Indicators

History of Indicators

The ancient Greeks and Romans spent lots of time trying to understand the World around them, and part of that time was spent trying to classify substances together into groups.

One of those groups were all the substances that tasted sour. In Latin (the language of ancient Rome) this group was called Acere, which means 'to make sour' and is where we get the word Acid.

Another one of those groups were all the substances that felt slippery. In Arabic (the language of the Arabian countries) this group was called Al-Qaliy, which means 'from the ashes' (ash is a source of a slippery chemical called Potash) and is where we get the word Alkali.


Indicators

Obviously either tasting or touching a chemical to find out what it is is a really bad idea. However, a much better method was discovered by fabric makers in France in the 16th century.

The fabric makers found that by adding certain chemicals to the dyes they were using, different colours could be made. When one chemical was added, the dye turned bright red, when a different chemical was added, the dye turned blue-green. The fabric makers didn't care why this happened, they were just happy it did!

A scientist called Robert Boyle wanted to know how the dye worked, however. He knew the fabric makers used plant dyes, so he started to experiment with different dyes and chemicals to see what happened.

When using the purple juice found in red cabbages, Boyle found that if it was mixed with an Acid, the juice went bright red, and when mixed with an Alkali, the juice turned blue-green. Boyle had found a way to test if a substance was an Acid or an Alkali using cabbage juice!

Litmus Paper

Boyle continued to work with the plant juice as an indicator, but found that it wasn't a simple way of doing it as the juice went bad quickly and kept getting spilt. He decided to soak paper in the juice of a plant called Lichen and then dry it out. The paper could then be used to test substances because it still changed colour. Boyle called this Litmus Paper.

In the image above, the Litmus Paper is being dipped into an Alkali.

Universal Indicator and the pH Scale

Knowing that a substance is an Acid or an Alkali is important, but it is also important to know how strong an Acid or Alkali is.

For example -

Coca Cola and Battery Acid are examples of Acid, however, one is safe to drink (Coke) and one is very much not (Battery Acid)!

By mixing several different Indicator chemicals together, it is possible to make an Indicator called Universal Indicator that shows the strength of an Acid or Alkali.

Universal Indicator can change through a total of 14 different colours to show the strength of an Acid or Alkali.

The diagram below shows the pH scale, which allows the colour to be matched to the Acid or Alkali strength:-

The image below shows some common household chemicals and their corresponding pH:-

An Indicator is a chemical which changes colour when added to an Acid or Alkali.


  1. Red cabbage juice turns red in Acids, blue-green in Alkalis and in neutral substances it remains purple.

  2. Litmus Paper turns red in Acids, blue-green in Alkalis and in neutral substances it remains unchanged.

  3. Universal Indicator turns a range different colours depending on whether it is in an Acid or an Alkali. Universal Indicator is the only indicator to show the strength of an Acid or Alkali.