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Certain coloured substances (many extracted from plants) have been found to change colour if added to an acid or alkaline solution. The colour change is reversed if the acid or alkali is neutralized. Substances that behave like this are known as indicators. Coloured extracts can be made from red cabbage or blackberries, but probably the most used indicator is litmus. This is extracted from lichens.

Litmus is purple in a neutral solution. When added to an acid solution, it turns red. Changing this red colour of litmus needs a chemical reaction. The molecules of the indicator are usually changed in the presence of the acid. Substances with the opposite chemical effect to acids are needed to reverse the change, and these are called alkalis. They turn litmus solution to blue.

Litmus can also be used in paper form, in which case it is called litmus paper. Here it comes in the blue and red forms. Litmus is a single chemical compound. It gives a single colour change.

Litmus is not the only single indicator that chemists find useful. Others that are used frequently are phenolphthalein and methyl orange. These indicators give different colour changes when in acidic and alkaline solutions.

Another commonly used indicator is the universal indicator (or full-range indicator). This is made from a mixture of dyes. Such an indicator is useful because it gives a range of colours (“spectrum”) depending on the strength of the acid or alkali added.

With a universal indicator, different acids produce a range of different colours. Indeed, solutions of the same acid with different concentrations (pH) give different colours.

The more acidic solutions (for example battery acid) turn the universal indicator bright red. A less acidic solution (for example vinegar) will only turn it orange-yellow. There are also colour differences produced with different alkali solutions. The most alkaline solutions give a violet colour while the less alkaline solutions give a blue colour. Many indicators are extracted from plants. Flowers and leaves of different plants have different colours such as hibiscus flower, red cabbage etc. These plant organs may be used to prepare indicators locally.

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