2.1.1 Litmus as an Indicator

Last updated at April 28, 2026 by

What is Litmus?

It is a natural substance obtained from lichens .

Key Facts and Examples
  • Litmus is available as solution and paper strips .
Example: Blue litmus paper and red litmus paper
  • Acidic substances turn blue litmus to red .
Example: Lemon juice, vinegar, tamarind water turn blue litmus red
  • Basic substances turn red litmus to blue .
Example: Soap solution, baking soda solution turn red litmus blue
  • Neutral substances do not change either litmus colour.
Example: Tap water, sugar solution, salt solution show no change
  • Acidic substances generally taste sour .
Example: Lemon, amla, tamarind have sour taste
  • Basic substances are slippery and taste bitter .
Example: Soap feels slippery, baking soda tastes bitter
Litmus Paper Testing Substance Type Blue Litmus Red Litmus Acidic substances Turns RED No change Basic substances No change Turns BLUE Neutral substances No change No change

Activity 2.1: Testing Substances with Litmus Paper

What you need:

  • Samples of lemon juice, soap solution, amla juice, tamarind water, vinegar, baking soda solution, lime water, tap water, washing powder solution, sugar solution, and salt solution
  • Blue litmus paper strips
  • Red litmus paper strips
  • Clean and dry white tile
  • Dropper

What to do:

  • Step 1: Take a strip of blue litmus paper and cut it into small pieces
  • Step 2: Spread these pieces on a clean and dry white tile
  • Step 3: Using a dropper, put one drop of each sample, one-by-one, on these litmus paper pieces
  • Step 4: Record your observations
  • Step 5: Repeat the same activity with pieces of red litmus paper and record your observations

Observations

What you see:

Substance Blue Litmus Red Litmus Nature
Lemon juice Turns red No change Acidic
Soap solution No change Turns blue Basic
Amla juice Turns red No change Acidic
Tamarind water Turns red No change Acidic
Vinegar Turns red No change Acidic
Baking soda solution No change Turns blue Basic
Lime water No change Turns blue Basic
Tap water No change No change Neutral
Washing powder solution No change Turns blue Basic
Sugar solution No change No change Neutral
Salt solution No change No change Neutral

Why this happens:

We have studied that litmus is an acid-base indicator that shows different colours in acidic and basic solutions . This is why different substances produce different colour changes.

  • Group A substances (lemon, vinegar, tamarind) are acidic
  • Group B substances (soap, baking soda, lime water) are basic
  • Group C substances (tap water, sugar, salt) are neutral
Substance Classification Acidic (Turn blue litmus red) Basic (Turn red litmus blue) Lemon juice Soap solution Vinegar Baking soda Tamarind water Lime water Amla juice Washing powder
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