Chapter 2 – Exploring Substances: Acidic, Basic, and Neutral

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-brownBaking-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.

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