A chemical reaction is nature's way of rearranging atoms. Reactants go in, bonds break and reform, and products come out — different substances with different properties. You witness chemical reactions every day without even thinking about it. Striking a match is a chemical reaction: friction ignites phosphorus, which combusts with oxygen to produce light, heat, and new compounds. Cooking an egg is a chemical reaction: heat denatures proteins, turning clear, runny egg white into an opaque solid permanently. Rusting is a chemical reaction: iron slowly combines with oxygen and moisture to form iron oxide. Unlike physical changes (melting ice, dissolving sugar), chemical reactions produce genuinely new substances.
Every chemical reaction obeys a fundamental rule: atoms are neither created nor destroyed, only rearranged. This is the law of conservation of mass, and it means that a balanced chemical equation has the same number of each type of atom on both sides. When methane (CH₄) burns in oxygen (O₂), it produces carbon dioxide (CO₂) and water (H₂O). The carbon, hydrogen, and oxygen atoms are all still there — just reassembled into different molecules. Energy changes accompany every reaction: exothermic reactions release energy (combustion, explosions), while endothermic reactions absorb it (photosynthesis, melting ice packs).
The speed of a chemical reaction depends on temperature, concentration, surface area, and whether a catalyst is present. Some reactions are instantaneous — mix acid and base and neutralization happens in milliseconds. Others take eons — the rusting of iron or the weathering of mountains. Inside your body right now, thousands of enzyme-catalyzed reactions are occurring every second: digesting food, producing energy, building proteins, repairing DNA. Life itself is an extraordinarily complex and beautifully orchestrated network of chemical reactions.