Making insoluble salts

An insoluble salt can be prepared by reacting two suitable solutions together to form a precipitate.

Determining suitable solutions

All nitrates and all sodium salts are soluble. This means a given precipitate XY can be produced by mixing together solutions of:

  • X nitrate
  • sodium Y

For example, to prepare a precipitate of calcium carbonate:

  • X = calcium and Y = carbonate
  • mix calcium nitrate solution and sodium carbonate solution together

calcium nitrate + sodium carbonate → sodium nitrate + calcium carbonate

Ca(NO3)2(aq) + Na2CO3(aq) → 2NaNO3(aq) + CaCO3(s)

It also works if potassium carbonate solution or ammonium carbonate solution is used instead of sodium carbonate solution. Remember that all common potassium and ammonium salts are soluble.

Question

Name two suitable solutions that, when mixed together, will produce a precipitate of silver chloride.

Silver nitrate solution and sodium chloride solution (or potassium chloride solution, or ammonium chloride solution).

An outline method

This is an outline method for making a pure, dry sample of an insoluble salt.

  1. mix together two suitable solutions (see above)
  2. use filtration to separate the precipitate as a residue from the solution
  3. wash the precipitate while it is in the filter funnel, with distilled water
  4. leave the washed precipitate aside or in a warm oven to dry

Reasons for each step

  • filtration separates insoluble substances from liquids and solutions
  • the precipitate is insoluble in water, so any remaining contaminating solution can be removed by washing it with distilled water
Two beakers, one containing a solid and liquid and the other with a filter funnel lined with filter paper inserted.

1. One beaker contains a mixture of solid and liquid, the other contains a funnel with filter paper

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