Final answer:
A chemical reaction is evidenced by a chemical change, which results in the formation of new substances with different properties, such as rusting of iron or the explosion of nitroglycerin.
Step-by-step explanation:
We can identify a chemical reaction by observing a chemical change, where the matter's identity is transformed into one or more different types of substances. For instance, rusting iron or the explosion of nitroglycerin are both classic examples of chemical changes because they produce substances with different properties and composition than the original materials. When chemical bonds break and atoms reorganize into new substances with new bonds, a chemical reaction occurs.
Some visual indicators that suggest a chemical reaction include:
- A change of color during the reaction.
- Production of a gas.
- Formation of a solid called a precipitate.
- An energy transfer, such as a temperature change or light emission.
In contrast to physical changes, which only affect the form or appearance of matter without altering its composition, a chemical change signifies that a substantive, molecular transformation has occurred.