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C3: Strong and Weak Acids and Alkalis
Acids and alkalis vary in strength as well as concentration.
Titrations can be used to find the amounts of acid or alkali in a
An acid can be defined as a proton donor. A base can be defined
as a proton acceptor.
Water must normally be present for a substance to act as an acid
or as a base.
Acids produce hydrogen ions in aqueous solution. The H+ ion is a
proton. In water this proton is hydrated and is represented as
Alkalis produce hydroxide ions, OH-(aq), in aqueous solutions.
Acids and alkalis are classified by the extent of their ionisation in
A strong acid or alkali is one that is completely ionised in water.
Examples of strong acids are hydrochloric, sulfuric and nitric
acids. Examples of strong alkalis are sodium and potassium
A weak acid or alkali is only partially ionised in water.
Examples of weak acids are ethanoic, citric and carbonic acids.
An example of a weak alkali is ammonia solution.
The volumes of acid and alkali solutions that react with each other
can be measured by titration using a suitable indicator:
strong acid + strong alkali any acidbase indicator
strong acid + weak alkali methyl orange indicator
weak acid + strong alkali phenolphthalein indicator.
If the concentration of one of the reactants is known, the results of
a titration can be used to find the concentration of the other