Evaporation is the transformation of a liquid into a gas, commonly seen when a puddle disappears or steam rises. This process is fundamental to the water cycle and industrial processes. The question often arises whether this change is chemical, creating a new substance, or physical, only altering its form. Understanding evaporation requires distinguishing between these two categories of change in matter.
Defining Physical and Chemical Changes
Scientists classify changes in matter based on whether the substance’s molecular identity is altered. A physical change involves altering the state, shape, or appearance of a material, but the chemical composition remains the same. Examples include melting, freezing, or breaking an object, where the substance is chemically identical before and after the process.
In contrast, a chemical change, or chemical reaction, involves breaking existing chemical bonds and forming new ones. This process creates an entirely new substance with different properties than the starting material. A change in composition is the defining feature of a chemical change, often signaled by signs like an unexpected color change or the release of gas.
The Scientific Classification of Evaporation
Evaporation is classified as a physical change because the water molecule (H₂O) remains H₂O throughout the entire process. The transition from liquid water to water vapor is a change of state, not a change in chemical composition. Adding energy, usually heat, to liquid water increases the kinetic energy of the molecules.
This energy is sufficient only to overcome the weak intermolecular forces, like hydrogen bonds, that hold the water molecules together in the liquid phase. The stronger covalent bonds within the H₂O molecule remain intact. Molecules that gain enough energy escape the liquid surface and become water vapor, which is chemically identical to the liquid water it originated from.
Everyday Examples of Chemical and Physical Changes
Physical changes are observed when a substance’s identity is preserved. Examples include glass shattering or ice melting. Dissolving sugar in water is also physical, as the sugar molecules disperse without chemically reacting. The sugar can be recovered by evaporating the water, demonstrating the reversibility often associated with physical changes.
Chemical changes create new materials that are difficult to reverse. Rusting iron, where iron reacts with oxygen to form iron oxide, is a key example. Burning wood is another chemical change, forming ash, carbon dioxide, and water vapor. The reaction of baking soda with vinegar also produces carbon dioxide gas, indicating a new chemical product has formed.