A chemical reaction is a process where one or more substances are transformed into different substances. This fundamental change involves the rearrangement of atoms within molecules, leading to the creation of new materials with distinct properties. Unlike physical changes, such as melting ice, a chemical reaction forms new substances. These transformations are happening constantly, both in nature and in engineered systems.
The Molecular Transformation
Chemical reactions involve changes at the molecular level. During a reaction, chemical bonds in the initial substances, known as reactants, must first break. This breaking of bonds requires an input of energy. Atoms or groups of atoms then rearrange, forming new chemical bonds to produce different substances called products.
Atoms are not created or destroyed during this process; they are simply reorganized. For a reaction to occur, reactant molecules must collide with sufficient energy and in the correct orientation to facilitate the breaking of old bonds and the formation of new ones. The products often possess properties significantly different from the original reactants.
Recognizing a Chemical Change
Certain signs indicate a chemical reaction has occurred. One common indicator is a change in color, such as the browning of a cut apple when exposed to air. The production of a gas, often seen as bubbles, signifies the formation of a new gaseous substance, not just a change in state like boiling. The appearance of a solid within a liquid solution, known as a precipitate, also points to a chemical change.
Changes in temperature often involve chemical reactions; the surroundings may become warmer or cooler. The emission of light or sound can also be evidence that a reaction is occurring. A new or altered odor suggests that new volatile compounds have been produced. While these are observable clues, they are consequences of the underlying molecular rearrangements.
Energy Shifts During Reactions
Chemical reactions involve energy changes, as energy is either absorbed from or released into the surroundings. Energy must be supplied to break bonds in reactants. Conversely, energy is released when new bonds form. The overall energy shift depends on the balance between energy required to break bonds and energy released when new bonds form.
Reactions that release more energy than they absorb are called exothermic reactions, making the surroundings warmer. Burning fuel is a common example, releasing heat and light. In contrast, endothermic reactions absorb more energy than they release, causing the surroundings to become cooler. An example is the dissolution of certain salts in water, which can lead to a drop in temperature.
Chemical Reactions All Around Us
Chemical reactions are ubiquitous, playing a role in many everyday phenomena. The rusting of iron is a familiar example of a slow chemical change. The burning of wood is a combustion reaction that releases heat and light. Cooking food, such as baking a cake or frying an egg, relies on chemical changes to alter the food’s texture, color, and flavor.
Biological processes also involve many chemical reactions. Photosynthesis in plants converts carbon dioxide and water into glucose and oxygen using light energy. Within our bodies, digestion breaks down food into simpler molecules through a series of chemical reactions. Even the operation of batteries involves chemical reactions that generate electrical energy. These examples highlight how chemical reactions are fundamental to life and technology.