Identify the Nature of Substances Using Indicators

What are Indicators?

Indicators are substances that change their own colour when added to an acidic or alkaline solution. Some of the common indicators used in the laboratory include red litmus, blue litmus, phenolphthalein, and methyl orange.

How Do Indicators Work in Solutions?

Blue litmus paper changes to red in an acidic medium but remains unchanged in a basic medium. Red litmus paper turns blue in a basic medium but remains unchanged in an acid. The colour of phenolphthalein remains colourless in acid, while it becomes pink in a base. Methyl orange is a reddish-orange indicator that turns red in acid but gives a yellow colour in a base.

To perform this experiment, apparatus and chemicals are required: Test tubes, HCl, H₂SO₄, CH₃COOH, NaOH, Ca(OH)₂, NH₄OH, and Lemon juice.

Procedure

We have to follow the given procedure to identify the acidic and basic nature of substances using different indicators.

• Take four test tubes and label them ‘A’, ‘B’, ‘C’ and ‘D’, and place them in the test tube stand.
• Put small strips of blue litmus paper, red litmus paper, a few drops of phenolphthalein and a few drops of methyl orange, respectively, in test tubes ‘A’, ‘B’, ‘C’ and ‘D’. 
• Add a few drops of dilute HCl to each of these test tubes.
• Record the change in the colour of the indicator in all four test tubes for the given sample.
• Repeat the above steps with dilute H₂SO₄, CH₃COOH, NaOH, Ca(OH)₂, NH₄OH and lemon juice.

In this experiment, we learnt that: 

• Blue litmus paper changes to red in an acidic medium but remains unaffected in a basic medium. 
• Red litmus paper turns blue in a basic medium but remains unaffected in an acidic medium.
• Phenolphthalein remains colourless in acid, while it becomes pink in a base.
• Methyl orange turns red in an acid but gives a yellow colour in a base.

FAQs on Identification of Substances Using Indicators Nature