When a litmus solution is neither acidic nor basic (neutral), its colour is:

Litmus solution is purple in colour when it is neutral, that is, when it is neither acidic nor basic. Litmus is the most common natural indicator used to test whether a substance is an acid or a base.

Litmus is a natural dye extracted from lichens, small organisms formed by a partnership between a fungus and an alga. It is used either as a solution or soaked into strips of paper (litmus paper). Its colour changes depending on whether the solution is acidic or basic:

In an acid, blue litmus turns red.

In a base, red litmus turns blue.

In a neutral solution, such as pure water or a sodium chloride solution, there is no change, and the litmus solution remains purple.

An indicator is a substance that shows, by a change in colour (or smell), whether a substance is acidic or basic. Other indicators include:

Natural indicators: turmeric (turns reddish-brown in a base), red cabbage juice and the petals of flowers such as hibiscus (China rose), which turn green in a base and dark pink or magenta in an acid.

Synthetic indicators: phenolphthalein (colourless in an acid, pink in a base) and methyl orange (red in an acid, yellow in a base).

Olfactory indicators: substances such as onion, vanilla and clove oil, whose smell changes in a base.

A universal indicator is a mixture of indicators that shows different colours for different pH values, from red (strongly acidic) through green (neutral, pH 7) to violet (strongly basic). The pH scale, which runs from 0 to 14, was introduced by the Danish chemist S.P.L. Sørensen in 1909.

Why not the other options?

• ✗Red is the colour of blue litmus in an acid.
• ✗Blue is the colour of red litmus in a base.
• ✗Green is the colour of a universal indicator at neutral pH, not of litmus.