C2aL16 Precipitation

Key Words

Insoluble - does not dissolve in water.

Ionic Equation - chemical equation that describes how ionic compounds split into ions.

Precipitate - insoluble salt made form soluble reactants.

Soluble - can dissolve in water

Grade E

We get a precipitate as a result of this reaction:

lead nitrate + potassium chromate ® potassium nitrate + lead chromate

Pb(NO₃)_{2 (aq)} + K₂CrO_{4 (aq)} ® 2KNO_{3 (aq)} + PbCrO_{4 (s)}

Both the reactant solutions are clear, but when they are mixed together, there is a yellow precipitate.  A solid has formed that is insoluble in water.

A similar thing happens when you treat hard water with sodium carbonate.  Hard water makes it difficult to get soaps to lather.  If we add sodium carbonate (washing soda) to the hard water, we get a solid precipitate of calcium carbonate.  (Huntingdonshire has some of the hardest water in the country.)

Grade C

We can make insoluble salts by reacting two soluble salts.  One of the products will be soluble, so we can wash the precipitate with distilled water.  We do this by filtering the precipitate and washing it with distilled water.

Some more precipitation reactions:

lead nitrate + sodium sulphate ® lead sulphate + sodium nitrate

silver nitrate + sodium bromide ® silver bromide + sodium nitrate

calcium nitrate + potassium carbonate ® calcium carbonate + potassium nitrate

The salts in red form precipitates.

Phosphates can be removed from waste water by treating it with calcium ions.  The precipitate is calcium phosphate.

Heavy metals cause toxic pollution if released with waste water.  Heavy metal carbonates are insoluble, so treating the water with sodium carbonate causes the heavy metals to precipitate out.  The sludge then can be recycled to extract the heavy metals.

Precipitation reactions are used to test for metals in solution.  Sodium hydroxide is added and the colour of the precipitate tells us what the metal is.

Grade A

Soaps are salts of sodium and fatty acids like stearic acid.  Fatty acids are made from animal fats or vegetable oils.  Stearate ions consist of a long hydrocarbon chain, but we will show it as St^-.   In hard water, the stearate ions react with calcium ions to form a precipitate which we know as scum.  The ionic equation shows the ions that react:

Ca²⁺ _(aq) + 2St^- _(aq) ® CaSt_{2 (s)}

Silver halide salts are insoluble.  If we add sodium halide solutions to silver nitrate, we get precipitates:

• silver chloride (white);

• silver bromide (cream);

• silver iodide (yellow).