For international standards and resources relating to expression of chemical reactions and concentration, and Australian conventions and resources, see Chemistry terminology.

Chemical equations

Chemical reactions are represented by chemical equations, shown either as part of the main body of the text (for short equations) or set separately from the text. If set separately, equations can be numbered, in the same way as mathematical equations (see Displaying equations in text):

NaOH + HCl → NaCl + H2O                                                                            (1)

A number of conventions are used for arrows within chemical equations. For example, a single arrow (→) is used to indicate a reaction that is proceeding in one direction, whereas 2 arrows in opposite directions (⇄) show a reaction that is proceeding in both directions:

2NO2 ⇄ N2O4

Two half arrows in opposite directions indicate a reaction at equilibrium:

2NO2 ⇌ N2O4

States of matter

States of matter are usually shown as symbols in brackets after reactants and products – gas (g), liquid (l), solid (s), aqueous (aq):

2Al(s) + 3H2SO4(aq) → 3H2(g) + Al2(SO4)3(aq)

These symbols are sometimes set as subscripts and sometimes as italics. Check the house style of the organisation or journal you are writing for. 

Chemical concentration

Terms to watch out for:

molecular mass, molecular weight

solute, solution, solvent

See all terms

Chemical concentration is usually measured in molecular weight equivalents (mole) dissolved in a litre of liquid. This is called the molarity and has the unit mol/L (mol L-1) or molar (M). Normality is another common measure of concentration and refers to the molar concentration divided by an equivalence factor. It is indicated by the unit N. Another measure is moles per kilogram, or molal, which has the unit m.

Do not italicise the units M (molar) or N (normal), but do italicise m (molal):

2 M NaCl     5 mM CuSO4     1 N NaOH     2 m CaCl2

Do not add a plural s to mole when written in full:

10 mole   not   10 moles

In reactions and equations, concentration can be denoted by square brackets:

[HCl] = 0.01M

However, do not use this abbreviation in running text or with the spelt-out name of the chemical:

The hydrochloric acid concentration was reduced   
not    
The [HCl] was reduced   or   The [hydrochloric acid] was reduced

See also Relative quantities for information on expressing measures of concentration such as weight/weight and parts per million).