Chapter 5 : Acids & Alkalis - Indicators

Dear students,

Today we will be learning about INDICATORS. What are indicators actually? If you search from the dictionary, indicators means "to point out" or "to specify something". Those are very general definitons of what the word indicator is. However, in Science, when you mention an indicator, it gives a whole different meaning to it.

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Indicators

There is a device called a pH meter which you can dip into a liquid and which will measure its pH. However, it is easier to use an indicator.

An indicator is a chemical that changes colour. Indicators are liquids, but they can be soaked into a type of paper similar to blotting-paper to form strips of indicator paper.

Just dip the indicator paper into the unknown liquid and it will change colour to show the pH.

The simplest indicator is Litmus (from the Litmus plant). It goes red in acids, blue in alkalis.

pH = less than 7	pH = more than 7

Universal indicator is a better indicator than litmus because it can show a greater range of colours. The chart below shows the approximate colour that universal indicator goes when put into liquids of different pH values.

pH = 3 or less	pH = 4 or 5	pH = 6	pH = 7 (neutral)	pH = 8 or 9	pH = 10 or more

Universal indicator is available as a liquid or soaked into absorbing paper. It is often used to work out the pH of soil samples to see which plants can be grown in a certain patch of ground.

There are other indicators, for example, methyl orange (which changes colour from colourless to orange at pH of about 3). They all have different pH values where they change colour, and each has its own uses.

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If you want to find out more on indicators, click on this website What are some natural acid/base indicators?

Remember, if you have fun learning about Science, you will learn about Science!