Precipitate Test – IGCSE Chemistry Definition

Precipitate tests form the core of qualitative analysis at IGCSE. Paper 4 and Paper 6 both include questions requiring candidates to identify ions by adding reagents and observing precipitate colours. The NaOH test for cations and the AgNO₃ test for halides are the two most important sets.

The 0620 definition

Precipitate tests are qualitative analysis techniques that identify ions in solution by forming insoluble precipitates of characteristic colours when specific reagents are added.

Cation tests with sodium hydroxide (NaOH) solution

Add a few drops of NaOH(aq) to the test solution:

Metal ion	Precipitate colour	Formula	Excess NaOH?
Cu²⁺	Blue	Cu(OH)₂	Insoluble (stays)
Fe²⁺	Green (pale/dark)	Fe(OH)₂	Insoluble
Fe³⁺	Red-brown / orange-brown	Fe(OH)₃	Insoluble
Al³⁺	White	Al(OH)₃	Dissolves (amphoteric)
Ca²⁺	White	Ca(OH)₂	Insoluble
Zn²⁺	White	Zn(OH)₂	Dissolves (amphoteric)

To distinguish Al³⁺ from Zn²⁺ (both give white precipitates that dissolve in excess NaOH): use ammonia solution instead. Both give white precipitates, but only Zn(OH)₂ dissolves in excess ammonia.

Cation test with ammonia (NH₃) solution

Metal ion	Precipitate	Excess NH₃?
Cu²⁺	Blue precipitate	Dissolves → deep blue solution
Zn²⁺	White precipitate	Dissolves → colourless solution
Al³⁺	White precipitate	Insoluble (stays)

Anion tests

Halide ions (add dilute HNO₃ then AgNO₃)

Ion	Precipitate	Colour
Cl⁻	AgCl	White
Br⁻	AgBr	Cream
I⁻	AgI	Yellow

Sulfate ion (add dilute HCl then BaCl₂)

SO₄²⁻ → white precipitate of BaSO₄

Carbonate ion (add dilute HCl)

CO₃²⁻ → effervescence; gas turns limewater milky (CO₂)

Worked exam question

A student adds NaOH solution to a pale green solution. A green precipitate forms that does not dissolve in excess NaOH. (a) Identify the metal ion. [1] (b) Write the ionic equation for the reaction. [1]

Mark scheme

(a) Iron(II) / Fe²⁺ [1]

(b) Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s) [1]

Common exam mistakes

• Confusing Fe²⁺ (green precipitate) with Fe³⁺ (red-brown precipitate) — remember: 2 = green, 3 = brown.
• Forgetting to add acid before AgNO₃ in halide tests — without dilute nitric acid, carbonates and sulfates could also give precipitates.
• Not stating whether the precipitate dissolves in excess NaOH — this is how you distinguish Al³⁺/Zn²⁺ from Ca²⁺.