Precipitation Reactions

A precipitation reaction produces an insoluble solid (precipitate) when two solutions are mixed. The precipitate forms because one of the possible products cannot dissolve. These reactions are used to identify ions and remove pollutants from water.

What Is a Precipitation Reaction?

A precipitate is an insoluble solid that forms when two solutions are mixed. The reaction that produces it is called a precipitation reaction.

The precipitate appears as a cloudy solid suspended in the solution, and can be collected by filtering.

General form:

$\text{solution A} + \text{solution B} \rightarrow \text{precipitate (s)} + \text{solution (aq)}$

A Classic Example

Lead(II) iodide precipitation:

Lead nitrate solution + potassium iodide solution →

$\text{Pb(NO}_3)_2\text{(aq)} + 2\text{KI(aq)} \rightarrow \text{PbI}_2\text{(s)} + 2\text{KNO}_3\text{(aq)}$

A bright yellow precipitate of lead(II) iodide forms immediately. The reaction is visually striking and is sometimes called "golden rain."

Copper(II) hydroxide:

Copper sulfate + sodium hydroxide:

$\text{CuSO}_4\text{(aq)} + 2\text{NaOH(aq)} \rightarrow \text{Cu(OH)}_2\text{(s)} + \text{Na}_2\text{SO}_4\text{(aq)}$

A pale blue precipitate of copper hydroxide forms.

Note

The coloured precipitates formed by transition metal hydroxides are used in lab tests to identify metal ions. For example:

Fe²⁺ gives a green precipitate with NaOH
Fe³⁺ gives a red-brown precipitate
Cu²⁺ gives a pale blue precipitate

Ionic Equations

In solution, ionic compounds exist as separate ions. We can write a net ionic equation that shows only the ions that actually react (the spectator ions cancel):

For copper hydroxide precipitation:

$\text{Cu}^{2+}\text{(aq)} + 2\text{OH}^-\text{(aq)} \rightarrow \text{Cu(OH)}_2\text{(s)}$

Uses of Precipitation

Water treatment: lime is added to hard water and rivers to precipitate out calcium and magnesium salts
Photography: silver halide precipitation creates light-sensitive surfaces
Removing toxic metal ions from industrial wastewater
Qualitative analysis: identifying unknown ions by the colour of their precipitates

Tip

State symbols in equations are key: (s) = solid precipitate, (aq) = dissolved in solution, (l) = liquid, (g) = gas. The precipitate always has (s) next to it.

Real world: Limescale in kettles and boilers

Hard water contains dissolved calcium hydrogen carbonate. When hard water is heated, the calcium ions precipitate out as calcium carbonate (CaCO₃) — the white, chalky limescale that builds up inside kettles, boilers, and pipes. This is a precipitation reaction triggered by heating. Limescale insulates heating elements, reducing efficiency, and costs billions in energy waste and pipe replacements every year.

Real world: Water treatment plants

Water treatment works use precipitation to remove dissolved impurities from drinking water. Lime (calcium hydroxide) is added to remove phosphate ions (from agricultural runoff) as calcium phosphate precipitate. Aluminium sulfate is added to cause smaller particles to clump together (flocculate) into a precipitate that can be filtered out. Precipitation chemistry literally makes tap water safe to drink.

Real world: Traditional photography and silver halides

Film photography relied on precipitation chemistry. Silver halide crystals (AgBr, AgCl) were precipitated onto photographic film by mixing silver nitrate solution with a halide salt. The tiny precipitate crystals are light-sensitive — when exposed to light, they darken. Digital cameras replaced this, but precipitation of silver halides was the basis of all photography for over a century.

Tip

Where do you see this in real life? Limescale in your kettle (calcium carbonate precipitating), water treatment plants (phosphate removal), old photographic film (silver halide precipitation), testing for metal ions in a school lab — precipitation reactions are the chemistry behind water purity and materials science.