solid, liquid, or gaseous state. Reactions also occur in solution. The simplest reactions are those which occur between molecules in the gas phase. The reason for this is that gas phase molecules do not have bonds between molecules or bonds to...