Solubility rules

Solubility

A substance's solubility is a measure of the maximum mass that will dissolve in a given volume of solvent, at a particular temperature. Substances that are very soluble have high solubilities. Substances that are insoluble or sparingly soluble (almost none dissolves) have low solubilities.

Solubility in water

The table summarises whether common ionic compounds are soluble or insoluble in water.

SolubleInsoluble
All common sodium, potassium and ammonium salts
All nitrates
Most common chloridesSilver chloride, lead chloride
Most common sulfatesLead sulfate, barium sulfate, calcium sulfate
Sodium carbonate, potassium carbonate, ammonium carbonateMost common carbonates
Sodium hydroxide, potassium hydroxide, ammonium hydroxideMost common hydroxides

The top two rows explain why so many salt solutions used in the laboratory are sodium or potassium compounds, or nitrates.

Precipitates

A precipitate is an insoluble product that forms when two solutions are mixed and react together. The reaction that produces a precipitate is called a precipitation reaction.

For example, a precipitate of lead iodide forms when potassium iodide solution and lead nitrate solution are mixed:

potassium iodide + lead nitrate → potassium nitrate + lead iodide

2KI(aq) + Pb(NO3)2(aq) → 2KNO3(aq) + PbI2(s)

Notice how important state symbols are in the balanced equations for precipitation reactions. Without the (s) for solid, it would not be obvious that PbI2 (lead iodide) was the precipitate.

A yellow precipitate of lead iodide.
A yellow precipitate of lead iodide

Predicting precipitates

The formation of a precipitate, when two solutions are mixed, can be predicted using the solubility rules in the table:

  • if the products meet the 'soluble' rules in the left hand column, no precipitate will form
  • if at least one product meets the 'insoluble' rule in the right hand column, a precipitate will form
Question

Silver nitrate solution is mixed with sodium chloride solution. Predict whether a precipitate forms, and name any precipitate.

A precipitate of silver chloride will form.

Question

Potassium sulfate solution is mixed with barium chloride solution. Predict whether a precipitate forms, and name any precipitate.

A precipitate of barium sulfate will form.