Preparation of Insoluble Salts (Precipitation)

An insoluble salt does not dissolve in water. We make it by precipitation: when two clear solutions are mixed, a solid forms and appears like tiny “snowflakes” in the liquid.

Key idea

Choose two soluble salts that supply the ions you want. When mixed, the desired insoluble salt forms as a solid (precipitate). Then separate, wash, and dry it.

Using solubility rules to predict a precipitate

• Always soluble: sodium, potassium, and ammonium salts; nitrates.
• Chlorides are soluble, except silver and lead (so AgCl and PbCl₂ are insoluble).
• Sulfates are soluble, except barium, calcium, and lead (BaSO₄, CaSO₄ slightly soluble, PbSO₄).
• Carbonates are insoluble, except sodium, potassium, and ammonium.
• Hydroxides are mostly insoluble, except sodium, potassium, ammonium (calcium is only slightly soluble).

Method (precipitation, filtration, washing, drying)

1. Select reagents: pick two soluble salts so that one provides the needed positive ion and the other the needed negative ion. Prefer sodium, potassium, or ammonium salts as sources because they are soluble.
2. Mix: pour the two solutions together and stir. A precipitate forms.
3. Filter: collect the solid on filter paper; the clear liquid (filtrate) passes through.
4. Wash: rinse the solid with a little distilled water to remove soluble impurities.
5. Dry: leave the solid on the filter paper or in a warm oven to dry.

Equations

General pattern (double replacement):

$$\text{Solution containing }X^+ + \text{solution containing }Y^- \;\rightarrow\; XY(s) + \text{another soluble salt}$$

Example (silver chloride):

$$\text{AgNO}_3(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq)$$

Net ionic: $\text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl}(s)$.

Another example: Barium sulfate for X-ray contrast is made by mixing barium chloride and sodium sulfate: $$\text{BaCl}_2(aq) + \text{Na}_2\text{SO}_4(aq) \rightarrow \text{BaSO}_4(s) + 2\,\text{NaCl}(aq)$$

Common misconceptions

• “Add acid to make the precipitate” — not needed for preparation; just mix the two correct solutions.
• “Skip washing” — unwashed precipitates contain trapped soluble impurities.