Hydrogen gas reacts with oxygen gas to form water. Chemists write that as 2H2 + O2 -> 2H2O. That is a chemical equation, a symbolic representation of a reaction. The formulas on the left are the reactants. The formulas on the right are the products. The arrow means yields or produces. The coefficients balance the equation, ensuring that the number of atoms of each element is the same on both sides. Matter is conserved.

Balancing equations is a fundamental skill. The coefficients must be whole numbers, and they must be in the simplest ratio. The equation for the combustion of methane is CH4 + 2O2 -> CO2 + 2H2O. One carbon, four hydrogens, and four oxygens on each side. States of matter can be indicated with symbols: (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous. Conditions like heat or a catalyst can be written above the arrow.

Chemical equations are used to calculate quantities. Stoichiometry uses the coefficients to convert between moles of reactants and moles of products. If you know how much of one substance reacts, you can calculate how much of another is produced. That is essential in industry, where yields and costs matter.

Equations can also be written as ionic equations, showing only the species that actually react. Net ionic equations omit spectator ions, which do not change during the reaction. They give a clearer picture of what is happening in solution.