Chemical Equation
A chemical equation uses formulas and an arrow to show the reactants that go into a chemical reaction and the products that come out.
Definition
A chemical equation is a way to write down a reaction using formulas: the starting substances go on the left, an arrow means "turns into," and the new substances go on the right. Coefficients give the relative numbers of each substance, state symbols in parentheses indicate solid $(s)$, liquid $(l)$, gas $(g)$, or aqueous $(aq)$, and a correct equation must be balanced so that the same number of each type of atom appears on both sides. The equation expresses the stoichiometry of the reaction, since balanced coefficients give molar ratios that satisfy conservation of mass and, for ionic and redox reactions, conservation of charge. Additional notation includes a double arrow for equilibrium, conditions or catalysts written over the arrow, an upward arrow for evolved gas and a downward arrow for precipitate; net ionic equations omit spectator ions, and thermochemical equations append the enthalpy change.
Example
$2H_2 + O_2 \rightarrow 2H_2O$ reads "two hydrogen molecules plus one oxygen molecule turn into two water molecules." Methane burning is $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$, with $1$ carbon, $4$ hydrogen, and $4$ oxygen atoms on both sides, and the $(g)$ tells you every substance is a gas. The full equation $AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)$ reduces to the net ionic equation $Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)$ once the spectator ions $Na^+$ and $NO_3^-$ are removed, and the Haber process is written $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$, $\Delta H = -92\text{ kJ}$, with the double arrow marking equilibrium.
Key Insight
The arrow is the most important symbol: it points from what you start with to what you end up with, and it is never an equals sign. An equation is a recipe written in molecules, so it gives proportions you can scale up, and the same equation describes one molecule of methane or a million cubic feet of natural gas. A balanced equation says nothing about mechanism; $2H_2 + O_2 \rightarrow 2H_2O$ does not mean two hydrogen molecules collide with one oxygen molecule, since the real process is a radical chain with many steps, and the equation is bookkeeping, not a movie.