Acids and Bases

  • Acid + Base -> Salt + Water


  • HCl + NaOH -> NaCl + H2O


  • Hydrochloric acid + Sodium Hydroxide -> Salt + Water


A Neutralisation reaction is one when an acid and a base react to form a salt.


Acids cause:

  • Lemons to be sour

  • Acid rain to dissolve sculptures

  • Cavities in your teeth

  • Digestion of food in the stomach

Properties of acids:

  • Tastes sour

  • Turns litmus paper red

  • Lemon juice (citric acid), vinegar (ethanoic acid)

Example of acids:

  • Hydrochloric acid

  • Sulfuric acid

  • Carbonic acid (fizzy drinks)

  • Ethanoic acid (vinegar)

  • Citric acid (lemon)


Properties of bases:

  • Tastes bitter

  • Turns litmus paper blue

  • pH greater than 7 

Examples of bases:

  • Sodium hydroxide (NaOH) - caustic soda

  • Calcium hydroxide (Ca(OH)2) - limewater when dissolved


  • an alkaline is a base that can dissolve in water


Arrhenius definition:


  • An acid is a substance that dissociates in water to produce H+ ions


  • HCl -> H+ + Cl-


General formula: 


  • HA -> H+ + A-

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Acids - 

  • Monobasic acids - HCl and HNO3 produce 1 H+ ion

  • Dibasic acids - H2SO4 produces 2 H+ ions

  • Tribasic acids - H3PO4 produces 3 H+ ions


  • Strong acids fully dissociate in water

  • Weak acids dissociate slightly in water

  • Hydrogen ions react with water to form a hydronium ion



  • A base is a substance that dissociates in water to produce OH- ions


  • NaOH -> Na+ + OH-


General formula:


  • XOH -> X+ + OH-


Problems with Arrhenius definition:

  • Hydronium ions exist rather than hydrogen ions

  • Definition is restricted to aqueous solutions

  • Doesn’t work for all acids and bases

  • Ammonia for example (Polar, and so it water)

Brønsted-Lowry theory:

  • An acid is a proton donor

  • A base is a proton acceptor


  • Strong acid - good donor

  • Weak acid - poor donor

  • Strong base - good acceptor

  • Weak base - poor acceptor


Water is polar - uneven distribution of electrons

Ammonia is slightly polar


Amphoteric substance - can act as an acid (donor) or a base (acceptor)


Advantages of Brønsted-Lowry theory:

  • No water present so no hydronium ions are formed

  • Not limited to aqueous solutions

  • Broader range of substances can be classified

  • Explains how substances can be amphoteric (behave as both acids and bases)