📌 In-Text "Let Us Enhance Our Learning" Questions
Q1. What are red and blue litmus papers made of? Why do they change colour?
They are paper strips dipped in litmus solution, a dye obtained from lichens. They change colour because litmus is an indicator that reacts with acids (turns red) and bases (turns blue).
Q2. Have you seen lichens on tree trunks?
Yes — lichens grow on tree bark in places with clean air and good rainfall. They look like greenish-grey patches and are very sensitive to pollution.
Q3. What other natural indicators can replace litmus?
Turmeric, red rose extract, china rose (gudhal), red cabbage juice, beetroot juice, and jamun (Indian blackberry) juice.
Q4. Are there substances whose smell changes in acids/bases?
Yes — these are called olfactory indicators. Examples: onion and vanilla extract. Their smell weakens or disappears in a base.
Q5. How can an ant bite be treated at home?
Apply moist baking soda or calamine lotion on the bite. Both are bases — they neutralise the formic acid injected by the ant and reduce burning.
Q6. How can we save fish from acidic factory waste?
Factories must add a base like lime (calcium hydroxide) to the waste before releasing it. The acid is neutralised, making the water safe for fish.
Q7. Can turmeric and soap solution be used for secret messages?
Yes! Write on white paper with turmeric paste — message is invisible. Spray soap solution (a base) → turmeric turns red, revealing the message.
📌 Exercise Questions (End of Chapter)
Q1. Red litmus paper turned blue when dipped in solution X. Which substance can reverse the colour back to red?
(i) Soap solution (ii) Sugar solution (iii) Vinegar (iv) Baking soda
Answer: (iii) Vinegar — Solution X is basic. Vinegar is acidic, so it neutralises the base and turns the paper red again.
Q2. Three solutions A, B, C turn red litmus blue. Their nature is:
(i) acidic, basic, neutral (ii) basic, acidic, neutral (iii) acidic, neutral, basic (iv) basic, basic, basic
Answer: (iv) basic, basic, basic — All three turn red litmus blue → all three are basic.
Q3. Red rose extract paper analysis — three containers gave different colours. Identify their nature.
Container 1 — green = Basic; Container 2 — pink = Neutral; Container 3 — red = Acidic.
Q4. A lab sample turned blue litmus red. What is its nature?
The sample is acidic — only acids turn blue litmus red.
Q5. How can a blindfolded student test a solution?
Use an olfactory indicator like onion juice or vanilla extract. Their smell changes / disappears in a base, which the student can identify by smell.
Q6. Why do we add lime to acidic soil?
Lime is a base. It neutralises the excess acid in the soil so plants can grow well.
Q7. Grape juice is mixed with red rose extract → it turns red. Then baking soda is added. What happens?
Baking soda is a base. It neutralises the acid in grape juice and makes the mixture basic — so red rose extract now turns green.
Q8. How can a secret message written with orange juice be revealed?
Spray red rose extract / beetroot juice / jamun extract. Orange juice is acidic, so the indicator turns red/pink, making the message visible.
Q9. How do you prepare a natural indicator at home?
Crush red rose petals (or beetroot/red cabbage) → add hot water → leave for 5–10 minutes → filter. The coloured liquid is your indicator.
Q10. Three liquids — vinegar, sugar solution, baking soda solution — were tested with turmeric paper. What changes?
Only baking soda solution turns turmeric paper from yellow to red (it is basic). Vinegar (acid) and sugar solution (neutral) cause no change.
Q11. Red rose extract turned liquid X green. Then amla juice was added — it became red. Explain.
Liquid X is basic (rose extract turns green in bases). Amla juice is acidic — it neutralises the base, then makes the solution acidic, so the indicator turns red.