A chemical property is a characteristic associated with a substance or reaction that is observable as it chemically changes or transforms into another substance. This chemical change produces a product that has a different composition than the original substance.
Some chemical properties are reactivity with water or other chemicals, flammability and heat of combustion. A piece of iron in the presence of air or water has a tendency to form rust. This rusting process is a chemical change.
An example of reactivity is a metal like zinc reacting with the acid hydrochloric acid. This reaction produces zinc chloride and hydrogen gas. In this reaction, the reactants are zinc and hydrochloric acid, and the products are zinc chloride and hydrogen gas, which have different compositions than the original reactants.