Chemical reactions occur when two or more substances interact to form new substances. These reactions can release energy in the form of heat or light, or absorb energy from their surroundings. For example, when hydrogen reacts with oxygen, it forms water, a common chemical reaction in nature.
In addition to the creation of new substances, chemical reactions also involve changes in the properties of the reactants. The products formed in a chemical reaction have different physical and chemical properties from the starting materials. Understanding these reactions is crucial in chemistry, as they form the basis for many processes in industries such as manufacturing, medicine, and environmental science.