What are the signs of a chemical reaction?

The signs of a chemical reaction include observable changes that indicate a transformation of substances. The correct choice highlights four key indicators: a color change, gas production, temperature change, and formation of a precipitate.

A color change often signifies that new chemicals have formed, as reactants undergo a transformation that alters their appearance. Gas production is another clear sign; bubbles or the release of gas indicate that a reaction is taking place. A temperature change is significant because many chemical reactions are either exothermic (releasing heat) or endothermic (absorbing heat), which can be felt or measured during the process. Lastly, the formation of a precipitate — a solid that forms from a liquid solution — is a definitive sign that a new substance has been created as a result of a chemical reaction.

Each of these signs serves as a reliable indicator of chemical activity, helping to differentiate chemical changes from physical changes, such as mixing or changing states of matter without altering the chemical composition. This understanding is fundamental in chemistry for identifying and predicting outcomes in various reactions.