CIE IGCSE Chemistry

Revision Notes

8.2.1 Preparation of Salts

Preparation, Separation & Purification of Salts

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
  • Other examples:
    • Sodium hydroxide reacts with hydrochloric acid to produce sodium chloride
    • Zinc oxide reacts with sulfuric acid to produce zinc sulfate

Preparing salts

  • Some salts can be extracted by mining but others need to be prepared in the laboratory.
  • There are two key ideas to consider when preparing salts:
    • Is the salt being formed soluble or insoluble in water?
    • Is there water of crystallisation present in the salt crystals?

Solubility of the common salts

Solubility of the common salts table, IGCSE & GCSE Chemistry revision notes

 

Preparing soluble salts

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

Preparation-of-soluble-salts, IGCSE & GCSE Chemistry revision notesDiagram 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

 

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

Equation of reaction:

Copper (II) Oxide + Dilute Sulfuric Acid → Copper (II) Sulphate + Water
CuO (s) + H2SO4 (aq) → CuSO4 (s) + H2O (l)

 

Method B: reacting a dilute acid and alkali

Using-titration-to-prepare-a-salt1, IGCSE & GCSE Chemistry revision notesDiagram 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 (phenolphthalein 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 appropriate colour
  • Note and record the final volume of acid in 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 to partially evaporate, leaving a saturated solution
  • Leave to crystallise decant excess solution and allow crystals to dry
Extended Only

Preparing Insoluble Salts

  • Insoluble salts can be prepared using a precipitation reaction
  • The solid salt obtained is the precipitate, thus in order to successfully use this method the solid salt being formed must be insoluble in water

Using two soluble reactants

Preparing-Insoluble-Salts, IGCSE & GCSE Chemistry revision notesDiagram showing the filtration of the mixture to remove the precipitate

 

Method:

  • Dissolve soluble salts in water and mix together using a stirring rod in a beaker
  • Filter to remove precipitate from mixture
  • Wash filtrate with water to remove traces of other solutions
  • Leave in an oven to dry

 

Preparation of Pure, Dry Lead (II) Sulfate Crystals using a precipitation reaction

 Soluble Salt 1 = Lead (II) Nitrate        

Soluble Salt 2 = Potassium Sulfate

Method:

  • Dissolve Lead (II) Nitrate and Potassium Sulfate in water and mix together using a stirring rod in a beaker
  • Filter to remove precipitate from mixture
  • Wash precipitate with water to remove traces of potassium nitrate solution
  • Leave in an oven to dry

Equation of reaction:

Lead (II) Nitrate + Potassium Sulfate → Lead (II) Sulfate + Potassium Nitrate
Pb(NO3)2  (s) + K2SO4 (s) → PbSO4 (s) + 2KNO3 (s)
Extended Only

Selecting a Method of Preparation

Selecting a method of preparation

  • When deciding the method of preparation, it is important to first know whether the salt being produced is soluble or insoluble
  • If it is soluble then it can be prepared using either method (A or B) for preparing a soluble salt
  • If it is insoluble then it must be prepared by using precipitation

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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