Class 7 Science | Curiosity| Chapter 2: Exploring Substances: Acidic, Basic, and Neutral| NCERT Solutions
Q1. A solution turns the red litmus paper blue. Excess addition of which of the following solutions would reverse the change?
(i) Lime water
(ii) Baking soda
(iii) Vinegar
(iv) Common salt solution
Answer: (iii) Vinegar
Only vinegar (acetic acid) is acidic and can neutralise the base, turning the blue litmus back to red.
Q2. You are provided with three unknown solutions labelled A, B, and C, but you do not know which is acidic, basic, or neutral. It turns blue after adding a few drops of red litmus solution to solution A. When a few drops of turmeric solution are added to solution B, it turns red. Finally, adding a few drops of red rose extract to solution C turns it green.
Based on the observations, which of the following is the correct sequence for the nature of solutions A, B, and C?
(i) Acidic, acidic, and acidic
(ii) Neutral, basic, and basic
(iii) Basic, basic, and acidic
(iv) Basic, basic, and basic
Answer: (iv) Basic, basic, and basic ✅
Q3. Observe and analyse Figs. 2.13, 2.14, and 2.15, in which red rose extract paper strips are used. Label the nature of solutions present in each of the containers.
In basic solutions, it turns greenish.
In neutral solutions, the strip does not change its colour.
Q4. A liquid sample from the laboratory was tested using various indicators:
Indicator Red litmus Blue litmus Turmeric
Change No change Turned red No change in colour
Based on the tests, identify the acidic or basic nature of the liquid and justify your answer.
Answer: Red litmus: No change
Red litmus turns blue in the presence of a base. Since there is no change, the liquid is not basic.
Blue litmus: Turned red
Blue litmus turns red in the presence of an acid. This indicates that the liquid is acidic.
Turmeric: No change in colour
Turmeric turns reddish-brown in the presence of a base. Since there is no change, this further confirms that the liquid is not basic.
Conclusion: The liquid is an acidic solution.
Q5. Manya is blindfolded. She is given two unknown solutions to test and determine whether they are acidic or basic. Which indicator should Manya use to test the solutions and why?
Answer: Manya should use olfactory indicators like vanilla or onion juice because:
Vanilla loses its smell in bases but stays the same in acids.
Onion juice loses its smell in bases but remains strong in acids.
Q6. Grape juice was mixed with red rose extract; the mixture got a tint of red colour. What will happen if baking soda is added to this mixture? Justify your answer.
Answer: 🔴 Red (acidic mixture) + � Baking soda (base) → Green/Blue colour change!
Grape juice contains weak organic acids, which is why red rose extract got a red colour when added. When baking soda (a base) is added to the grape juice and rose extract mixture (which is acidic), the red colour will likely change to rose pink and then to green or blue. It neutralises the effect of acid. An excess of baking soda makes the mixture basic and turns it green.
Q7. Keerthi wrote a secret message to her grandmother on her birthday using orange juice. Can you assist her grandmother in revealing the message? Which indicator would you use to make it visible?
Answer: Grandmother can apply blue litmus dye (liquid)on he paper, the letters will turn blue.
Q8. How can natural indicators be prepared? Explain by giving an example.
Answer: Natural indicators are made from plants that change colour in acids or bases. Here’s how to make one using red cabbage.
Materials Needed:
½ red cabbage (chopped)
Hot water
A strainer
Clear cups/jars
Steps:
1. Boil the cabbage: Soak chopped red cabbage in hot water for 10–15 mins.
2. Strain it: Pour the purple liquid into a jar—this is your indicator juice!
3. Test it!
- Add to acidic things (lemon juice, vinegar) → Turns pink/red!
- Add to basic things (soap, baking soda) → Turns blue/green/yellow!
Q9. Three liquids are given to you. One is vinegar, another is a baking soda solution, and the third is a sugar solution. Can you identify them only using turmeric paper? Explain.
Answer: Yes, you can identify all three liquids using turmeric paper! Here’s how:
Vinegar (Acidic): Turmeric paper stays yellow (no change in acids).
Baking Soda Solution (Basic): Turmeric paper turns reddish-brown (bases change their colour).
Sugar Solution (Neutral): Turmeric paper stays yellow (no reaction with neutral substances).
So, the liquid that turns turmeric paper red is baking soda, while vinegar and sugar solution keep it yellow—but you can taste a drop (sugar is sweet, vinegar is sour!) to tell them apart.
Q10. The extract of red rose turns the liquid X green. What will the nature of liquid X be? What will happen when an excess of amla juice is added to liquid X?
Answer: Since red rose extract turns green in Liquid X, it means X is basic (like soap or baking soda solution), as rose extract changes colour in bases.
Adding Excess Amla Juice:
Amla juice is acidic. When added to Liquid X (a base), it will neutralise the base, turning the green colour back to the original shade (likely pink/red of the rose extract).
Q11. How can substances around us be classified based on their nature?
Answer: Substances around us may be classified as acidic, basic, and neutral in nature.
Q12. What natural extracts can be used to determine whether a substance is acidic or basic?
Answer: Extracts of lichen, red rose, red hibiscus, purple cabbage, turmeric, etc., can be used to indicate the nature of substances.
Q13. What substances are called that change colour in acidic and basic solutions?
Answer: Substances that show different colours in acidic and basic solutions are called acid-base indicators.
Q14. How do acids and bases affect the colour of litmus paper?
Answer: Acids turn the colour of blue litmus to red. Bases turn the colour of red litmus to blue.
Q15. How does red rose extract behave in acidic and basic solutions?
Answer: Extract of red rose gives red colour in acidic solutions and green colour in basic solutions.
Q16. What happens to turmeric’s colour in basic, acidic, and neutral solutions?
Answer: The yellow colour of turmeric turns red in basic solutions but remains unchanged in acidic and neutral solutions.
Q17. What happens when an acid and a base react with each other?
Answer: An acid and a base neutralise each other, forming salt and water, along with the evolution of heat.
Q18. How can neutralisation be useful in solving everyday problems?
Answer: Many day-to-day problems like a red ant bite, acidic or basic nature of soil, and industrial waste can be attempted to be managed by the process of neutralisation.
Ch 5 Acids, Bases and Salts(MCQs)
Ch 5 Acids, Bases and Salts (Worksheet)
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