Litmus tells us three things

On 28 February, National Science Day, Ashwin and Keerthi went to their school science fair. At the gate each got a plain white sheet of paper. A volunteer sprayed a liquid on it and, like magic, the words Welcome to the Wonderful World of Science appeared. By the end of this chapter you will know the trick.

In Activity 2.1 you put one drop of each of these on blue litmus paper and on red litmus paper: lemon juice, soap solution, amla juice, tamarind water, vinegar, baking soda solution, lime water, tap water, washing powder solution, sugar solution and salt solution.

Litmus is a natural dye that comes from lichens. A lichen is two living things living together: a fungus and an alga. The alga is green and makes food from sunlight; the fungus gives it shelter and protection. Each helps the other. Lichens grow on rocks and trees where there is plenty of rain and clean air. Litmus comes as a solution and as paper strips, in two colours: blue and red.

The drops sort into three groups:

A memory trick: B for B. A base turns red litmus blue.

Litmus shows one colour in acids and another in bases, so it is called an acid-base indicator. Some indicators come from nature; others, called synthetic indicators, are made in laboratories, and you will meet them in higher grades.

One word to keep apart: lime water is not lemon (nimbu) juice. It is made by mixing lime (chuna) in water, leaving it for about an hour, and filtering it.