Making Salts (Oxford AQA IGCSE Combined Science Double Award)

Making Salts

Using metals

• Not all metals are suitable to make salts

• Metals such as copper which are below hydrogen in the reactivity series will not react with acids

• It is too dangerous for reactive metals such as sodium to be mixed with acids

  • The reaction is explosive

• Zinc is an appropriate metal to make a salt

zinc + hydrochloric acid → zinc chloride + hydrogen

Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)

• In this reaction the hydrogen ion in the acid is replaced by the metal to form the salt

Using bases

• A salt is a compound that is formed when the hydrogen atom in an acid is replaced by a metal

• For example, if we replace the H in HCl with a potassium atom, then the salt potassium chloride is formed, KCl

• Salts are an important branch of chemistry due to the varied and important uses of this class of compounds

• These uses include fertilisers, batteries, cleaning products, healthcare products and fungicides

• Some salts can be extracted by mining but others need to be prepared in the laboratory

Adding acid to an insoluble base or insoluble carbonate

 Method

• Add dilute acid into a beaker and heat using a Bunsen burner flame

• Add the insoluble metal, base or carbonate, a little at a time, to the warm dilute acid and stir until the base is in excess (i.e. until the base stops disappearing and a suspension of the base forms in the acid)

• To check the acid has been neutralised touch the glass rod onto indicator paper

• Filter the mixture into an evaporating basin to remove the excess base to form the salt solution

copper(II) oxide + sulfuric acid → copper(II) sulphate + water

CuO (s) + H₂SO₄ (aq) → CuSO₄ (aq) + H₂O (l)

Using titrations

• Titrations can be used to prepare a soluble salt e.g. sodium chloride from an acid and alkali

• The acid and alkali are reacted together in a neutralisation reaction

• When the acid and alkali are completely neutralised only a salt and water will be present in the solution

hydrochloric acid + sodium hydroxide → sodium chloride + water

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

Steps to prepare a soluble salt

 Method:

• Use a pipette to measure a fixed volume of alkali into a conical flask and add a few drops of an indicator 

• Add the acid into the burette and note the starting volume

• Add the acid very slowly from the burette to the conical flask until the indicator changes to the appropriate colour

• Note and record the final volume of acid in the burette and calculate the volume of acid added (final volume of acid - initial volume of acid)

• Add this same volume of acid into the same volume of alkali without the indicator to form the salt solution

How to Make Salt Crystals

• Once the salt solutions are formed solid salts can be formed by crystallisation

• This is done by directly heating the solution until salt crystals begin to appear

• This forms a saturated solution

  • To check the solution is saturated dip a cold, glass rod into the solution and see if crystals form on the end

• The filtrate can be left in a warm place to dry and crystallise

• There may be some solution remaining, so this can be carefully poured away and the crystals can be blotted dry with paper towel

Making Salt Crystals From a Salt Solution