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4.4 - Chemical changes
4.4.2 - Reactions of acids
4) The pH scale and neutralisation
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When substances dissolve in water, they
dissociate
into their individual ions:
Hydroxide
ions,
OH-
(aq), make solutions alkaline.
Hydrogen
ions,
H+
(aq), make solutions acidic.
The pH scale is a measure of the acidity or
alkalinity
of an
aqueous
solution:
A solution with a pH of
7
is neutral.
Aqueous solutions with a pH less than 7 are
acidic.
The closer to a pH of
zero
, the
stronger
the acid.
Aqueous solutions with a pH of more than 7 are
alkaline.
The closer to a pH of 14, the
stronger
the
alkali.
The pH of a solution can be measured using a
pH probe
or
universal indicator.
Indicators are dyes that change colour depending on whether they are in
acidic
or
alkaline
solutions:
Litmus
changes colour from red to blue or vice versa.
Universal indicator
is a mixture of dyes that shows a range of colours to indicate how acidic or alkaline a substance is.