Preparing Soluble Salts (Cambridge O Level Chemistry)

Salts

• 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

Naming salts

• The name of a salt has two parts
  • The first part comes from the metal, metal oxide or metal carbonate used in the reaction
  • The second part comes from the acid
• The name of the salt can be determined by looking at the reactants
• For example hydrochloric acid always produces salts that end in chloride and contain the chloride ion, Cl^-
• • Sodium hydroxide reacts with hydrochloric acid to produce sodium chloride
  • Zinc oxide reacts with sulfuric acid to produce zinc sulfateOther examples:

Preparing salts

• Some salts can be extracted by mining but others need to be prepared in the laboratory
• The method used depends on the solubility of the salt being prepared
  

Preparing soluble salts

Method A: Adding acid to a solid metal, insoluble base or insoluble carbonate

Diagram showing the preparation of soluble salts

 

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)
• Filter the mixture into an evaporating basin to remove the excess base
• Heat the solution to evaporate water and to make the solution saturated. Check the solution is saturated by dipping a cold, glass rod into the solution and seeing if crystals form on the end
• Leave the filtrate in a warm place to dry and crystallize
• Decant excess solution and allow crystals to dry or blot to dry with filter paper

Example: Preparation of pure, hydrated copper(II) sulfate crystals using method A

Acid = dilute sulfuric acid 

Insoluble base = copper(II) oxide

Method:

• Add dilute sulfuric acid into a beaker and heat using a bunsen burner flame
• Add copper(II) oxide (insoluble base), a little at a time to the warm dilute sulfuric acid and stir until the copper (II) oxide is in excess (stops disappearing)
• Filter the mixture into an evaporating basin to remove the excess copper(II) oxide
• Leave the filtrate in a warm place to dry and crystallize
• Decant excess solution
• Blot crystals dry with filter paper

Equation of reaction:

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

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

 Method B: Reacting a dilute acid and alkali (soluble base)

Diagram showing the apparatus needed to prepare a salt by titration

 Method:

• Use a pipette to measure the alkali into a conical flask and add a few drops of indicator (thymolphthalein or methyl orange)
• 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 (starting volume of acid - final volume of acid)
• Add this same volume of acid into the same volume of alkali without the indicator
• Heat the resulting solution in an evaporating basin to partially evaporate, leaving a saturated solution (crystals just forming on the sides of the basin or on a glass rod dipped in and then removed)
• Leave to crystallise, decant excess solution and allow crystals to dry