A chemical reaction is a fundamental process where reactants transform into products. This transformation involves breaking existing chemical bonds and forming new ones, leading to a complete rearrangement of atoms. The resulting products possess distinct chemical properties from the starting materials. The evidence of this molecular change is often visible through specific, observable changes in the environment.
The Formation of New Substances
One indicator that a chemical reaction has occurred is the creation of new matter. The physical appearance of bubbling or fizzing, which is not caused by boiling, signals the production of a gas. This occurs when atoms from the reactants recombine to form a gaseous compound that escapes the solution. A common example is mixing baking soda with vinegar, which rapidly produces bubbles of carbon dioxide gas.
A second sign of new matter formation is the formation of a precipitate, a solid that appears and settles out of a liquid solution. This occurs when two clear liquid solutions are mixed, and the dissolved ions react to form a new compound that is insoluble in the solvent. This insoluble product separates from the liquid, often appearing as a cloudy suspension or a powder.
Observable Energy and Sensory Changes
The exchange of energy with the surroundings provides a third observable sign, typically seen as a change in temperature. Reactions that release heat energy into the environment, making the container feel hot, are called exothermic reactions. Conversely, endothermic reactions absorb heat energy from the surroundings, causing a noticeable drop in temperature. The energy required to break bonds and the energy released when new bonds form dictates whether the reaction is endothermic or exothermic.
A fourth indicator of a chemical change is a change in color that is permanent and not simply due to mixing. When atoms rearrange, the new product molecules absorb and reflect light differently than the starting materials. For instance, the rusting of iron involves a chemical reaction with oxygen and water, changing the metal’s surface from a metallic gray to a reddish-brown iron oxide.
The fifth sign encompasses two sensory observations: the production of light or a change in odor. Some reactions release energy in the form of light, a phenomenon called chemiluminescence, such as the glow from a glow stick. A new or distinct odor suggests that a volatile compound has been created as a product.
Differentiating Chemical from Physical Changes
Observing these signs is important because a chemical change must be distinguished from a physical change, where the substance changes form but not its chemical identity. A physical change, like melting ice, alters the appearance or state of matter but leaves the molecular composition intact.
A chemical reaction, by contrast, is characterized by a change in the substance’s molecular composition, resulting in products with entirely new properties. The presence of one or more of the five signs confirms that the transformation is chemical, meaning the change is often permanent and not easily reversible back to the original reactants. This ability to form a new substance is the fundamental distinction that these five signs help scientists recognize.