A chemical reaction is a process where atoms and molecules are rearranged, breaking old chemical bonds and forming new ones. The starting materials, known as reactants, are transformed into different materials called products. Because this transformation involves changes at the molecular level, it is not always visually obvious that a reaction has occurred. Scientists rely on observable signs, or evidence, to confirm that a chemical change, rather than a physical change like melting or boiling, has taken place.
Changes in Energy State
A primary indicator of a chemical reaction is a change in the energy state, often experienced as a difference in temperature. The rearrangement of chemical bonds during a reaction results in either the release or absorption of energy. Reactions that release energy into the surroundings, typically as heat, are known as exothermic reactions, causing the environment to feel warmer.
Combustion, such as burning wood, is a common exothermic process where stored chemical energy is converted into heat and light. Conversely, reactions that absorb energy from their surroundings, causing a drop in temperature, are called endothermic reactions. Instant cold packs illustrate this, as chemicals react to pull heat from the environment, making the pack feel cold. Energy release can also manifest as light, a phenomenon called chemiluminescence, utilized in glow sticks. Explosions, which involve rapid chemical transformations, release significant energy almost instantaneously, often producing sound waves.
Alterations in Physical Appearance
A change in color is one of the most readily observed pieces of evidence that a chemical reaction has taken place. This visual shift occurs because the newly formed product molecules absorb and reflect light differently than the original reactants. The color change must be unexpected and lasting, distinguishing it from simple mixing, like combining paints.
A familiar example is the browning of a cut apple, an oxidation reaction that forms new, darker compounds. Another instance is the formation of rust, where iron reacts with oxygen and water to create a reddish-brown iron oxide compound. Cooking meat also causes a color change, as the iron in the protein myoglobin oxidizes, turning the familiar red color to brown.
Formation of New Phases
The appearance of a substance in a different state of matter that was not present initially offers strong proof of a chemical change. This involves the creation of a gas or a solid, representing a new phase formed from the reactants. The most common sign of gas production is effervescence, which is the bubbling that occurs when a gas is released from a liquid solution.
The classic reaction between baking soda and vinegar is an example, as it rapidly produces carbon dioxide gas, visible as fizzing bubbles. This gas is a new molecular product of the reaction, not simply steam. The production of a solid from the mixing of two liquids is another definitive sign, referred to as precipitate formation. A precipitate is an insoluble solid that “falls out” of the liquid solution, indicating that a new compound has been synthesized. For instance, when blue copper sulfate solution is mixed with sodium hydroxide solution, a solid blue precipitate of copper hydroxide quickly forms and settles at the bottom of the container.