Preparing Soluble Salts (Cambridge International IGCSE Science)

Spec point — Describe the preparation, separation and purification of soluble salts by reaction of an acid with: an alkali by titration excess metal excess insoluble base excess insoluble carbonate

Preparing soluble salts

How to name a salt

• The name of salt has two parts:

  1. The first part comes from the metal, metal oxide or metal carbonate used in the reaction
  2. 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

What is a salt?

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

  • For example, replacing H in HCl with potassium gives potassium chloride, KCl

• Salts have many uses including:

  • Fertilisers
  • Batteries
  • Cleaning products
  • Healthcare products
  • Fungicides

• The method used depends on:

  • The solubility of the salt being prepared
  • Whether the base is insoluble or soluble (alkali)

Preparing soluble salts

• There are two main methods of preparing a soluble salt:

• Method A: Using excess solid reactant

  • This includes three reactions:

    1. Using excess metal
    2. Using excess insoluble base
    3. Using excess insoluble carbonate

• Method B: Using an alkali (by titration)

  • This is for soluble bases such as sodium hydroxide

• The method used depends on:

  • Whether the base is insoluble or soluble (alkali)
  • Whether a metal is used

Method A: Using excess solid reactant

This method is used when the reactant is a solid in excess. Any unreacted solid is removed by filtration, then the salt is crystallised.

Method A1: Using excess metal

• This is for metals that react with dilute acids

1. Add dilute acid to a beaker
2. Add the metal in small pieces while stirring until no more reacts

• This means that the metal is in excess

1. Filter the mixture to remove the excess metal
2. Transfer the filtrate (salt solution) to an evaporating basin
3. Heat the evaporating basin gently until the solution is concentrated

• Check the solution is saturated by dipping a cold, glass rod into the solution and seeing if crystals form on the end

1. Leave the basin in a warm place to crystallize
2. If necessary, decant any excess liquid
3. Dry the crystals with filter paper

Example reaction - preparing magnesium sulfate crystals:

magnesium + sulfuric acid → magnesium sulfate + hydrogen

Mg (s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)

Method A2: Using excess insoluble base

• This is for insoluble bases such as metal oxides

1. Warm dilute acid gently in a beaker
2. Add the insoluble base slowly while stirring until no more reacts

• This means that the insoluble base is in excess

1. Filter the mixture to remove the excess insoluble base
2. Transfer the filtrate (salt solution) to an evaporating basin
3. Heat the evaporating basin gently until the solution is concentrated

• Check the solution is saturated by dipping a cold, glass rod into the solution and seeing if crystals form on the end

1. Leave the basin in a warm place to crystallize
2. If necessary, decant any excess liquid
3. Dry the crystals with filter paper

*Diagram showing the preparation of soluble salts *

• Example reaction - preparing copper(II) sulfate:

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

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

Method A3: Using excess insoluble carbonate

• This is for metal carbonates that do not dissolve in water

• The method is exactly the same as using excess insoluble base:

  • Warm dilute acid gently in a beaker
  • Add the insoluble carbonate slowly while stirring until no more reacts (carbonate in excess)
  • Filter to remove the excess insoluble carbonate
  • Transfer the filtrate (salt solution) to an evaporating basin
  • Heat the evaporating basin gently until the solution is concentrated
  • Leave to crystallize, decant any excess liquid, and dry the crystals

• Example reaction - preparing calcium chloride:

calcium carbonate + hydrochloric acid → calcium chloride + carbon dioxide + water

CaCO₃(s) + 2HCl (aq) → CaCl₂(aq) + CO₂(g) + H2O (l)

Method B: Using an alkali (by titration)

• This is for soluble bases such as sodium hydroxide

1. Use a pipette to place alkali in a conical flask
2. Add a few drops of indicator (e.g. phenolphthalein)
3. Fill a burette with the acid and record the starting volume
4. Add the acid slowly while swirling until the indicator changes colour (end point)

• Record the final volume

1. Repeat the titration without indicator, using the same measured volume of acid
2. Heat the neutral solution in an evaporating basin until concentrated

• Check the solution is saturated by dipping a cold, glass rod into the solution and seeing if crystals form on the end

1. Leave the basin in a warm place to crystallize
2. If necessary, decant any excess liquid
3. Dry the crystals with filter paper

Diagram showing the apparatus needed to prepare a salt by titration

• Example reaction - preparing sodium chloride crystals:

sodium hydroxide + hydrochloric acid → sodium chloride + water

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

Exam Hint Use Method B (titration) if the base is soluble (alkali)

Use Method A (excess solid) if adding a solid in excess, then filtering and crystallising:

• A1 for excess metal
• A2 for excess insoluble base
• A3 for excess insoluble carbonate