A chemical reaction is the transformation of one set of chemical substances (reactants) into a different set of substances (products). This occurs through the rearrangement of atoms and the breaking and forming of chemical bonds, fundamentally changing the chemical identity and properties of the original materials. In contrast, a physical change, such as melting ice, alters a substance’s form or state but not its molecular composition. Observing a chemical reaction relies on detecting macro-level changes—like shifts in appearance or energy—that serve as evidence that a microscopic transformation has taken place.
Changes in Appearance
The formation of new substances often results in a visible color change due to how the resulting material interacts with light. New molecular structures have different electron energy levels, causing them to absorb and reflect light wavelengths differently than the starting reactants. A common example is the rusting of iron, where the metal reacts with oxygen and water to form iron oxide, transforming the shiny gray metal surface into a reddish-brown color. Another instance is the Statue of Liberty’s green patina, which formed as the copper exterior reacted with air and moisture to create hydrated copper carbonate.
A distinct visual change is the formation of a precipitate, an insoluble solid that separates from a liquid solution. This solid appears when two liquids are mixed and the newly formed compound cannot dissolve in the solvent. The precipitate may make the solution appear cloudy or milky before settling to the bottom. For example, mixing lead(II) nitrate with potassium iodide yields a brilliant yellow precipitate. In a biological context, kidney stones involve a similar precipitation reaction where minerals solidify out of the urine.
Energy Release or Absorption
All chemical reactions involve an exchange of energy, often detected as a change in temperature. Reactions that release energy into the surroundings, typically as heat, are classified as exothermic reactions and cause the surrounding temperature to rise. The most familiar exothermic example is combustion, such as burning wood or fuel, where stored chemical energy is rapidly converted into heat and light.
Alternatively, some reactions absorb energy from the surroundings, resulting in a temperature drop that makes the container feel cold to the touch; these are known as endothermic reactions. A common example is the chemical cold pack, which contains a salt like ammonium nitrate that dissolves in water and draws heat from the immediate environment. Energy can also be released in forms other than heat, such as the emission of light or sound.
The light emission from a chemical reaction, known as chemiluminescence, occurs when the reaction produces a molecule in a high-energy, excited state. This unstable molecule returns to a more stable state by releasing its excess energy as a photon of light. Glow sticks operate on this principle, where two chemicals mix to produce light without generating significant heat. Sound can also be a product of a chemical change, often accompanying a rapid, highly energetic reaction like an explosion or the loud hissing sound produced when calcium oxide reacts with water.
Formation of Gases or Odors
The sudden appearance of bubbles or fizzing indicates that a new gaseous substance has been produced by the reaction. This must be distinguished from the physical change of boiling, where a liquid simply turns into its gaseous state due to heat. A common example is the acid-base reaction between vinegar and baking soda. When combined, the mixture fizzes vigorously as it generates carbon dioxide gas.
The formation of new, volatile chemical compounds is often detected as a distinct change in odor. This indicator is prominent in food spoilage, where microorganisms break down complex molecules into simpler, smellier components. For instance, bacteria decompose proteins in meat, releasing foul-smelling amines. Similarly, the rancid smell of old oils and fats is caused by lipid oxidation, a reaction that produces volatile aldehydes and ketones.