What is a chemical reaction?
A chemical reaction is a process in which atoms in substances are rearranged to form different substances. Starting materials are reactants and resulting materials are products. Ordinary chemical reactions neither create nor destroy atoms.
Bonds, equations and conservation
Some chemical bonds break and new ones form during a reaction, potentially exchanging energy. Balancing an equation requires equal numbers of each element’s atoms on both sides; changing a formula’s subscripts would instead change the substance. Combustion, precipitation, acid–base and oxidation–reduction are different ways to classify reactions. Categories can overlap: one reaction may fit more than one description. Total mass is conserved in a closed system; a visible loss of mass from an open vessel as gas escapes does not violate this principle.
Rate and observation
Temperature, concentration, surface area and catalysts can affect reaction rates. A catalyst may provide another pathway and change the rate without being consumed in the net equation. Color change, gas release, a precipitate or temperature change may signal chemical transformation, but appearance alone cannot establish a product’s identity. Melting ice is generally a physical change; rusting forms new substances. Safe laboratory work avoids mixing unknown chemicals and assesses possible heat or gas production beforehand.