A chemical reaction is a fundamental process in chemistry where substances, called reactants, transform into different substances called products. During this process, atoms are rearranged as chemical bonds are broken and new bonds are formed. For example, hydrogen gas reacts with oxygen gas to form water molecules.
Chemical reactions can be classified into four main types. Synthesis reactions combine two or more substances to form a single compound. Decomposition reactions break down a compound into simpler substances. Single replacement involves one element replacing another in a compound, while double replacement involves the exchange of ions between two compounds.
All chemical reactions involve energy changes. In exothermic reactions, energy is released to the surroundings, making them feel warm. Examples include combustion and cellular respiration. In endothermic reactions, energy is absorbed from the surroundings, making them feel cool. Examples include photosynthesis and ice melting. The energy diagram shows how reactants transform into products with different energy levels.
Chemical equations must be balanced according to the Law of Conservation of Mass. This means the same number of each type of atom must appear on both sides of the equation. We use coefficients to balance equations, never changing the chemical formulas themselves. For example, the reaction of hydrogen and oxygen to form water requires two hydrogen molecules and one oxygen molecule to produce two water molecules.
Chemical reactions have countless applications in our world. In industry, they are used to manufacture plastics, refine metals, and produce pharmaceuticals. In biology, reactions like photosynthesis convert sunlight into chemical energy, while digestion breaks down food for nutrients. In daily life, we rely on chemical reactions for cooking food, cleaning with detergents, and powering combustion engines. Understanding chemical reactions helps us harness these processes for human benefit.