Solubility rules and predicting precipitates

Why solubility rules are the engine of the whole procedure

• Every precipitation test works the same way: you add an ion that will form an insoluble compound with the ion you are looking for.
• If you know which combinations are insoluble, you can predict every test in the procedure and explain why it works — which is what separates Excellence from a lookup.

The rules

Learn these in this order. Rules higher in the list override rules lower down.

Rule	Soluble	Exceptions (insoluble)
1. Group 1 ions (Na+, K+, Li+) and ammonium (NH4+)	all soluble	none
2. Nitrates (NO3−)	all soluble	none
3. Chlorides, bromides, iodides	soluble	Ag+, Pb2+
4. Sulfates (SO42−)	soluble	Ba2+, Pb2+, Ca2+ (slightly)
5. Carbonates (CO32−)	insoluble	Group 1 and NH4+
6. Hydroxides (OH−)	insoluble	Group 1; Ba(OH)2 soluble, Ca(OH)2 slightly

• Rules 1 and 2 are the overriding ones: sodium carbonate is soluble despite rule 5, because rule 1 comes first.

Using the rules to predict a precipitate

• Write the ions present in both solutions being mixed.
• Pair them the other way round — each cation with the other anion.
• Check each new pair against the rules.
• Any pair that is insoluble is the precipitate.

For example, mixing barium chloride and sodium sulfate solutions:

• Ions present: Ba²⁺, Cl⁻, Na⁺, SO₄²⁻.
• New pairings: BaSO₄ and NaCl.
• Rule 4 says sulfates are soluble except barium — so BaSO₄ precipitates.
• Rule 1 says sodium salts are all soluble — so NaCl stays in solution.

Writing the ionic equation

• The ionic equation shows only the ions that actually change. Spectator ions are left out.
• Include state symbols: this is the only way the equation shows that a precipitate formed.

$$\text{Ba}^{2+}(aq) + \text{SO}_4^{2-}(aq) \rightarrow \text{BaSO}_4(s)$$

• Never write the sodium or chloride ions in this equation. They were in solution before and are in solution after, so they are spectators.
• Getting the charges balanced matters: $\text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl}(s)$ is 1 : 1, but $\text{Pb}^{2+} + 2\text{Cl}^- \rightarrow \text{PbCl}_2(s)$ needs two chlorides.

The colours of the common precipitates

Colour is what distinguishes one precipitate from another, so this table is the heart of the identification.

Precipitate	Colour
AgCl	white (darkens to grey/purple in light)
AgBr	cream
AgI	yellow
BaSO4, PbSO4, CaSO4	white
CaCO3, most carbonates	white
CuCO3	blue-green
Cu(OH)2	pale blue
Fe(OH)2	green
Fe(OH)3	red-brown
Al(OH)3, Zn(OH)2, Mg(OH)2, Ca(OH)2	white

• Notice how many are white. Colour alone will identify Cu²⁺ or Fe³⁺, but it can never distinguish Al³⁺ from Zn²⁺ from Mg²⁺ — which is why the next page's complex ion tests exist.