Soluble salts: Soluble salts can be made from acids by reacting them with solid insoluble substances, such as metals, metal oxides, hydroxides or carbonates. The solid is added to the acid until no more reacts and the excess solid is filtered off to produce a solution of the salt.

The idea

A soluble salt can be prepared by reacting an acid with an insoluble solid: a metal, metal oxide, hydroxide or carbonate. The solid is added in small portions to the warm acid and stirred until no more dissolves, so the acid is fully used up and a little solid remains. That excess solid is then removed by filtration, leaving a solution containing the salt. The method works because the solid is insoluble while the salt is soluble, so unreacted solid can be separated physically. For example, copper(II) oxide is added to warm dilute sulfuric acid until excess black powder settles; filtration gives copper(II) sulfate solution. The solution can then be crystallised to obtain the solid salt.

What earns the marks

• Choose an insoluble reactant that matches the required salt, for example a metal, metal oxide, hydroxide or carbonate of the appropriate metal.
• Add the solid in small portions to the acid, warming if needed, and stir until no more dissolves.
• Recognise that excess solid is added deliberately so that all the acid reacts and the solution is neutral.
• Filter the mixture to remove unreacted excess solid, collecting the salt solution as the filtrate.
• Identify that the salt remains in solution because it is soluble, while the excess reactant is removed as residue.
• Link the method to a later step such as crystallisation or evaporation to obtain the solid salt.

Common mistakes and how to put them right

• Adding all the solid at once and assuming the reaction is complete.

  Put it right: Add portions gradually and stop when solid remains undissolved, showing the acid is used up.

• Filtering before excess solid has been added, leaving unreacted acid in the solution.

  Put it right: Continue adding solid until no more reacts, then filter.

• Choosing a soluble reactant such as sodium hydroxide solution, which cannot be removed by filtration.

  Put it right: Select an insoluble metal, oxide, hydroxide or carbonate for this method.

• Confusing residue and filtrate.

  Put it right: The excess solid is the residue on the filter paper; the salt solution is the filtrate.

Exam tips

• Name the specific reactants and the salt produced, rather than describing the method only in general terms.
• Explain the purpose of each step: excess solid ensures complete reaction; filtration separates unreacted solid from the salt solution.
• Use the terms residue and filtrate precisely when describing separation by filtration.
• If asked why the mixture is warmed or stirred, link this to increasing the rate of reaction and helping the solid dissolve.