When a chemical reaction takes place, there are several changes that may occur. These visible changes are called the indicators of a chemical reaction. There are several indicators of reaction:
A change in colour is a sign of a chemical reaction. For example, frying an egg is a chemical reaction with a colour change - colourless to white.
Gas Given Off
During a chemical reaction, a gas can be produced. This can be seen by bubbles forming in the reaction or by a coloured gas being produced (such as smoke).
If two solutions are mixed and a solid is formed, this is called a precipitate. A precipitate is an insoluble solid formed by reacting two liquids. For example, when milk goes very sour, a solid is formed.
An energy change involves a change in temperature (either up or down). For example, the burning of a match involves a release of both heat and light.
The indicators of chemical reactions shown here all involve a chemical change - for example, the burning of a match. These are different to physical changes (such as boiling water) which always end up with the same substance, but sometimes in a different state (liquid to gas); it's easy to confuse those with a chemical reaction. which always ends up with new substances, so watch out!
Remember that, in a chemical reaction, one or more new substances are produced!