Balanced chemical equations

A balanced chemical equation represents a chemical reaction using the formulae of the reactants and products. It shows the number of units of each substance involved.

State symbols

Balanced chemical equations sometimes include state symbols in brackets after each formula. They show the physical state of that substance.

State symbolMeaning
(s)Solid
(l)Liquid
(g)Gas
(aq)Aqueous solution

An aqueous solution forms when a substance dissolves in water.

State symbols are useful because they show what a substance is like. For example:

  • H2O(l) is liquid water but H2O(g) is steam
  • HCl(g) is hydrogen chloride gas but HCl(aq) is hydrochloric acid

Balancing an equation

If an equation is written by just replacing names with formulae, it may not be balanced. The numbers of atom of each element on the left must be the same as they are on the right. This follows from the law of conservation of mass - the mass of all the atoms must be the same in the reactions and the products.

To balance an unbalanced equation, add numbers to the left of one or more formulae. Here is one way to work out how to do this for the reaction between nitrogen and hydrogen.

StepResult
Check to see if there are an equal number of atoms of each element on both sides. There is not.N2 + H2 → NH3
There are two nitrogen atoms on the left but only one on the right, so put a big 2 on the left of the NH3.N2 + H2 → 2NH3
Check again. There are two hydrogen atoms on the left but (2 × 3) = 6 on the right, so put a big 3 in front of the H2.N2 + 3H2 → 2NH3
Check again to see if there are equal numbers of each element on both sides. There is.(Two nitrogen atoms and six hydrogen atoms)
Add the state symbols if asked to do so.N2(g) + 3H2(g) → 2NH3(g)
curriculum-key-fact
Balanced chemical equations only show formulae, not names. A balancing number multiplies all the atoms in the substance next to it.