Question 1
A solution turns red litmus blue. Which solution in excess would reverse this change: lime water, baking soda, vinegar, or salt solution?
Steps only
Vinegar is acidic and neutralises the basic solution.
Answer
(iii) Vinegar.
CLASS 7 · SCIENCE · CHAPTER 2
Exploring Substances: Acidic, Basic, and Neutral
Simple answers to all 12 end questions, chapter activities, the additional chapter question, and three exploratory projects.
23 worked answersComplete chapter answers
23 worked answers · short steps first, explanations when you need them
Let Us Enhance Our Learning
Textbook pages 20–21
Question 2
Red litmus turns blue in A; turmeric turns red in B; and red-rose extract turns green in C. What is the nature of A, B, and C?
Steps only
A—basic; B—basic; C—basic.
Answer
(iv) Basic, basic, and basic.
Question 3
Use the red-rose extract paper strips in Figures 2.13–2.15 to label each solution acidic, basic, or neutral.
Steps only
Green—basic; no change—neutral; deep red—acidic.
Answer
2.13 basic; 2.14 neutral; 2.15 acidic.
Question 4
A liquid does not change red litmus, turns blue litmus red, and does not change turmeric. Is it acidic or basic? Explain.
Steps only
Blue litmus turns red, so the liquid is acidic.
Answer
Acidic.
Question 5
Manya is blindfolded and must test two unknown solutions to find out whether they are acidic or basic. Which indicator should she use, and why?
Steps only
Use an olfactory indicator such as onion strips; compare the smell safely.
Answer
An olfactory indicator works through smell rather than colour.
Question 6
Suggest materials for writing a hidden message and a spray liquid to reveal it. Give possible combinations and colours.
Steps only
Soap solution + turmeric spray → red writing; soap solution + red-rose spray → green writing; lemon juice + red-rose spray → red writing.
Answer
Choose a writing liquid and an indicator that changes colour with it.
Question 7
Grape juice mixed with red-rose extract turns red. What happens when baking soda is added? Explain.
Steps only
Baking soda neutralises the acid; enough baking soda makes the rose extract green.
Answer
The red colour changes towards green as the mixture becomes basic.
Question 8
How can Keerthi’s grandmother reveal a secret message written with orange juice? Which indicator can make it visible?
Steps only
Apply red-rose extract; the acidic orange juice should show a red mark.
Answer
Use red-rose extract as the indicator.
Question 9
How can a natural indicator be prepared? Explain with an example.
Steps only
Soak crushed rose petals in hot water and filter; the extract can indicate acids and bases.
Answer
Red-rose extract is a natural acid-base indicator.
Question 10
Can turmeric paper alone identify vinegar, baking-soda solution, and sugar solution? Explain.
Steps only
Turmeric identifies baking soda as a base but cannot distinguish vinegar from sugar solution.
Answer
No. Turmeric alone cannot identify all three.
Question 11
Red-rose extract turns liquid X green. What is X like, and what happens when excess amla juice is added?
Steps only
X is basic; excess acidic amla makes the mixture acidic and the extract red.
Answer
X is basic; excess amla juice makes it acidic.
Question 12
Complete the soil flowchart: what indicator can test soil; what may the soil be; how can basic and acidic soil be treated?
Steps only
Test with an indicator. Basic soil—add compost/manure; acidic soil—add lime.
Answer
Soil may be acidic, basic, or neutral; treat only after testing.
Chapter activities and learning prompts
Textbook pages 8–19, 21
Question Activity 2.1
Group lemon, soap, amla, tamarind, vinegar, baking soda, lime water, tap water, washing powder, sugar, and salt solutions as acidic, basic, or neutral using litmus.
Steps only
Acidic: lemon, amla, tamarind, vinegar. Basic: soap, baking soda, lime water, washing powder. Neutral: tap water, sugar, salt.
Answer
Group each sample by how it changes red and blue litmus.
Question Activity 2.2
Name common acids found in lemon, curd, tamarind, and vinegar.
Steps only
Citric, lactic, tartaric, acetic.
Answer
Lemon—citric; curd—lactic; tamarind—tartaric; vinegar—acetic acid.
Question Activities 2.3–2.5
What colour changes do red-rose extract and turmeric show in acidic, basic, and neutral solutions?
Steps only
Rose: acid red, base green. Turmeric: base red; acid and neutral stay yellow.
Answer
The indicators show different colour patterns and have different limits.
Question Activity 2.7
What happens when an acid is mixed with a base in suitable amounts?
Steps only
Acid + base → salt + water + heat.
Answer
Neutralisation forms salt and water and releases heat.
Question Section 2.3
How can neutralisation help with acidic soil, basic soil, and acidic industrial waste?
Steps only
Acidic soil—lime; basic soil—compost/manure; acidic waste—careful treatment with a base.
Answer
Neutralisation can reduce excess acidity or basicity when done safely.
Question Opening question
How could a clear soap solution and turmeric spray make a hidden message appear on white paper?
Steps only
Write with soap solution; turmeric spray turns red where it touches the basic writing.
Answer
The indicator reveals the dried soap message.
Question Activity 2.6
How can onion-treated strips be tested with an acidic liquid and a basic liquid? What should be recorded?
Steps only
Compare onion-treated strips before and after adding tamarind water and baking-soda solution; record the smell.
Answer
The odour can help distinguish acidic and basic samples.
Question Dive Deeper
Why does vinegar make bubbles on eggshell or marble, while soap solution does not?
Steps only
Vinegar reacts with calcium carbonate and releases carbon dioxide bubbles; soap solution does not.
Answer
The bubbles are carbon dioxide gas.
Exploratory Projects
Textbook pages 22
Question Project 1
How can acidic or basic materials and natural indicators be used to make a rangoli pattern?
Steps only
Test safe acidic and basic samples with a natural indicator, then use the colour changes to create a labelled pattern.
Answer
Use colour changes from natural indicators to make the pattern.
Question Project 2
How can water samples from rain, taps, or rivers be compared to find whether they are acidic, basic, or neutral?
Steps only
Label water samples, test each with the same indicator, compare colours, and record observations.
Answer
Classify each sample from its indicator result; observations may vary.
Question Project 3
How can a soil sample be tested to find whether it is acidic, basic, or neutral?
Steps only
Mix soil with water, let solids settle, and test the liquid with an indicator.
Answer
Use the indicator result to classify the soil; test carefully before treating it.