Salt preparations

Acids do not just sit in bottles waiting to be classified — they react. Chapter 16 (u2:c16) gave you the map: how to tell an acid from an alkali, how the pH scale bands them, and what neutralisation looks like ion by ion. This chapter is about what acids actually do: how they attack metals, how they are neutralised by bases and carbonates, and how those reactions are turned into a laboratory skill — making salts, on purpose, pure and dry. When the hydrogen of an acid is replaced by a metal (or by the ammonium ion), the compound that forms is called a salt — and by the end of this chapter you will be able to make several of them from scratch.

Four ideas organise everything that follows. First, the reactions of acids: with metals above hydrogen, with metal oxides and hydroxides, and with carbonates — each with its own summary equation and its own observations. Second, the solubility rules: which ionic compounds dissolve in water and which do not, because the method you choose for making a salt depends entirely on whether your starting materials and your product are soluble or insoluble. Third, the three preparation methods: crystallising a soluble salt from an insoluble reactant, running a titration when both reactants are solutions, and precipitating an insoluble salt out of two solutions. Fourth, the theory underneath it all: acids and bases defined by proton transfer — an idea that explains why ammonia, with no hydroxide ions of its own, still neutralises an acid.