Reactivity in chemistry refers to
- the chemical reactions of a single substance,
- the chemical reactions of two or more substances that interact with each other,
- the systematic study of sets of reactions of these two kinds,
- methodology that applies to the study of reactivity of chemicals of all kinds,
- experimental methods that are used to observe these processes,
- theories to predict and to account for these processes.
The chemical reactivity of a single substance (reactant) covers its behaviour in which
- it decomposes,
- it forms new substances by addition of atoms from another reactant or reactants,
- reactions in which it interacts with two or more other reactants to form two or more products.
The chemical reactivity of a substance can refer to
- the variety of circumstances (conditions that include temperature, pressure, presence of catalysts) in which it reacts, in combination with
- the variety of substances with which it reacts,
- the equilibrium point of the reaction (i.e. the extent to which all of it reacts),
- the rate of the reaction.
A responsible discussion of chemical reactivity can be found in any standard textbook on physical chemistry.