Making insoluble salts

Insoluble salts do not dissolve in water. They can be made by mixing appropriate solutions of ions together.

Soluble and insoluble salts

 

Soluble	Insoluble
All nitrates	None
Most sulfates	Lead sulfate, barium sulfate
Most chlorides, bromides and iodides	Silver chloride, silver bromide, silver iodide, lead chloride, lead bromide, lead iodide
Sodium carbonate, potassium carbonate	Most other carbonates
Sodium hydroxide, potassium hydroxide	Most other hydroxides

Notice that all nitrates and most chlorides are soluble. This is why many of the chemicals you use in the laboratory are nitrates or chlorides. If you want to make an insoluble salt, you can react together two soluble salts in a precipitation reaction.

Making an insoluble salt

Silver chloride is insoluble - you can see this from the table. You need a soluble silver salt and a soluble chloride salt to make it. Silver nitrate and sodium chloride are both soluble. When you mix their solutions together, you make soluble sodium nitrate and insoluble silver chloride:

• silver nitrate + sodium chloride → sodium nitrate + silver chloride
• AgNO₃(aq) + NaCl(aq) → NaNO₃(aq) +AgCl(s)

The silver chloride appears as tiny particles suspended in the reaction mixture - it forms a precipitate. The precipitate can be filtered, washed with water on the filter paper, and then dried in an oven.

Remember: if you want to make an insoluble salt XY, mixing X nitrate with sodium Y will always work. In the example above, X is silver and Y is chloride.

Using precipitation reactions

Precipitation reactions can be used to remove unwanted ions in solution. This is useful for treating drinking water and waste water.