Chemistry · Salts · Precipitation method

Preparing an insoluble salt

Two soluble salt solutions are mixed. Their ions exchange partners; the insoluble salt drops out as a solid (a precipitate). Filter, wash, dry — and you have the pure insoluble salt.

Step 1

Mix the two solutions in a beaker

Step 2

Filter the mixture (filter paper in funnel)

Step 3

Wash residue with distilled water

Step 4

Dry the residue (oven or air)

Lead iodide: recipe

cation source: Pb(NO₃)₂(aq)
anion source: KI(aq)
Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)

Exam-style questions

1. When making an insoluble salt by precipitation, why are TWO SOLUBLE salts mixed?

2. After filtering, the RESIDUE on the filter paper is washed with distilled water. Why?

3. Lead iodide forms a yellow precipitate. What is the ionic equation?

4. Why CAN'T the 'acid + insoluble base' method be used to make lead iodide?