Physical Changes vs. Chemical Changes Quiz

When Substances Change Without Changing Chemistry: Physical Changes

Imagine taking a bar of pure chocolate and snapping it in half. The chocolate now comes in two pieces, but each piece is still made of the same type of molecules: cocoa, milk, and sugar. This process of breaking chocolate into pieces doesn't change the chemical composition of the chocolate, but it does alter its physical form.

A physical change is a transformation during which the matter changes its state, shape, or appearance, but the composition of atoms and molecules within the substance remains unaltered.

Some common examples of physical changes include:

• Melting ice to water: The water and ice are both made of H₂O molecules, but the ice turns into a liquid state.
• Boiling water: The water turns into steam, but the water and steam are both composed of H₂O molecules.
• Rusting iron: The iron reacts with oxygen in the atmosphere, forming a new oxide layer, but the iron and the oxide are made of the same elements.

Physical changes are typically reversible. If you freeze the water from the earlier example, it will turn back into ice. However, some changes, like the melting of a crayon or the burning of a candle, can result in a new substance that cannot be reversed. In these cases, a chemical reaction has occurred, and a new chemical composition has been formed.

In contrast, a chemical change, or chemical reaction, involves the breaking and forming of chemical bonds, resulting in a different type of matter. During a chemical change, the substance's identity changes, such as when iron rusts, and the rust is an entirely different substance than the iron.

In summary, physical changes and chemical changes are distinguished by whether or not the chemical composition of the substance changes. Physical changes result in the substance taking on a new state, shape, or appearance without altering the types of atoms present, while chemical changes result in the formation of a new substance with a different chemical composition.