Activity 2.1 – Litmus test (Table 2.1)
Question
Fill the table after putting one drop of each sample on blue and red litmus paper.
Answer
| S. No. | Sample | Blue litmus turns | Red litmus turns |
|---|---|---|---|
| 1 | Lemon juice | Red | No change (stays red) |
| 2 | Soap solution | No change (stays blue) | Blue |
| 3 | Amla juice | Red | No change |
| 4 | Tamarind water | Red | No change |
| 5 | Vinegar | Red | No change |
| 6 | Baking-soda solution | No change | Blue |
| 7 | Lime-water | No change | Blue |
| 8 | Tap water | No change | No change |
| 9 | Washing-powder solution | No change | Blue |
| 10 | Sugar solution | No change | No change |
| 11 | Salt solution | No change | No change |
| 12 | (Any other: Cola drink) | Red | No change |
Sorting after Activity 2.1 (Table 2.2)
Question
Place the samples into Group A, Group B and Group C.
Answer
| Group A – turn blue litmus red (Acids) | Group B – turn red litmus blue (Bases) | Group C – no change (Neutrals) |
|---|---|---|
| Lemon juice, Amla juice, Tamarind water, Vinegar, Cola drink | Soap solution, Baking-soda solution, Lime-water, Washing-powder solution | Tap water, Sugar solution, Salt solution |
Activity 2.2 – Acids in food
Question
Write the common acid in each food.
Lemon ______, Curd ______, Tamarind ______, Vinegar ______
Answer
-
Lemon – Citric acid
-
Curd – Lactic acid
-
Tamarind – Tartaric acid
-
Vinegar – Acetic acid
Activity 2.4 – Red-rose indicator (Table 2.3)
Question
Note colour changes after adding samples to red-rose extract.
Answer
| S. No. | Sample | Colour after adding | Nature of sample |
|---|---|---|---|
| 1 | Lemon juice | Pink/Red | Acidic |
| 2 | Soap solution | Green | Basic |
| 3 | Amla juice | Pink/Red | Acidic |
| 4 | Tamarind water | Pink/Red | Acidic |
| 5 | Vinegar | Pink/Red | Acidic |
| 6 | Baking-soda solution | Green | Basic |
| 7 | Lime-water | Green | Basic |
| 8 | Tap water | No change | Neutral |
| 9 | Washing-powder solution | Green | Basic |
| 10 | Sugar solution | No change | Neutral |
| 11 | Salt solution | No change | Neutral |
Activity 2.5 – Turmeric paper (Table 2.4)
Question
Record colour of each sample on turmeric paper.
Answer
| S. No. | Sample | Colour on turmeric paper | Nature |
|---|---|---|---|
| 1 | Lemon juice | No change (yellow) | Acidic |
| 2 | Soap solution | Reddish-brown | Basic |
| 3 | Amla juice | No change | Acidic |
| 4 | Tamarind water | No change | Acidic |
| 5 | Vinegar | No change | Acidic |
| 6 | Baking-soda solution | Reddish-brown | Basic |
| 7 | Lime-water | Reddish-brown | Basic |
| 8 | Tap water | No change | Neutral |
| 9 | Washing-powder solution | Reddish-brown | Basic |
| 10 | Sugar solution | No change | Neutral |
| 11 | Salt solution | No change | Neutral |
Can turmeric paper show acids?
No. It becomes red only with bases, so it cannot separate acids from neutral liquids.
Ashwin’s Guru-Purnima card
Question
Which solution could he use for writing on turmeric-coated paper?
Answer
He could write with soap solution (or any mild base like baking-soda water).
When the teacher later sprayed turmeric liquid, the writing turned red-brown and became clear.
Activity 2.6 – Onion strips (olfactory indicator)
Question
Describe the smell before and after adding tamarind water (acid) and baking-soda solution (base).
Answer
-
Before adding anything: strong onion smell.
-
After tamarind water (acid): smell stays.
-
After baking-soda solution (base): smell goes away / very weak.
So acids keep the onion smell; bases remove it.
Activity 2.7 – Mixing lemon juice, litmus and lime-water
Question
Why does the colour change from red back to blue, and back to red again?
Answer
-
Lemon juice is an acid, so blue litmus turns red.
-
Lime-water is a base; when enough is added it neutralises the acid, so the solution becomes basic and looks blue again.
-
Adding one more drop of lemon juice makes it acidic once more, turning it red again.
This shows a neutralisation reaction:
Acid + Base → Salt + Water + heat.
Neutralisation in Daily Life
Situation 1 – Ant bite
Answer: An ant injects formic acid. Rubbing baking-soda paste (base) neutralises the acid and stops the pain.
Situation 2 – Acidic soil
Answer: Mix lime (calcium oxide / hydroxide) in the soil. The base neutralises extra acid so plants grow well.
Situation 3 – Factory waste in lake
Answer: If waste water is acidic, add a safe basic substance (like slaked lime) before releasing it. This protects fish.
Wrap-up (Magic welcome sign)
Question
How did the hidden words appear?
Answer
The paper was first written with a mild base (for example soap water).
Later it was sprayed with turmeric solution (yellow).
Turmeric turns red-brown where it touches the base, so the words showed up.
LET US ENHANCE OUR LEARNING (Exercise)
1. Multiple choice
A solution turns red litmus blue. Which liquid will reverse it?
Answer: (iii) Vinegar – it is an acid and will turn blue litmus back to red.
2. Multiple choice
Correct order of nature for A, B, C?
A turns red litmus blue → Basic
B turns turmeric red → Basic
C turns red-rose extract green → Basic
Answer: (iv) Basic, Basic, Basic
3. Figures 2.13–2.15 with red-rose strips
| Figure | Colour seen | Nature of solution |
|---|---|---|
| 2.13 – strip looks red | Acidic | |
| 2.14 – strip looks green | Basic | |
| 2.15 – strip shows no change | Neutral |
4. Table of indicator results
Blue litmus turns red, others no change → the liquid is Acidic (acids change blue litmus to red).
5. Blindfolded Manya
Answer: She should use onion-soaked cloth strips (olfactory indicator). Sight is not needed; a base will remove the onion smell, an acid will keep it.
6. Hidden-message combinations
| Writing liquid (on paper) | Spray bottle liquid | Final colour of words |
|---|---|---|
| Soap solution (basic) | Turmeric water | Red-brown |
| Baking-soda solution (basic) | Red-rose extract | Green |
| Lemon juice (acidic) | Blue-litmus solution | Red |
7. Baking-soda added to grape-juice + rose-extract mix
The mixture is slightly acidic (red). Baking-soda (a base) will neutralise and then make it basic, so the colour will turn green.
8. Revealing orange-juice message
Spray the paper with blue-litmus solution. Orange juice is acidic, so the letters will change the spray to red and become visible.
9. Making a natural indicator (example: turmeric)
Grind turmeric, add little water to make a paste, dip filter paper, dry it. The yellow paper turns red-brown with bases and stays yellow with acids or neutrals.
10. Identifying vinegar, baking-soda and sugar with turmeric paper
-
Drops that turn paper red-brown → Baking-soda solution (base)
-
Drops with no change could be vinegar (acid) or sugar (neutral).
Heat a little: vinegar smells sharp; sugar does not. Thus you can tell all three.
11. Liquid X turns red-rose extract green
Liquid X is Basic.
Adding plenty of amla juice (acid) will neutralise it; extra acid will finally make the mixture pink/red.
12. Soil flow-chart completion
-
Indicator: Red-rose extract or Litmus paper
-
Treat basic soil with compost / manure (adds mild acids)
-
Treat acidic soil with lime (slaked lime or quick lime)
-
Soil can be Basic or Acidic (the two blanks at top).
DIVE DEEPER – Vinegar vs Soap on egg-shell
Question
Why do bubbles form with vinegar but not with soap?
Answer
Egg-shell and marble both contain calcium carbonate.
-
Vinegar (acid) reacts with calcium carbonate and makes carbon-dioxide gas bubbles, so you see fizzing.
-
Soap solution (basic or neutral) does not react with calcium carbonate, so no bubbles appear.