2.1.1 Litmus as an Indicator

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

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:

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