Physical changes only change the appearance of a substance, not it's the chemical composition. Chemical changes cause a substance to transform into a real substance with a new chemical formula. Chemical changes are also known as chemical reactions. The "ingredients" of a reaction are called reactants, and the final results are called products. A physical change affects a substance's physical properties, and a chemical change affects its chemical properties. Many physical changes are reversible (such as heating and cooling), whereas chemical changes are often irreversible or only reversible with an additional chemical change.