A chemical change is a fundamental process in which a substance is transformed into an entirely new substance with a different chemical composition and properties. The most direct and universally accepted scientific synonym for a chemical change is a chemical reaction. This transformation is distinct because the atoms of the original materials are rearranged to form new molecules, which is not merely a change in appearance or state of matter.
The Term Chemical Reaction
The term “chemical reaction” precisely describes the molecular mechanics underlying a chemical change. This process involves the breaking of existing chemical bonds within the starting materials and the formation of new bonds to create the final products. The substances present at the beginning are called reactants, and the newly formed substances are known as the products. Products possess properties that are chemically distinct from the reactants.
For example, when iron rusts, the metal (reactant) reacts with oxygen to form iron oxide (the product). The original metallic iron is replaced by a brittle, reddish-brown compound, demonstrating a permanent shift in chemical identity. Similarly, the burning of wood is a chemical reaction where components react with oxygen to produce ash, carbon dioxide, and water vapor.
Observable Signs of Transformation
Since atoms and molecules are too small to observe directly, scientists look for observable signs that a chemical reaction has occurred. One common indication is an unexpected change in color, such as when a banana peel darkens, signaling the formation of new chemical compounds. The formation of a solid within a liquid mixture, known as a precipitate, is another sign that two dissolved substances have reacted to form a new, insoluble product.
The evolution of a gas, visible as bubbles or fizzing, also points to a chemical transformation, provided it is not caused by boiling. A change in energy, felt as a shift in temperature, is a reliable indicator. Reactions that release heat and sometimes light are termed exothermic, while those that absorb heat from their surroundings are called endothermic.
Comparison to Physical Changes
Comparing chemical changes to physical changes helps solidify the definition. A physical change alters the form, state, or appearance of a substance without changing its chemical identity. For instance, melting an ice cube into liquid water is a physical change because the substance remains H2O molecules throughout the process; only the arrangement of the molecules changes. Cutting a piece of paper or dissolving sugar in water are also examples of physical changes.
In contrast, physical changes generally do not involve the breaking or forming of chemical bonds. The defining difference is that a physical change, like boiling water or crushing a can, can often be reversed with relative ease, and no new substance is created. A chemical change, such as baking a cake or the souring of milk, results in a product with a unique chemical formula, marking the formation of a truly new material.