The difference between a physical and a chemical change often confuses people. When a material changes its appearance, it is not always clear whether it has simply been reshaped or if its fundamental nature has been altered. Clarifying this distinction requires looking beyond mere appearance to understand how scientists categorize these changes at the molecular level.
Defining Physical Change
A physical change alters the form, size, or appearance of a substance without changing its core chemical composition. This means that the atoms and molecules of the material remain structurally identical before and after the transformation. For example, when an ice cube melts into liquid water, the substance is still composed of \(\text{H}_2\text{O}\) molecules, only their arrangement and energy level have changed.
Changing the shape of an object, such as crushing an aluminum can or tearing a piece of paper, is a clear example of a physical change. These actions merely rearrange the existing particles without breaking or forming any chemical bonds between atoms. Changes of state, including melting, boiling, freezing, and condensation, are also physical changes because they only modify the spacing between molecules. Physical changes are often easily reversible; for instance, liquid water can be refrozen back into solid ice.
Defining Chemical Change
A chemical change, also known as a chemical reaction, results in the formation of one or more entirely new substances with properties different from the starting material. This transformation occurs because the molecular bonds within the original substances are broken and then reformed in new ways. The resulting products possess a new chemical identity.
Scientists identify a chemical change by observing specific evidence. One common indicator is an unexpected color change, such as when a freshly cut apple turns brown due to oxidation. The formation of a gas, often seen as bubbling or fizzing, also signals a chemical reaction, provided the change is not merely a liquid boiling.
The production of heat or light indicates energy is being released (exothermic) or absorbed (endothermic) as molecular bonds are rearranged. Finally, the formation of a precipitate—a new, insoluble solid that settles out of a liquid mixture—is a definitive sign that a chemical change has occurred.
Comparing the Changes Using Common Tests
The most practical way to distinguish between a physical and a chemical change is to apply the “molecular identity test.” If the molecules of the substance are the same before and after the change, it is physical; if the molecules are different, it is chemical. Tearing a sheet of paper is a physical change because the smaller pieces are still cellulose molecules, whereas burning paper is a chemical change because the cellulose is converted into new molecules like ash and carbon dioxide.
A change in shape is a physical change because it only involves rearranging the existing molecules in space. Bending a metal wire changes its form, but the metal atoms remain bonded to one another in the same way, meaning the wire is still the same element. Furthermore, a change in shape is not accompanied by any definitive chemical indicators, such as the production of light or a new precipitate.
Melting a stick of butter is a physical change because the liquid butter molecules are identical to the solid butter molecules. However, baking that butter into a cake causes chemical reactions that produce new substances with entirely different properties.