Bases and Alkali

Definition of bases:

v  Oxides or hydroxides of metal that reacts with an acid to form a salt and water only

Definition of alkali:

v  A metal hydroxide which is soluble in water and produces hydroxide ions (OH^-) in water

v  All alkalis are bases but not all bases are alkalis

Two types of alkali:

v  Strong alkali

Ø  Is one that ionises/dissociates completely in water to produce hydroxide ions

Ø  No molecules left

v  Weak alkali

Ø  Is one that ionises/dissociates incompletely/partially in water to produce few hydroxide ions

Ø  Most of the alkali molecules remain as molecules

Properties of bases:

v  Have a soapy feel and a bitter taste.

v  Can change the colour of indicators (e.g. turn red litmus paper blue).

v  Bases can react with acids to form salt and water only – Neutralisation.

Reaction with acids:

v  Base + acid → salt + water

v  NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l)

Reaction with ammonium salts:

v  Base + ammonium salt → salt + ammonia + water

v  Example: NaOH (aq) + NH₄Cl (s) → NaCl (aq) + NH₃ (g) + H₂O (l)

v  Observation:

Ø  A colourless, pungent gas is evolved. 

Ø  Gas turns damp red litmus paper blue

Colour change of indicators:

v  Litmus paper: Turns red litmus paper blue

v  Methyl orange: Turns methyl orange yellow

v  Phenolphthalein: Turns pink

v  Universal indicator: Between green to violet