In its simplest form, a chemical reaction is the process that results in the change of one set of substances into another. Chemical reactions are either spontaneous, which means they require no energy input, or they are nonspontaneous, which means they need a catalyst. A chemical reaction can be identified by essential chemical change however; when that change isn't easily distinguished, there are other characteristics you can observe.
The change of state is one of the clearest characteristics of a chemical reaction; for example, water boiling is a nonspontaneous reaction, with the catalyst of heat, which turns the water into vapor or steam. The water changes from a liquid into a gas. Equally, the equivalent change of gas to liquid or liquid to solid -- for instance, water into ice -- is a clear example of a chemical reaction.
Temperature Input or Output
Chemical reactions will very often involve some kind of change in temperature, whether it is a result or catalyst of change. For instance, the burning of wood is a chemical reaction between the fuel, oxygen and heat, meaning the reaction has used the input of heat energy. This chemical reaction, in turn, produces more heat, therefore, both the input and output of heat are characteristics of a chemical reaction.
Changes in Color
Chemical reaction often presents unnatural changes in color. For instance, when metal rusts or oxidizes, the color of it will change, however slowly, from its natural silver or bronze appearance to an orange and red color. The chemical reaction of rust usually takes a long period of time, so the only way of noticing deterioration, is in the change in texture and the change in color.
Release of Gas
The release of gas is a clear characteristic of a chemical reaction, whether it is a liquid bubbling violently or a smell you can observe. Bubbling usually occurs when heat is introduced to a liquid; however, hydrogen peroxide bubbles when exposed to oxygen, meaning the chemical combination can have violent results without the need for heat. Sulfur is odorless but when it is combined with hydrogen, no bubbling takes place, but it becomes a highly toxic gas with an incredibly foul smell.