Indicators & the pH Scale

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Identifying Acids

It is not possible to observe directly whether a substance releases hydrogen ions or not. So we need other methods of identifying acids.

Indicators are chemicals, made from dyes extracted from plants, which change colour when placed in an acidic solution.

Litus is a chemical extracted from plants called lichens, and it is either blue or red. Litmus is usually a strip of paper. Blue litmus indicator changes to pink or red when placed in an acidic solution.

When Universal Indicator, which comes as either a solution or a paper strip, is placed in acidic solutions it turns red or pink or orange or yellow. The colour Universal Indicator turns can be used to identify the strength of an acid solution. Red indicates a very strong acid, pink a strong acid, orange a weak acid and yellow a very weak acid.

Measuring Acidity on the pH Scale

A more accurate scale for measuring the acidity of solutions has been developed - the pH scale. pH indicates the abundance of hydrogen ions in a solution. The pH scale runs from 0 to 14. A solution with pH = 0 is the strongest acidic solution you can get. Solutions with pH's from 0 to 6 are acidic, but the acidity of solutions decreases as the pH number rises.

A solution with pH = 7 is said to be neutral. This is the pH of pure water.

Solutions with pH's above 7 are called basic. As you move from 8 to 18 solutions get more basic. A solution with pH = 14 is the most basic solution you can get. A base is a substance which is the chemical opposite of an acid. A base will remove 'acid particles' (hydrogen ions) from a solution. The colour Universal Indicator turns can be used to identify the pH of a solution.

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