Making salts (3): precipitation

Soluble or insoluble?

Insoluble salts are made by mixing two soluble compounds.

The solid obtained when solutions of two soluble compounds are mixed is called precipitate.

We have to know which compounds are soluble in water and which are insoluble.

We are using solubility rules, shown in the table below.

If we want to make an insoluble salt, e.g. lead chloride, we have to:identify the ions present in the insoluble salt – lead

(Pb2+) and chloride (Cl-)use solubility rules to choose soluble compounds

including these ions – e.g. lead nitrate for lead and sodium chloride for chloride

add one solution to the otherfilter off the precipitate then wash and dry the solid

Soluble compounds Insoluble compounds

all salts of Group I elements

all nitrates

all ammonium salts

most chlorides, bromides and iodides

chlorides, bromides and iodides of silver and lead

most sulphates sulphates of calcium, barium and lead

Group I hydroxides and carbonates are soluble(calcium hydroxide is slightly soluble)

most hydroxides and carbonates

Group I and II oxides react with water

most metal oxides

What happens in a precipitation reaction?

By looking at the reaction between lead nitrate and sodium chloride as an example, we can explain why a solid precipitates.

Pb(NO3)2(aq) + 2NaCl → PbCl2(s) + 2NaNO3(aq)

In a solution of ionic compound, the ions are free to move.

Solution of lead nitrate contains lead ions and nitrate ions that are separate from one another, moving freely and randomly throughout the water.

The water molecules help to keep them in solution.A similar thing happens with solution of sodium chloride.When we mix the solutions, lead nitrate is precipitated.The lead ions in solution have a greater attraction for

chloride ions than the water molecules that keep them in solution.