Distinguish soluble and insoluble salts, describe the preparation of a soluble salt from an acid, and prepare an insoluble salt by precipitation

What this dot point is asking

The syllabus wants you to know which salts are soluble and which are insoluble, to describe how to prepare a soluble salt from an acid and an excess of a solid, and to prepare an insoluble salt by precipitation. The method you choose depends entirely on whether the salt dissolves. This dot point puts the reactions of acids to practical use, and the step order matters for the marks.

The answer

Solubility rules

You need a few rules to decide whether a salt is soluble:

• All common sodium, potassium, and ammonium salts are soluble.
• All nitrates are soluble.
• Most chlorides are soluble (silver chloride and lead chloride are not).
• Most sulfates are soluble (barium sulfate and lead sulfate are not).
• Most carbonates are insoluble (except sodium, potassium, and ammonium).

If the salt is soluble, make it from an acid. If it is insoluble, make it by precipitation.

Preparing a soluble salt from an acid and excess solid

For a soluble salt of an insoluble base, metal, or carbonate:

1. Warm the acid in a beaker.
2. Add the solid (metal, metal oxide, or carbonate) a little at a time, stirring, until excess solid remains and no more reacts. Using excess ensures all the acid is used up, so the salt is not contaminated with acid.
3. Filter to remove the excess solid; the salt solution is the filtrate.
4. Evaporate some water to make the solution saturated, then leave it to crystallise on cooling.
5. Filter off the crystals and dry them between filter paper.

Preparing an insoluble salt by precipitation

For an insoluble salt, mix two soluble solutions that together provide the right ions:

1. Mix a solution containing the positive ion with a solution containing the negative ion. The insoluble salt forms at once as a precipitate.
2. Filter to collect the precipitate as the residue.
3. Wash it with a little distilled water to remove soluble impurities.
4. Dry the salt.

For example, barium chloride solution plus sodium sulfate solution gives insoluble barium sulfate.

Examples in context

Example 1. Making garden chemicals. Copper(II) sulfate, used in some fungicides, is a soluble salt prepared in school exactly as in industry in principle: react the acid with excess base, filter, then crystallise. The blue crystals that form on cooling are a familiar sight in the laboratory.

Example 2. Barium meal for X-rays. Barium sulfate is an insoluble salt made by precipitation. Because it does not dissolve, it is safe to swallow as a coating that shows up on X-rays of the gut, even though barium ions in solution would be poisonous. Its insolubility is exactly why precipitation is the right preparation method.

Try this

Q1. State whether each salt is soluble or insoluble: sodium chloride, silver chloride, potassium nitrate. [3 marks]

• Cue. Sodium chloride is soluble (sodium salt), silver chloride is insoluble, potassium nitrate is soluble (all nitrates and potassium salts are soluble).

Q2. Explain why excess copper(II) oxide is added to sulfuric acid when preparing copper(II) sulfate. [2 marks]

• Cue. The excess ensures all the acid is used up, so the salt solution is not contaminated with leftover acid, and the excess solid is removed later by filtering.

Q3. Name the method used to prepare lead(II) iodide, an insoluble salt, and the type of reaction involved. [2 marks]

• Cue. The method is precipitation, mixing two soluble solutions (one supplying lead ions, one supplying iodide ions) so the insoluble salt forms as a precipitate.