Base - GCSE Chemistry Definition

What is a base?

In GCSE Chemistry, a base is any substance that reacts with an acid to form water and a salt only.

base + acid → water + salt

Common examples of bases are metal oxides like magnesium oxide and metal hydroxides like sodium hydroxide.

magnesium oxide + hydrochloric acid → water + magnesium chloride

sodium hydroxide + sulfuric acid → water + sodium sulfate

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

base + acid ⟶ water + carbon dioxide + salt

magnesium carbonate + nitric acid → water + carbon dioxide + magnesium nitrate

Neutralisation reactions

Acids have a pH below 7 and bases have a pH above 7. A neutralisation reaction happens when an acid reacts with a base or an alkali to make a solution that is neutral, pH 7. In these reactions, water is always made.

• Acids contain hydrogen ions, H⁺.

• Bases or alkalis contain hydroxide ions, OH⁻.

When they react, these ions join to make water:

H⁺ (aq) + OH^– (aq) ⟶ H₂O (l)

This equation is often called the ionic equation of neutralisation. It is the same for all neutralisation reactions.

What is an alkali?

If a base dissolves in water, it is called an alkali. Bases dissolve in water to form alkaline solutions.

For example:

• Copper(II) oxide, CuO

  • Is a base

  • Is not an alkali because it does not dissolve in water

• Sodium hydroxide, NaOH

  • Is a base

  • Is also an alkali because it dissolves in water

Base Revision Resources to Ace Your Exams

To explore cracking further, you should check out the revision notes on Acid-Base Reactions bases and practise our exam questions to ensure success. Flashcards on acid are also a great way to revise the wider topic of hydrocarbons before an exam. 

Explore our GCSE Chemistry revision resources.