A chemical reaction occurs when substances called reactants are transformed into new substances known as products through the rearrangement of atoms. This process fundamentally changes the chemical identity of the materials involved, meaning the original substances are consumed to create entirely different ones. Recognizing this change is different from observing a physical change, such as water boiling, where the material’s identity remains the same. Identifying a true chemical transformation relies on observing specific, measurable changes in the system.
Observable Energy Shifts
One of the clearest indicators of a chemical reaction is a sudden and noticeable shift in energy, often manifesting as a change in temperature. Reactions that release energy into the surroundings, causing the temperature to rise, are classified as exothermic. A common example is the burning of wood, where the stored chemical energy is rapidly converted into heat and light. Conversely, some reactions absorb energy from the surroundings, resulting in a measurable drop in temperature; these are called endothermic reactions. Energy shifts can also present as the production of light without significant heat, a phenomenon known as chemiluminescence, which is utilized in glow sticks.
Formation of New Physical States
The spontaneous appearance of a new physical state in a liquid mixture strongly suggests a chemical change has occurred. One sign is the evolution of a gas, visible as bubbling or fizzing, which indicates a volatile product is escaping the solution. When baking soda and vinegar are mixed, the resulting vigorous bubbling is the release of carbon dioxide gas, a new substance that was not present in the original liquids. This gas production is only an indicator of a chemical reaction when it is not caused by heating a liquid to its boiling point. Another distinct sign is the formation of a precipitate, a solid that suddenly appears and settles out of a previously clear liquid solution.
Changes in Color and Odor
A distinct and unexpected change in color frequently signals that the original reactants have been consumed and new products have formed. These new molecules absorb and reflect light differently than the starting materials, resulting in a visible shift in hue. For instance, when oxygen reacts with chemicals in a freshly sliced apple, the surface turns reddish-brown, indicating the creation of new compounds. The emergence of a new odor is another sensory clue that a chemical reaction has occurred. This odor is produced when one of the products is a volatile substance that easily transitions into a gas phase, such as the hydrogen sulfide gas that causes the smell of a rotten egg.
Confirming the Creation of New Substances
While these observable signs are suggestive, they do not offer absolute confirmation that a chemical reaction has taken place. Certain physical changes, such as dissolving a brightly colored salt, can sometimes mimic a color change or slight temperature shift. The ultimate evidence of a chemical reaction is the formation of a completely new substance with a unique chemical identity. A strong indication that a true chemical change has happened is the difficulty in reversing the process to reclaim the original reactants. When multiple signs, such as a color change combined with gas production and a temperature shift, are observed simultaneously, the evidence for a chemical transformation becomes significantly stronger.