Chemical Vs. Physical Reactions

Reactions between two or more molecules result in physical or chemical changes. Physical changes alter matter's appearance and chemical changes alter matter's composition.

A reaction happens when two or more molecules, or groups of atoms, interact. The result depends on the type of molecule and how they interact. The interaction causes either a physical or a chemical change.

If a physical change occurs, the matter involved remains the same at its molecular level. The molecules rearrange, but the internal structure stays intact. It may look different but it has the same material.

The change from water to ice shows an example of physical change. Lowering the temperature of the water causes it to freeze and to take on a different shape, but it still contains hydrogen and oxygen.

A chemical change occurs when a reaction causes a change at a molecular level. During this reaction, bonds between atoms break or new ones form. After the reaction, the matter's chemical composition changes, and a new substance forms.

Rusting shows an example of a chemical change. Rusting occurs when iron (Fe), interacts with oxygen (O). The resulting rust, or iron oxide, has a different chemical composition from its original components.