Hinglish

Litmus as an Indicator

Class 7 Science · Chapter 2: Exploring Substances: Acidic, Basic, and Neutral · NCERT Curiosity section 2.1–2.1.1

Learn

At a science fair, Meher and Kabir are handed blank white sheets. A volunteer sprays them with a liquid, and the words "Welcome to the Wonderful World of Science" appear! At the "Colourful World of Substances" stall they decide to find out how. Nature, it turns out, is our science laboratory.

They put drops of lemon juice, soap solution, vinegar, baking soda solution, lime water, sugar and salt solutions on small pieces of blue and red litmus paper. Some samples turn blue litmus red, some turn red litmus blue, and some change neither.

Substances that turn blue litmus red are acidic; those that turn red litmus blue are basic; those that change neither are neutral. Litmus comes from lichens (a fungus and an alga living together). Because it shows different colours in acids and bases, it is an acid-base indicator.

blue → red: acidic · red → blue: basic · no change: neutral

Acidic foods taste sour: lemon and orange have citric acid, amla has ascorbic acid (vitamin C), tamarind has tartaric acid, curd has lactic acid and vinegar has acetic acid. Bases feel soapy and often taste bitter, though not everything bitter (like karela) is basic. Never taste an unknown substance!

Explore: the litmus test.

  1. Test each sample with litmus and sort it.

Remember

  • Acids turn blue litmus red; bases turn red litmus blue; neutral substances change neither.
  • Litmus, from lichens, is an acid-base indicator.

Quick check

A liquid turns red litmus blue. What is its nature?

  1. basic
  2. acidic
  3. neutral
  4. it cannot be told
Answer

basic. Red → blue means basic.

Think about it

Rain water, river water and tap water: how could you find whether each is acidic, basic or neutral?

One answer

Put a drop of each on blue and red litmus paper (or in an indicator like red rose extract) and compare the colour changes.

Practice

Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.