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Indicators are substances that show a colour change when acids or alkalis are added to them. The most common acid-base indicators are,
  • Litmus paper
  • Phenolphthalein
  • Methyl orange
Test with a litmus paper:
The most common indicator is litmus, which is derived from lichens. We mostly use it in the form of litmus paper, which comes in three colours: blue, red and purple.
Acids convert blue litmus into red, while base or alkali converts red litmus into blue, as shown below:
Test for acid and base using litmus papers
When a red litmus paper is immersed in an acidic solution, it has no effect. Similarly, when a blue litmus paper is immersed in a basic solution, it has no effect, as shown below.
No effects on the litmus
When litmus papers are used to test a neutral solution, the same result (as shown in the above picture) occurs.
Neutral solutions are those in which the concentrations of hydrogen and hydroxide ions are the same. Examples, sodium chloride solution, sugar solutions and water.
Note: Litmus paper is usually used to determine the pH of liquids. Litmus does not react with solid substances in the majority of cases. However, the paper can be used for solids when the litmus is moistened with distilled water before exploring the substance. On the other hand, we make a solution of the given sample using distilled water. So, we can use litmus paper to perform an acid-base test.
Test with phenolphthalein indicator:
Phenolphthalein is colourless in an acid medium. The colour of phenolphthalein in a basic medium is pink.
Test for acid and base using phenolphthalein indicator
Test with methyl orange indicator:
Methyl orange is pink in an acid medium. Methyl orange appears yellow in a basic medium.
Test for acid and base using methyl orange indicator
Acid-base indicators:
Colour in acid
Colour in base
Litmus paper
    Blue → Red
Red → Blue
Methyl orange
Other natural indicators are turmeric powder, and China rose.