Preparing Soluble Salts (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

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Preparing soluble salts

What is a salt?

  • 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
  • The method used depends on the solubility of the salt being prepared

How to name a salt

  • The name of 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 soluble salts

  • There are two methods of preparing a solution salt:
    • Method A
      • Adding acid to a solid metal, insoluble base or insoluble carbonate
    • Method B
      • Reacting a dilute acid and alkali (soluble base)

Method A

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

7-2-1-preparation-of-soluble-salts-1

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

  • 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

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

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

Method B

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

7-2-1-titration-and-forming-salt

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
  • Record the starting volume of acid in the burette
  • Add the acid very slowly from the burette to the conical flask until the indicator changes to the appropriate colour
  • Record the final volume of acid in the burette
  • Calculate the volume of acid added
    • Final volume of acid - initial volume of acid
  • Add this same volume of acid to 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

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Caroline

Author: Caroline

Expertise: Physics Lead

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.