Mammoth Memory

Acids, alkalis, bases and salts: a summary


An acid is a compound that has dissolved in water and has a pH of less than 7.

Properties of acids:

  • Will always be in a water solution
  • Can be corrosive
  • Turn litmus paper red
  • Weak acids turn universal indicator yellow, strong acids turn it red
  • React with bases or alkalis to make water and a salt. This is called neutralisation.
  • Chemical formula usually start with an H.

Examples of acids:



A base is any substance that can neutralise an acid.        

Properties of bases:

  • Usually compounds contain a metal, but not always (ammonia is a base but does not contain metal).
  • Can be caustic (can burn your skin if handled). E.g. if you rub washing powder into your skin, it will burn
  • May or may not dissolve in water (if a base dissolves in water, it is also an alkali)
  • React with acids to make water and a salt. This is called neutralisation.

Examples of bases:



Alkali (solutions) are bases that have dissolved in water (some bases don’t dissolve in water). They have a pH of more than 7.

Properties of alkalis

  • Can be corrosive
  • Turn litmus paper blue
  • Weak alkalis turn universal indicator blue, strong alkalis turn purple
  • React with acids to make water and a salt. This is called neutralisation.
  • Chemical formulae usually end with -OH

Examples of alkalis:

  • Sodium hydroxide
  • Ammonium hydroxide
  • Bleach (sodium hypochlorite)

Don’t get confused with…

Alkali metals

Alkali metals react with water, but are not alkalis themselves. They are alkali metals – the word ‘metal’ is vital to define what they are. When they react with water they create alkalis by making a brand new compound that dissolves in the water (they also let off hydrogen gas).



Alkaline is a word used to describe a solution with a pH of more than 7. It is the opposite of ‘acidic’.

There is technically no such thing as ‘an alkaline’.

In this context, the word ‘alkaline’ should be followed by ‘solution’ to be clear (however the word ‘alkaline’ can also be used to describe an ‘alkaline substance’ or ‘alkaline soil’ etc.)

Don’t get confused with…

Alkaline earth metals

This is a group in the periodic table that can react with other substances to create bases. They are not alkalis. The words ‘earth’ and ‘metal’ are vital in defining what they are.



A salt is a compound that is created by the neutralisation reaction between an acid and a base (or alkali).

Salts cannot be created in any other way

The bonds between the two elements in a salt are always ionic.

Examples of salts:

  • Table salt (sodium chloride)
  • Epsom salt (magnesium sulphate)
  • Potassium nitrate
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