Why is evaporation of water a physical change? In terms of substances, why is evaporation of water a physical change and not a chemical change?
The Correct Answer and Explanation is:
Evaporation of water is considered a physical change because it does not alter the chemical composition of the substance. In a physical change, the substance undergoes a change in its physical state (such as from liquid to gas) without any change in its molecular structure. Evaporation occurs when water molecules at the surface gain enough energy (usually from heat) to overcome the forces holding them together in the liquid phase and escape into the air as water vapor.
In terms of substances, during evaporation, the water molecules (H2O) remain the same before and after the process. No new substances are formed, and the chemical bonds between hydrogen and oxygen atoms within each water molecule are not broken. The process only involves a change in the physical state of water—from liquid to gas.
To further distinguish this from a chemical change, consider what happens in a chemical reaction: bonds between atoms are broken and reformed to create a new substance with different chemical properties. For example, if water undergoes electrolysis, the water molecules split into hydrogen and oxygen gases, which are two entirely different substances. In evaporation, however, there is no change in the composition of the water; it remains H2O throughout the process.
Since evaporation only involves a change in the physical state and no new substance is formed, it fits the definition of a physical change. Additionally, physical changes are typically reversible; if you cool the water vapor, it will condense back into liquid water, showing that the process is reversible without changing the chemical makeup of the water.
