Litmus Test

The litmus test uses litmus paper, or solution, to test the acidity of a solution. Litmus is a natural pH indicator that comes from lichens. Litmus is a mixture of dyes and these dyes can be soaked into the paper to make litmus paper. The color of the litmus paper, and the pH changes the paper detects, depend on how the litmus paper was made.

Red and blue litmus paper can be dipped into a solution to determine if it is acidic, basic, or neutral, as shown in Figure 1. Blue litmus paper can be used to test for carboxylic acids.

• Red litmus paper turns blue when dipped into a basic solution (pH > 7).
• Blue litmus paper turns red when dipped into an acidic solution (pH < 7).
• Both red and blue litmus paper do not change colour when placed into a neutral solution (pH = 7)

Materials

• Test tubes and test tube rack, or beakers
• Blue and red litmus paper
• Compound to be tested

Safety

Acidic and basic solutions are corrosive, gloves should be worn for safety.

Procedure

1. Put a drop of the liquid compound, or the compound in solution, on moist blue or red litmus paper.
2. For red litmus paper:
   • If the red color of the litmus paper changes to blue, then the compound is basic.
3. For blue litmus paper:
   • If the blue color of the litmus paper changes to red, then the compound is acidic.
   • This could mean that a carboxylic group or phenolic group is present in the compound.
4. If the red and blue litmus paper do not change color then the compound is neutral.

Figure 1 - The colours of litmus paper after being dipped in basic, acidic, and neutral solutions.

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References:

• NCERT Lab Manual for Functional Group tests