A chemical reaction occurs when what happens among chemicals?

A chemical reaction typically occurs when different substances interact to form new products. This interaction often happens when chemicals are mixed together. When substances are combined, their molecules can collide and interact, leading to a reorganization of their atomic structure. This may involve breaking old bonds and forming new ones, resulting in different chemical compounds than those initially present.

The act of mixing several chemicals can lead to various outcomes, such as a change in temperature, color, gas production, or precipitate formation, indicating that a reaction has taken place. This is fundamental in chemistry where the nature of the products formed highlights the importance of mixing as a precursor to chemical reactions.

The other choices describe processes that do not inherently result in chemical reactions. Heating substances might speed up reactions or cause physical changes but does not guarantee a reaction will happen. Purification involves removing impurities, typically without altering the chemical composition of the substance itself. Dissolving a substance can lead to physical changes without chemical reactions occurring, as the molecules remain intact in a solution. Thus, mixing is the most direct cause that can initiate a chemical reaction among different chemicals.