Chemistry is the field of science dedicated to the study of matter and the transformations it undergoes. A chemical change, also known as a chemical reaction, involves converting initial substances into one or more entirely new substances with different chemical properties. The defining characteristic of this process is the formation of new compounds that were not present in the original material.
What Chemical Change Means at the Molecular Level
A chemical change is fundamentally a process of atomic rearrangement that takes place at the molecular level. It begins with initial substances, known as reactants, which must collide with sufficient energy to start the transformation. This requires breaking existing chemical bonds within the reactant molecules, a process that demands an input of energy.
The constituent atoms then reorganize themselves into new molecular structures, resulting in the formation of new chemical bonds. These final substances are called products. The formation of new bonds releases energy, and the overall energy balance determines if heat is absorbed or released during the reaction.
The identity of the atoms remains unchanged throughout this transformation; they are simply shuffled and connected in a different order. This aligns with the Law of Conservation of Mass, which dictates that matter is neither created nor destroyed during a chemical reaction. Because the atoms are rearranged, the products possess a chemical composition and set of properties entirely different from the starting materials.
Observable Signs of Chemical Change
Chemical changes often provide macroscopic evidence that a reaction has taken place, even though the reordering of atoms occurs at a microscopic scale. One common indicator is a change in temperature, where heat is either released (exothermic) or absorbed (endothermic). The sudden appearance of bubbles or fizzing, not caused by boiling, often signals the production of a new gaseous substance.
Other signs include the formation of a precipitate—a new solid material suspended in a liquid mixture—or a permanent and unexpected change in color. Less common indicators are the production of light or a notable change in odor.
These visible signs are indicators, not absolute proof, that a chemical change has occurred. For example, mixing paints causes a color change, and boiling water produces bubbles, but neither is a chemical reaction. A true chemical change is confirmed only when the composition of the matter has permanently altered to form a new substance.
Distinguishing Chemical and Physical Changes
The distinction between a chemical change and a physical change hinges on whether the substance’s identity is permanently altered. A physical change affects only the form or appearance, such as chopping wood into smaller pieces. The wood remains chemically the same, even though its shape and size are different.
In contrast, a chemical change results in a substance with a fundamentally new composition, such as when wood is burned and transforms into ash, smoke, and carbon dioxide. Physical changes, like melting ice or dissolving salt, are easy to reverse by simple means.
Chemical changes are difficult or impossible to reverse using simple physical methods because the original reactants no longer exist. For example, rusted iron cannot be easily converted back to pure iron metal. Chemical reactions also involve significantly larger energy shifts, either absorbing or releasing much more heat or light than physical changes.
Everyday Illustrations of Chemical Change
Chemical changes drive many processes necessary for life and technology. The metabolism of food, for example, is a complex sequence of chemical reactions that break down complex molecules into simpler ones, releasing energy for the body’s use. This process relies on breaking and forming new chemical bonds to sustain biological function.
Oxidation is the rusting of metal, where iron reacts with oxygen and water in the atmosphere to produce iron oxide, a new compound with a distinctly different reddish-brown color and texture. Cooking and baking also involve chemical transformations, such as when heat causes proteins in an egg to denature and coagulate, creating a permanent change in texture. Combustion, like burning natural gas, is a rapid chemical reaction that combines fuel with oxygen to produce heat, light, and new substances like carbon dioxide and water vapor.