Describe what a salt is, distinguish soluble and insoluble salts, and outline the preparation of a soluble salt from an acid and an insoluble base

What this dot point is asking

SEAB wants you to say what a salt is, to know which salts dissolve and which do not, and to outline the laboratory preparation of a pure, dry soluble salt from an acid and an insoluble base. The central idea is that a salt is what you get when the hydrogen of an acid is replaced by a metal, and that making a pure salt means removing every leftover starting material.

The answer

What a salt is

A salt is the compound formed when the hydrogen in an acid is replaced by a metal (or by another positive ion). The acid used decides the type of salt:

• hydrochloric acid makes chlorides,
• sulfuric acid makes sulfates,
• nitric acid makes nitrates.

So sulfuric acid plus zinc oxide makes the salt zinc sulfate.

Soluble and insoluble salts

Some salts dissolve in water (soluble) and some do not (insoluble). A few useful rules:

• all sodium, potassium and ammonium salts are soluble,
• all nitrates are soluble,
• most chlorides and sulfates are soluble (with a few exceptions),
• most carbonates are insoluble.

Whether a salt is soluble decides how you prepare it. Soluble salts are usually made by reacting an acid with a metal, a base or a carbonate, then crystallising the solution.

Preparing a soluble salt from an acid and an insoluble base

A common method makes a soluble salt from an acid and an insoluble base (such as a metal oxide). The steps are:

1. add excess (extra) base to warm acid until no more dissolves, so all the acid is used up,
2. filter off the leftover excess base, leaving the salt solution,
3. heat the solution to evaporate some water and concentrate it,
4. leave it to cool so crystals form (crystallisation),
5. remove the crystals and pat them dry.

Using excess base guarantees no acid is left, and filtering removes the leftover solid, so the final salt is pure.

Examples in context

Example 1. Garden and farm fertilisers. Many fertilisers are soluble salts such as ammonium nitrate and potassium sulfate. They are made by neutralising acids with the right base, then crystallising, so plants can take up the dissolved nutrients through their roots.

Example 2. Why some salts can be made by simply mixing solutions. An insoluble salt such as silver chloride is made by mixing two soluble salt solutions so the insoluble salt drops out as a solid (a precipitate). This is a different method from the acid-plus-base route used for soluble salts.

Try this

• Cue. Name the salt made from hydrochloric acid and zinc oxide. Zinc chloride.
• Cue. Explain why excess copper(II) oxide is added when making copper(II) sulfate. To ensure all the acid reacts, leaving no acid in the salt.
• Cue. State the method used to remove the unreacted solid base. Filtration.