Properties of acids and bases

Acids (aq)

  • Are electrical conductors
  • Turn blue litmus red
  • Have a sour taste
  • React with active metals to form hydrogen gas and a salt
  • React with carbonates and hydrogencarbonates to form CO2, a salt and water
  • React with metal oxides and hydroxides to form water and a salt

Bases (aq)

  • Are electrical conductors
  • Turn red litmus blue
  • Neutralise acids
  • Taste bitter
  • Feel slippery

Arrhenius Theory

  • Acids are substances that release H+ ions in aqueous solution
  • HCl (aq) –> H+(aq) + Cl (aq)
  • Bases are substances that release OH ions in aqueous solution
  • NaOH(aq) –> Na+(aq) + Cl (aq)
  • Arrhenius’ theory is limited: it cannot explain bases such as carbonates or ammonia that react with acids but do not form OH ions in solution.

Bronsted-Lowry Theory

  • Acids are substances that will donate a proton.
  • Bases are substances that will accept a proton.
  • The acid will react with the base to form a conjugate base and a conjugate acid.


  • HCl(aq) + H2O (l) Cl (aq) + H3O+(aq)
  • HCl donates a proton, therefore it is the acid, forming the conjugate base, Cl
  • H2O accepts a proton, therefore it is the base, forming the conjugate acid, H3O+
  • This reaction is reversible – if it reversed, HCl would be the conjugate acid, while Cl is the base etc.
  • An acid forms a conjugate base when it donates a proton.
  • A base forms a conjugate acid when it accepts a proton.
  • In the example, HCl and Cl is a conjugate acid-base pair.


Weak acids yield strong conjugate bases.

Strong acids yield weak conjugate bases.

 Terms and definitions

  • Strong = completely ionise/dissociate in aqueous solution
  • Weak = partially ionises/dissociates in solution
  • Concentrated = high number of particles per volume
  • Dilute = a low number of particles per volume


  • Multiprotic acids
  • Monoprotic acids are able to donate only one proton – HCl.
  • Diprotic can donate two – H2SO4
  • Triprotic – H3PO4

Amphiprotic or amphoteric substances are those that can act both as a base and an acid, depending on the circumstances.

Self-ionisation of water

Water self-ionises:

2 H2O ⇌ H3O+ + OH

A neutral solution is defined as one which the hydrogen ion and the hydroxide ion concentrations are equal.

At 25 oC, this occurs at pH 7.

However, if temperature changes, the solution may be neutral at a pH other than 7.

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