The difference between a chemical change and a physical change lies in how the change affects the composition a substance. A chemical change occurs on the molecular level and produces a new substance. However, a physical change rearranges molecules and does not produce a new substance.
Chemical changes occur during chemical reactions, and these changes are not reversible. Some of the chemical reactions produce heat, whereas others require heat for them to occur. When a chemical reaction takes place, the molecules are rearranged and the reaction is followed by a change in energy as new products are produced.
Indicators that show that a chemical change has occurred include change of color, odor, temperature and composition. Other signs include formation of gases or precipitation, when heat is given off during the reaction and when change is impossible to reverse.
Physical changes do not alter the chemical composition of a chemical substance. A physical change, unlike a chemical change, is reversible. For example, when salt dissolves in water, it can be recovered by letting the water evaporate. A physical change results in a change in physical properties. Some of these changes in physical properties include changes in durability, crystal form, texture, size and shape.