When water is heated on a stove, the process of boiling converts liquid water into gaseous water vapor (steam). This transformation is categorized as a physical change, not a chemical one. The distinction lies in what happens to the water molecule itself during the energy transfer.
Identifying Physical and Chemical Changes
Scientists classify changes in matter based on whether the fundamental identity of the substance is altered. A physical change is a transformation that affects the form, appearance, or state of a substance without altering its molecular structure. Examples of physical changes include melting, freezing, and dissolving, where the substance remains chemically the same throughout the process.
Conversely, a chemical change involves a deep transformation at the molecular level, where chemical bonds are broken and new ones are formed. This results in the creation of an entirely new substance with a different chemical composition and set of properties.
The Effect of Heat on Water Molecules
The application of heat during boiling directly increases the energy within the liquid water. This added energy is absorbed by the individual water molecules, causing them to move and vibrate much faster, which is an increase in their kinetic energy. Liquid water molecules are held together by relatively weak attractive forces between molecules called hydrogen bonds.
As the water temperature approaches the boiling point, the molecules gain enough kinetic energy to overcome these intermolecular attractions. They separate from their neighbors and escape into the surrounding atmosphere as a gas. This transition from a liquid to a dispersed gas is a change in the physical state, or phase, of the substance.
Why Water Remains Water
Boiling is a physical change because the individual water molecule (H₂O) remains intact. The heat energy supplied is only sufficient to break the weak hydrogen bonds between molecules, not the much stronger covalent bonds that hold the two hydrogen atoms to the single oxygen atom within the molecule. A molecule of liquid water is chemically identical to a molecule of water vapor.
For a chemical change to occur, the H₂O molecule itself would need to be broken apart. A distinct process, such as electrolysis, demonstrates this difference clearly. Electrolysis uses electrical energy to break the internal covalent bonds, decomposing water into two entirely new substances: hydrogen gas (H₂) and oxygen gas (O₂).
These resulting gases possess properties vastly different from water, confirming a true chemical change has taken place. Boiling water, which produces steam that is still H₂O, does not result in any new substance and therefore remains a physical phase transition.