A chemical reaction transforms reactants into products by breaking and reforming chemical bonds. The resulting products have properties entirely unlike the starting materials. Confirming that this molecular rearrangement has occurred requires observable signs, or evidence, that the original substances no longer exist.
Setting the Stage: Chemical vs. Physical Changes
Identifying the evidence of a chemical reaction distinguishes a chemical change from a physical change. A physical change alters a substance’s form or state (like water freezing) but not its chemical identity. Conversely, a chemical change forms one or more chemically new substances, fundamentally altering the underlying chemical composition. This change is often difficult or impossible to reverse.
Indicators Involving Energy Transfer
A noticeable change in temperature is a sign of a chemical reaction, signifying an energy transfer between the chemicals and their surroundings. Reactions that release energy are known as exothermic reactions. When an exothermic process occurs, the mixture feels hot, such as during combustion, or calcium chloride dissolved in water.
In contrast, reactions that absorb energy from the surroundings are termed endothermic reactions, causing the immediate environment to feel cooler. For example, dissolving ammonium nitrate in water or the biological process of photosynthesis absorb heat energy, leading to a drop in temperature.
Energy transfer can also be observed through the production of light or sound. Light is often produced by chemiluminescence, where energy released from the reaction excites a product molecule, causing it to emit a photon of light. This process is responsible for the glow emitted by glow sticks, which produce light without significant heat. Sound is a less common indicator, observed as a form of energy release in highly exothermic, rapid reactions, such as the loud expansion of gas during an explosive transformation.
Indicators Involving Matter Transformation
The formation of a new gaseous substance is a clear sign of matter transformation. This is visually confirmed by the appearance of bubbles or fizzing within a liquid, or the evolution of smoke or an odor. The gas produced is a new chemical compound. A familiar example is the bubbling that occurs when baking soda is mixed with vinegar, releasing carbon dioxide gas.
Another indicator is the formation of a precipitate, which is a solid product that appears when two solutions are mixed. The precipitate forms because the new product created by the reaction is insoluble in the liquid solvent, causing it to separate and settle or remain suspended. For instance, mixing silver nitrate with sodium chloride causes the ions to rearrange and form silver chloride, a white solid precipitate.
A distinct and permanent change in color is strong evidence of a chemical change, as it signals an alteration in the electron structure of the molecules. This color shift must be a permanent alteration, not merely the result of mixing two different colored liquids. The browning of a sliced apple or the change in color of a chemical indicator when exposed to an acid or base are examples that confirm the formation of new chemical entities.