∞ generated and posted on 2016.01.01 ∞
Change of one substance into a different substance in a manner that involves a change in the presence or orientation of electrons relative to more than one atom.
That is, chemical reactions involve, more or less, the making and breaking of chemical bonds. The have the property of the conversion of chemical reactants/reagents/substrates into one or more chemical products. Note that such conversion may or may not require a net input of energy, though typically some energy, in the form of activation energy, is required to get a chemical reaction started.
Note that ionization, the complete loss or gain of one or more electrons by molecules or atoms (or, indeed, ions) usually involves the gain (or loss) or electrons by other entities so very much represents a chemical reaction. On the other hand, it is possible to heat substances, particularly atoms, until ionization occurs, which is more a physical phenomenon rather than chemical.