A chemical reaction is the process where atoms and molecules of one or more substances rearrange to form entirely new substances with different properties. While this fundamental change occurs at an invisible, molecular level, the event leaves behind observable evidence that confirms a transformation has taken place. Scientists rely on six primary indicators to confirm that a chemical reaction has occurred, signaling the successful creation of new matter.
Changes in Physical State
One of the clearest signs of a chemical reaction involves a change in the physical state of the materials, which is distinct from a simple phase transition like melting or boiling. This transformation results in a new product that is physically different from the starting materials. These changes signal that the atoms have recombined to form a substance with different solubility or gaseous properties.
Formation of a Precipitate
A precipitate is a solid substance that appears out of a liquid solution when two clear liquids are mixed. This occurs because the ions from the two starting solutions rearrange to form a new compound that is insoluble in the solvent, causing it to solidify and become visible. For example, mixing silver nitrate and sodium chloride solutions results in the instant formation of a white, insoluble solid called silver chloride.
Production of Gas
The release of gas, often seen as bubbling or “effervescence,” is a strong indicator that a chemical reaction has taken place. This happens when the reaction’s new product is a gas that escapes the liquid or solid reactants. A classic example is combining baking soda and vinegar, which produces visible carbon dioxide bubbles. It is important to distinguish this bubbling from boiling, as boiling is merely a physical phase change.
Indicators of Energy Transfer
Chemical reactions involve the breaking and forming of chemical bonds, processes intrinsically linked to energy. The breaking of existing bonds requires an input of energy, while the formation of new bonds releases energy. This net transfer of energy into or out of the reaction system provides two distinct signs of a chemical change.
Change in Temperature
A noticeable change in temperature is a common sign of a chemical reaction, indicating that the system’s energy has shifted. Reactions that release energy into the surroundings, causing the temperature to rise and the container to feel hot, are called exothermic reactions. Conversely, endothermic reactions absorb energy from the surroundings, causing the temperature to drop and the container to feel cold. In an endothermic process, the energy required to break the reactants’ bonds is greater than the energy released when the products’ bonds form.
Production of Light or Sound
In some highly energetic reactions, the released energy is not solely heat but also takes the form of visible light or audible sound. The sudden, bright flash seen when magnesium metal burns is an example of light production. Reactions that produce light without significant heat, such as the chemical process inside a glow stick, are known as chemiluminescence. The rapid release of gaseous products in an explosion generates a shockwave perceived as a loud sound.
Visible Transformations
The final two signs of a chemical reaction involve transformations easily observed by the senses. These changes confirm that the initial reactants have been fundamentally altered and a new substance with different sensory properties has been created.
Change in Color
A permanent, non-mixing-related change in color indicates that the reactants have rearranged to form a new substance with different light absorption and reflection properties. For instance, the transition of iron metal from gray to reddish-brown rust signals the formation of iron oxide through a chemical reaction with oxygen. This is different from a physical change, like simply mixing blue and yellow paint.
Change in Odor
The formation of new molecules can result in a distinct change in the smell of the substances involved. A change in odor signals that the chemical composition has been altered, resulting in the production of volatile compounds. The smell of rotten eggs, caused by the formation of hydrogen sulfide, or the aroma of baking bread, produced by fermentation, both signal a chemical change has occurred.