A chemical equation is a symbolic statement of a chemical transformation. Species written to the left of the reaction arrow are reactants; those to the right are products.

For example,

$$2\mathrm{H_2}+\mathrm{O_2}\rightarrow2\mathrm{H_2O}$$

states that hydrogen and oxygen are converted into water.

The numbers placed before formulas are stoichiometric coefficients. A coefficient multiplies the entire formula: $2\mathrm{H_2O}$ represents two water molecules or two moles of water, not a different chemical species.

Ordinary chemical reactions rearrange atoms rather than creating or destroying nuclei. A valid chemical equation must therefore contain the same number of atoms of each element on both sides. In the example,

• hydrogen: $2\times2=4$ atoms on the left and $2\times2=4$ on the right;
• oxygen: $2$ atoms on the left and $2\times1=2$ on the right.

If charged species are written explicitly, net electric charge must also balance.

Physical-state labels may be appended when useful:

$$\mathrm{(s)},\quad\mathrm{(l)},\quad\mathrm{(g)},\quad\mathrm{(aq)},$$

for solid, liquid, gas and a species dissolved in aqueous solution.

The arrow records the net transformation. It does not by itself describe the microscopic reaction mechanism, reaction rate, equilibrium position or energy change; those are separate properties of the chemical process.