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Acids and Alkalis

What is pH?

pH is a measure of how acidic or how alkaline a solution is.

The pH of a solution depends on the
degree of ionisation of an acid or alkali
as well as how concentrated or dilute it is.
The pH scale goes from 1 to 14
with 1 being very strongly acidic and 14 being very strongly alkaline.
A pH of 7 is neutral.

You can measure the pH of a solution using universal indicator.
Just as litmus paper will be red for an acid and blue for an alkali,
so universal indicator is a mixture of indicators
that will give a different colour for a different pH.

Indicator Colour for pH Scale

Any acid will have a pH of less than 7.
Any alkali will have a pH of more than 7.

Strong acids (HCl or H2SO4 or HNO3) have a pH of 1 (red).

Weak acids have a pH of 3 to 4 (orange).
Examples of weak acids are ethanoic acid (vinegar),
citric acid (lemon juice) and rain water.
Rain water has a natural pH of 5·5 (see carbonic acid).

Water and salts are neutral. They have a pH of 7 (green).

A weak alkali (ammonia) has a pH of 11 to 12 (blue).

A strong alkali (NaOH) has a pH of 14 (purple).

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