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Chapter Title: Exploring Substances: Acidic, Basic, and Neutral
Subject: Science | Grade: 7 | Based on: NCERT/CBSE Curriculum
Total Marks: 50 | Time: 1 hour 30 minutes
Section A: Multiple Choice Questions (1 × 20 = 20 Marks)
1. Which of the following is an acidic substance?
A) Baking soda
B) Lemon juice
C) Lime water
D) Soap solution
2. What colour does red litmus turn in a base?
A) Blue
B) Red
C) Green
D) Yellow
3. A neutral substance:
A) Changes both litmus colours
B) Turns blue litmus red
C) Turns red litmus blue
D) Does not change any litmus
4. Which is not a natural indicator?
A) Turmeric
B) Litmus
C) Red rose extract
D) Methyl orange
5. Turmeric turns reddish-brown when in contact with:
A) Acid
B) Salt
C) Base
D) Water
6. Vinegar contains:
A) Acetic acid
B) Citric acid
C) Lactic acid
D) Oxalic acid
7. Which substance is used to treat acidic soil?
A) Baking soda
B) Lime
C) Vinegar
D) Salt solution
8. In a neutralisation reaction, what is released besides salt and water?
A) Electricity
B) Sound
C) Heat
D) Light
9. Which indicator shows a change in smell with acids and bases?
A) Red cabbage
B) Onion
C) Litmus
D) Turmeric
10. What is the nature of tap water?
A) Acidic
B) Basic
C) Neutral
D) Salty
11. Which flower extract gives green colour in basic media?
A) Marigold
B) Rose
C) Hibiscus
D) Red rose
12. A solution turns blue litmus red. What is its nature?
A) Neutral
B) Basic
C) Acidic
D) Weak base
13. Which of the following is a base?
A) Lemon juice
B) Vinegar
C) Baking soda
D) Tamarind juice
14. What is used to test the pH nature of the soil?
A) Soap solution
B) Red cabbage extract
C) Milk
D) Salt water
15. Which of the following does not react with turmeric?
A) Lime water
B) Vinegar
C) Soap solution
D) Baking soda
16. Neutralisation occurs between:
A) Acid and water
B) Base and water
C) Salt and acid
D) Acid and base
17. The acid in orange juice is:
A) Acetic acid
B) Citric acid
C) Formic acid
D) Sulphuric acid
18. Which natural indicator changes colour in the presence of bases but not acids?
A) Red cabbage
B) Onion
C) Turmeric
D) Rose extract
19. Which indicator is made from lichens?
A) Turmeric
B) Litmus
C) Methyl orange
D) Onion
20. Which of the following best describes red rose extract in a neutral solution?
A) Red
B) Green
C) No change
D) Blue
Section B: One-Mark Questions (1 × 10 = 10 Marks)
1. What is the taste of bases?
2. Name a basic substance used in cooking.
3. What happens when an acid reacts with a base?
4. What is an olfactory indicator?
5. Give one use of neutralisation in agriculture.
6. What is the colour of turmeric in acids?
7. Name a synthetic indicator.
8. What acid is present in curd?
9. What is the nature of sugar solution?
10. What is the function of an indicator?
Section C: Two-Mark Questions (2 × 10 = 20 Marks)
1. Differentiate between acids and bases with two points.
2. Write the equation of a neutralisation reaction and explain.
3. How is turmeric paper prepared? Mention its use.
4. Why is it important to neutralise factory waste before releasing it into rivers?
5. Explain how litmus paper works as an indicator.
6. List two common examples of neutral substances.
7. What precautions should be taken while testing unknown substances?
8. Write two applications of neutralisation in everyday life.
9. How does red rose extract behave in acidic and basic solutions?
10. Explain the use of lime in correcting acidic soil.
Section D: Higher Order Thinking Skills (HOTS) (3 × 5 = 15 Marks)
1. A student added lime water to lemon juice and noticed a change in colour using red cabbage extract. Explain the science behind it.
2. Manya accidentally spilled soap on her turmeric-stained shirt, and it turned red. Why did this happen?
3. Devise a classroom experiment to show neutralisation using household items.
4. How can farmers test whether their soil is too acidic or basic using natural indicators? Propose a method.
5. A mixture gives no change with litmus but changes red rose extract to green. What is the likely nature of the substance and what test will you use to confirm it?
📝 Answer Key: Exploring Substances
Section A: MCQs
- B
- A
- D
- D
- C
- A
- B
- C
- B
- C
- D
- C
- C
- B
- B
- D
- B
- C
- B
- C
Section B: One-Mark Answers
- Bitter
- Baking soda
- Salt + Water + Heat
- Indicator that changes smell in acid/base
- Lime is added to reduce acidity of soil
- Yellow (no change)
- Phenolphthalein
- Lactic acid
- Neutral
- To determine acidic/basic/neutral nature
Section C: Two-Mark Answers
- Acids taste sour, turn blue litmus red; Bases taste bitter, turn red litmus blue.
- Acid + Base → Salt + Water + Heat. Example: HCl + NaOH → NaCl + H₂O
- Turmeric mixed with water, spread on paper, dried to test for bases (turns red).
- Factory waste may be acidic and harm aquatic life; neutralisation prevents pollution.
- Blue litmus turns red in acid; red litmus turns blue in base.
- Tap water, sugar solution.
- Do not taste or touch; use indicators with caution.
- Treating ant bites, soil correction.
- Red in acid, green in base.
- Lime neutralises soil acidity, improving plant growth.
Section D: HOTS Sample Answers
- Acid (lemon) and base (lime water) neutralise; red cabbage changes from purple to green in base.
- Soap is basic; turmeric turns reddish-brown in base.
- Use vinegar (acid) and baking soda (base); observe fizz and add red cabbage extract for colour change.
- Extract red cabbage juice, add to soil water extract; observe colour to determine nature.
- The solution is basic (green from red rose); confirm with turmeric (should turn red).
