Solubility Rules: How to Predict Precipitates

Pour a clear solution of silver nitrate into a clear solution of sodium chloride and a thick white solid instantly appears. Mix potassium iodide with lead nitrate and you get a startling bright yellow. These solids are precipitates — insoluble products that form when ions meet in solution.

The solubility rules let you predict when that will happen, without doing the experiment.

What “soluble” means here

When an ionic compound dissolves in water, it separates into its ions. “Soluble” in these rules roughly means more than about 1 g dissolves in 100 mL of water; “insoluble” means very little does. (Nothing is truly completely insoluble — the rules describe practical behaviour.)

The rules

Always soluble:

1. Compounds of group 1 ions (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) and ammonium (NH₄⁺).
2. All nitrates (NO₃⁻), acetates (CH₃COO⁻) and chlorates / perchlorates.

Usually soluble, with exceptions:

3. Chlorides, bromides and iodides (Cl⁻, Br⁻, I⁻) — except with silver (Ag⁺), lead(II) (Pb²⁺) and mercury(I) (Hg₂²⁺).
4. Sulfates (SO₄²⁻) — except with barium, strontium, lead(II) and calcium (calcium sulfate is only slightly soluble), and silver is also only slightly soluble.

Usually insoluble, with exceptions:

5. Carbonates (CO₃²⁻), phosphates (PO₄³⁻) and sulfides (S²⁻) — except with group 1 ions and ammonium.
6. Hydroxides (OH⁻) and oxides — except group 1 and ammonium; calcium, strontium and barium hydroxides are slightly to moderately soluble.

When two rules conflict, the one listed first takes priority. Sodium carbonate is soluble because rule 1 (sodium) outranks rule 5 (carbonates); silver nitrate is soluble because rule 2 (nitrates) has no exceptions.

A memory aid

A short version worth memorizing:

Salts of sodium, potassium and ammonium, and all nitrates, dissolve. Silver and lead are the troublemakers.

Most of the exceptions to rules 3 and 4 involve silver or lead.

Predicting a precipitation reaction

1. Write the ions present in each solution.
2. Swap partners to find the two possible new combinations.
3. Check each new combination against the rules.
4. If either is insoluble, it precipitates. If both are soluble, no reaction occurs.

Example 1: silver nitrate + sodium chloride

• Ions: Ag⁺, NO₃⁻, Na⁺, Cl⁻
• New combinations: AgCl and NaNO₃
• AgCl: chloride with silver → insoluble (rule 3 exception)
• NaNO₃: soluble (rules 1 and 2)

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

Example 2: potassium nitrate + sodium chloride

• New combinations: KCl and NaNO₃ — both soluble.
• No reaction. You just get a solution containing four kinds of ions.

Example 3: barium chloride + sodium sulfate

• New combinations: BaSO₄ and NaCl
• BaSO₄: sulfate with barium → insoluble

This reaction is used as a test for sulfate ions: add barium chloride solution and look for a white precipitate.

Net ionic equations

In example 1, the Na⁺ and NO₃⁻ ions start dissolved and finish dissolved. They don’t actually take part — they’re spectator ions. Remove them and you get the net ionic equation, which shows what really happens:

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

This tells you something useful: any soluble silver salt plus any soluble chloride will give the same precipitate.

Where precipitation matters

• Qualitative analysis: identifying ions in an unknown solution by the colour of the precipitates they form (white AgCl, cream AgBr, yellow AgI, blue Cu(OH)₂, green Fe(OH)₂, red-brown Fe(OH)₃).
• Water treatment: removing phosphate and heavy metals by precipitating them.
• Hard water: calcium and magnesium ions precipitate as limescale (calcium carbonate) inside kettles and pipes, and as scum with soap.
• Medicine: barium sulfate is so insoluble that patients can swallow it safely as a contrast agent for X-rays, even though soluble barium compounds are toxic.
• Kidney stones are often precipitated calcium oxalate or calcium phosphate.

Quick answers

Is calcium sulfate soluble? Only slightly — about 0.2 g per 100 mL. It’s usually listed as an exception to the sulfate rule.

Why does barium sulfate not poison patients? Because so little dissolves that the amount of free barium ions is tiny.

What’s the difference between a precipitate and a suspension? A precipitate is a solid formed by a reaction in solution; it usually settles or can be filtered off.

Related

Precipitation reactions are a type of double displacement reaction — see types of chemical reactions. The equation balancer will balance the full molecular equation.