Matter constantly undergoes change. Some changes are simple alterations to a substance’s physical presence, such as when water freezes into ice or evaporates into vapor. However, a deeper transformation occurs when the identity of the matter itself is fundamentally altered. This reorganization of material, where original substances cease to exist and entirely new ones are created, is known as a chemical change.
Defining Chemical Change
A chemical change is a process where one or more starting substances, called reactants, are converted into one or more new and distinct substances, known as products. This transformation is fundamentally a chemical reaction that results in products possessing chemical properties entirely different from the original materials. For instance, when iron rusts, the metallic iron reacts with oxygen in the air to form iron oxide, which is a brittle, reddish-brown compound with none of the strength or luster of the original metal.
Consider the simple combination of baking soda and vinegar, which produces a vigorous fizzing action. The original substances, sodium bicarbonate and acetic acid, are converted into water, a salt, and carbon dioxide gas. This process involves the rearrangement of atoms, resulting in the formation of molecules with new chemical formulas and characteristics.
Observable Evidence of Chemical Change
While the core change is invisible at the atomic level, several macroscopic indicators can signal that a chemical transformation has taken place. The production of a gas, often seen as bubbling or fizzing, is a common sign, such as the carbon dioxide released when an antacid tablet dissolves in water. Another strong indicator is an unexpected color change, where the final product has a hue unrelated to the starting materials, like the browning of a cut apple exposed to air.
Energy changes frequently accompany chemical reactions, manifesting as either the release or absorption of heat. Reactions that release heat are known as exothermic reactions, causing the surrounding area to warm up, such as the combustion of a candle. Conversely, endothermic reactions absorb heat from the surroundings, causing a noticeable drop in temperature.
The formation of a precipitate, a new solid that appears when two clear liquids are mixed, also points to a chemical reaction. Other indicators include the production of light, as seen in a glow stick, or the emission of a new, distinct odor.
The Difference Between Chemical and Physical Change
Distinguishing a chemical change from a physical change is crucial, as only the former results in the creation of a new substance. A physical change alters the form or state of matter but leaves its chemical composition unchanged. For example, when an ice cube melts, it changes from solid water to liquid water, but the molecules remain H₂O in both states. Tearing a piece of paper or shattering a pane of glass are other examples where the material is physically altered but remains chemically identical.
In contrast, a chemical change involves a fundamental alteration in the molecular structure of the substances involved. The combustion of wood is a clear chemical change because the wood (cellulose) is converted into ash, smoke, and gases like carbon dioxide and water vapor. Unlike many physical changes, which are often easily reversible, chemical changes are typically difficult or impossible to undo without another subsequent reaction.
The Molecular Mechanism
At the atomic scale, a chemical change is a sophisticated process involving the breaking and forming of chemical bonds between atoms. The process begins with the reactants, whose existing bonds must be broken apart to free up the atoms for rearrangement. This initial step requires an input of energy, often referred to as activation energy, which is needed to overcome the stability of the original bonds.
Once the initial bonds are broken, the atoms rearrange themselves into new groupings and form new chemical bonds to create the products. The formation of new bonds releases energy, and the overall energy balance determines whether the process is exothermic or endothermic. If the energy released by forming the new product bonds is greater than the energy required to break the reactant bonds, the reaction releases heat. This shuffling of atoms ensures that the types and numbers of atoms remain constant, in accordance with the law of conservation of mass.