Litmus: Origin and Forms

Introduction

Litmus is probably the most famous indicator in the world. The phrase "litmus test" is even used in everyday speech to mean a simple test that gives a clear answer. Yet many people do not know that litmus comes from a living thing. This page explains where litmus comes from, the different forms in which it is sold, and why it has been a favourite of chemists and school laboratories for centuries.

Core explanation

Origin: a dye from lichens. Litmus is a mixture of coloured substances extracted from certain lichens . A lichen is not a single plant; it is a partnership between a fungus and an alga (or a cyanobacterium) living together. Lichens grow as crusty, leafy or shrubby patches on rocks, walls and tree bark. Species of Roccella , which grow on rocky sea coasts, have been an important source of litmus.

A long history. Dyes from lichens were used to colour cloth for centuries, and by the 1500s and 1600s people had noticed that these dyes changed colour with acids and alkalis. The chemist Robert Boyle described their use as indicators in the 1600s. Litmus has been used to test acidity ever since.

How the dye is made. Traditionally, the lichens were ground and allowed to ferment slowly in the presence of an alkaline substance and air. This process changes colourless substances in the lichen into the blue-purple pigments of litmus. The dye was then dried into small cakes. Today litmus is made by specialist suppliers under controlled conditions.

The forms of litmus.

Form What it is Colour as supplied Used for --- --- --- --- Litmus solution Dye dissolved in water Purple Adding drops to a sample Blue litmus paper Paper soaked in alkaline litmus and dried Blue Testing for acids Red litmus paper Paper soaked in acidified litmus and dried Red Testing for alkalis Neutral litmus paper Paper soaked in neutral litmus Purple Testing for both, less common

Litmus colours. In an acidic solution litmus is red . In an alkaline solution it is blue . In a neutral solution the dye is purple , a colour between the two.

Why litmus paper is so popular. Paper strips are cheap, light, easy to store and simple to use. Only one drop of the test liquid is needed, so very little sample is used and there is less risk of spills. The paper keeps well if stored in a closed container away from damp air and laboratory fumes.

Step-by-step reasoning

To choose the right form of litmus:

1. Decide what you want to find out: whether a solution is acidic, alkaline, or either. 2. For "is it an acid?", use blue paper, which turns red in acid. 3. For "is it an alkali?", use red paper, which turns blue in alkali. 4. To test a larger liquid sample by colour in a tube, use litmus solution.

Visual explanation

Picture a grey-green crusty lichen on a seaside rock, then a small jar of deep purple liquid, then two strips of paper beside it: one pink-red and one blue. That chain shows the journey from living lichen to dye to the two familiar test papers.

Real-world analogy

Red and blue litmus papers are like two different keys for two different locks. The blue key "opens" only when it meets an acid and turns red. The red key "opens" only when it meets an alkali and turns blue. Carrying both keys lets you test for either.

Real-world example

Gardeners, aquarium keepers and home brewers often keep litmus paper in their kits because it is cheap and needs no equipment. Litmus paper is also used to check moist gases: damp red litmus paper turns blue in ammonia gas, a common laboratory test for that alkaline gas.

Why?

Why are litmus papers supplied in two colours? A strip can only show a change it is able to make. Blue paper is already blue, so it cannot show that a solution is alkaline; it can only turn red. Having both colours lets you detect both acids and alkalis.

Common misconception

"Litmus is a synthetic chemical invented in a factory." Litmus is a natural indicator made from lichens. It was used long before modern synthetic indicators such as phenolphthalein were developed.

Worked example

Question: A student has only blue litmus paper. Can she show that a solution is alkaline?

Reasoning: Blue litmus stays blue in both alkaline and neutral solutions. It only changes, to red, in acid.

Answer: No. Blue litmus can show that the solution is not acidic, but it cannot distinguish alkaline from neutral; she needs red litmus paper as well.

Quick check

1. From what kind of living thing is litmus obtained? Answer: Lichens, which are partnerships of a fungus with an alga or cyanobacterium.

Exam focus

Remember: litmus is red in acid, blue in alkali and purple in neutral solution. Know that it is a natural indicator from lichens and that red and blue papers are used together. Questions often ask what each paper can and cannot show.

Advanced insight

Litmus is not one pure compound but a mixture of several related dyes, including substances called orceins and azolitmin. Because it is a natural mixture, its colour change is gradual and it is less precise than purified synthetic indicators, which is one reason chemists prefer synthetic indicators for accurate work.

Summary

Litmus is a natural indicator made from lichens and has been used for centuries. It is red in acid, blue in alkali and purple when neutral. It is sold as purple litmus solution and as red and blue litmus paper. Blue paper tests for acids; red paper tests for alkalis. It is cheap, simple and widely used.

Practice questions

1. What colour is litmus solution in a neutral solution? Answer: Purple. 2. Which litmus paper would you use to test whether a liquid is acidic, and what change would you see? Answer: Blue litmus paper, which turns red if the liquid is acidic. 3. Give two reasons why litmus paper is popular in schools. Answer: It is cheap and easy to use, and it needs only a drop of sample. 4. Why must litmus paper be stored in a closed container? Answer: Damp air and acidic or alkaline fumes in the laboratory could change its colour and spoil it.