Definition of a Base

Specification Point 3.9:
  • Recall that a base is any substance that reacts with an acid to form a salt and water only.
  • Bases have pH values of above 7.
  • A base which is water soluble is referred to as an alkali.
  • Bases are substances which can neutralise an acid, forming a salt and water only.

Definition of an Alkali

Specification Point 3.10:
  • Recall that alkalis are soluble bases.
  • Many bases are insoluble in water but the ones that do dissolve in water are called alkalis.
  • They thus form an alkaline solution.
  • Examples of alkalis are soluble metal hydroxides such as NaOH and Ca(OH)2.

Reactions of Acids

Specification Point 3.11:
  • Explain the general reactions of aqueous solutions of acids with:
    a) metals
    b) metal oxides
    c) metal hydroxides
    d) metal carbonates to produce 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.

Reactions of Acids and Metals

  • Only metals above hydrogen in the reactivity series will react with dilute acids. 
  • When acids react with metals they form a salt and hydrogen gas:


Reactions of Acids and Metals table, Edexcel GCSE Chemistry

Reactions of Acids with Metal Oxides and Metal Hydroxides

  • Metal oxides and metal hydroxides act as bases
  • When they react with acid, a neutralisation reaction occurs. 
  • In all acid-base neutralisation reactions, salt and water are produced.


Acids Metals reactions table

Reactions of Acids with Metal Carbonates 

  • Acids will react with metal carbonates to form the corresponding metal salt, carbon dioxide and water.


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Morgan Curtin Chemistry

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.