Making Salts (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Neutralisation

• When an acid reacts with a base, a neutralisation reaction occurs

• A base is a chemical that neutralises acids

• Examples of bases include:

  • Metal oxides, e.g. copper(II) oxide, CuO

  • Metal hydroxides, e.g. sodium hydroxide, NaOH

  • Metal carbonates, e.g. calcium carbonate, CaCO₃ 

• Bases have pH values above 7

• Many bases are insoluble in water

• Some bases dissolve in water and are called alkalis because they form an alkaline solution

  • Examples of alkalis are soluble metal hydroxides such as sodium hydroxide, NaOH (aq), and calcium hydroxide, Ca(OH)₂ (aq)

Neutralisation reactions

• In all acid-base neutralisation reactions, salt and water are produced:

acid + base ⟶ salt + water

• If the base is a metal carbonate, carbon dioxide is also produced:

acid + base ⟶ salt + water + carbon dioxide

• The identity of the salt produced depends on the acid used and the positive ions in the base

  • Hydrochloric acid produces chlorides

  • Sulfuric acid produces sulfates

  • Nitric acid produces nitrates

Reactions of acids with metal oxides

• Metal oxides also act as bases by reacting with acids in a neutralisation reaction to produce the corresponding metal salt and water

• The following are some specific examples of reactions between acids and metal oxides:

2HCl + CuO ⟶ CuCl₂ + H₂O

H₂SO₄ + Na₂O⟶ Na₂SO₄ + H₂O

2HNO₃ + MgO ⟶ Mg(NO₃)₂ + H₂O

Reactions of acids with metal hydroxides

• Metal hydroxides also act as bases by reacting with acids in a neutralisation reaction to produce the corresponding metal salt and water

• The following are some specific examples of reactions between acids and metal hydroxides:

HCl + LiOH ⟶ LiCl + H₂O

H₂SO₄ + 2NaOH ⟶ Na₂SO₄ + 2H₂O

HNO₃ + KOH ⟶ KNO₃ + H₂O

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

• Hydrochloric acid always produces salts that end in chloride and contain the chloride ion, Cl^–

• Sulfuric acid always produces salts that end in sulfate and contain the sulfate ion, SO₄^2–

• Nitric acid always produces salts that end in nitrate and contain the nitrate ion, NO₃^–

Naming salts examples

• Salts have no overall charge since the sum of the charges on the ions is equal to zero

• If you know the ions present in a salt you can identify the formula from balancing the charges

Naming salts using ions table

Preparing soluble salts

• A soluble salt can be made from the reaction of an acid with an insoluble base

• During the preparation of soluble salts, the insoluble reactant is added in excess to ensure that all of the acid has reacted

• If this step is not completed, any unreacted acid would become dangerously concentrated during evaporation and crystallisation

• The excess reactant is then removed by filtration to ensure that only the salt and water remain

• Since all of the acid has reacted and the excess solid base has been removed then the solution left can only be salt and water

• The water is evaporated by heating until small crystals begin to appear

  • This is typically once half of the water is left 

  • Allowing the filtered solution to evaporate slowly over a period of days results in the formation of larger crystals 

• If a carbonate was used as the solid base instead of an oxide or hydroxide, then any carbon dioxide gas produced would have been released into the atmosphere

• A common example is the preparation of copper(II) sulfate which can be made with copper(II) oxide and dilute sulfuric acid:

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

Copper sulfate crystals

Photo by Wikimedia commons (opens in a new tab)

• Acids can react with metal to produce the salt if:

  • The metal is above hydrogen in the reactivity series

  • The metal is not too reactive, which could result in a dangerous reaction