Class 7 General Science – Chapter 2: Acids, Bases and Salts (AP SCERT, Pages 41–74) – Answers. AP 7th Science Unit 2 Acids, Bases and Salts Answers and Solutions. This post covers complete solved answers, activity tables, and textbook exercise solutions for the AP SCERT Class 7 General Science Chapter 2 "Acids, Bases and Salts". Topics include acids and bases, natural indicators (litmus, red rose extract, turmeric paper), olfactory indicators, neutralisation reaction, and everyday applications of neutralisation such as ant stings, acidic/basic soil treatment, and industrial waste management. These AP SCERT Class 7 Science solutions are ideal for AP TET, DSC, and school exam preparation, covering keywords like acid base indicator class 7, neutralisation reaction examples, litmus test class 7, red rose extract indicator, turmeric paper indicator, AP SCERT Class 7 Science Chapter 2 solutions.
📘 Chapter 2: Acids, Bases and Salts
Page 44–46 | Activity 2.1: Testing Samples with Litmus Paper
| S.No. | Name of the Sample | Colour of Blue Litmus Paper | Colour of Red Litmus Paper |
|---|---|---|---|
| 1 | Lemon juice | Turns Red | No change (remains red) |
| 2 | Soap solution | No change (remains blue) | Turns Blue |
| 3 | Usiri (Amla) juice | Turns Red | No change |
| 4 | Tamarind water | Turns Red | No change |
| 5 | Vinegar | Turns Red | No change |
| 6 | Baking soda solution | No change | Turns Blue |
| 7 | Lime water | No change | Turns Blue |
| 8 | Tap water | No change | No change |
| 9 | Washing powder solution | No change | Turns Blue |
| 10 | Sugar solution | No change | No change |
| 11 | Salt solution | No change | No change |
Table 2.2: Grouping
| Group A (Acidic – turns blue litmus red) | Group B (Basic – turns red litmus blue) | Group C (Neutral – no change) |
|---|---|---|
| Lemon juice, Usiri (Amla) juice, Tamarind water, Vinegar | Soap solution, Baking soda solution, Lime water, Washing powder solution | Tap water, Sugar solution, Salt solution |
Prediction (Group C nature): Tap water, sugar solution and salt solution do not change either litmus paper, so they are neutral substances — neither acidic nor basic.
Page 50 | Activity 2.2: Acids Present in Common Foods
Lemon — Citric acid | Curd — Lactic acid | Tamarind — Tartaric acid | Vinegar — Acetic acid
Page 54–56 | Activity 2.3 & 2.4: Red Rose Extract as Indicator
| S.No. | Name of Sample | Colour of Red Rose Extract | Nature of Substance |
|---|---|---|---|
| 1 | Lemon juice | Red | Acidic |
| 2 | Soap solution | Green | Basic |
| 3 | Usiri (Amla) juice | Red | Acidic |
| 4 | Tap water / Sugar / Salt solution | No change (pink) | Neutral |
Answers to questions:
• Yes — the samples turning the extract red are the same as Group A (acidic) samples.
• Yes — the samples turning the extract green are the same as Group B (basic) samples.
• Yes — the samples showing no colour change are the same as Group C (neutral) samples.
Page 58–60 | Activity 2.5: Turmeric Paper as Indicator
| S.No. | Name of Sample | Colour of Turmeric Paper |
|---|---|---|
| 1 | Lemon juice | No change (remains yellow) |
| 2 | Soap solution | Turns Red / Reddish-brown |
| 3 | Amla juice | No change |
| 4 | Baking soda / Lime water / Washing powder solution | Turns Red |
Turmeric paper is used as an indicator for basic substances only. It cannot differentiate between acidic and neutral substances, since both leave it unchanged.
Krishna's greeting card: He wrote the message using soap solution (a basic solution), which turns turmeric paper red, making the message visible.
Page 62 | Activity 2.6: Onion as an Olfactory Indicator
The odour of the onion-soaked strip changes (becomes weaker/different) when tamarind water (acidic) is added, and the onion smell disappears/reduces greatly when baking soda solution (basic) is added — because the basic solution neutralises/masks the acidic-smelling volatile compound. This shows onion acts as an olfactory indicator — its odour changes only in a basic medium.
Page 64 | Activity 2.7: Neutralisation Experiment
Blue litmus solution added to lemon juice turns red (acidic). On slowly adding lime water, the colour changes from red back to blue — showing the acid has been neutralised by the base. Adding a fresh drop of lemon juice again turns it red, because the acid now exceeds the base again.
Equation: Acid + Base → Salt + Water + Heat
Page 66 | Let Us Wrap Up!
Yes — the spray liquid could be a turmeric solution (which is yellow and colourless-looking when dry) and the message could have been pre-written using a soap solution (a basic, colourless liquid) on the paper. When the turmeric spray touches the soap-written areas, it turns red, revealing the hidden message, while the rest of the paper stays yellow.
✍️ Let Us Enhance Our Learning (Page 68–74)
Q1.
Match the fruits with the acids naturally present in them.
Q → 3 (Amla → Ascorbic acid and Citric acid)
R → 1 (Tamarind → Tartaric acid)
S → 4 (Tomato → Citric acid and Oxalic acid)
Answer: A. P–2, Q–3, R–1, S–4
Q2.
a) Acid-base indicators: Substances that show different colours when mixed with acidic and basic solutions are called acid-base indicators. They help us find out whether a substance is acidic or basic. Example: Litmus.
b) Neutralisation: The reaction between an acid and a base, in which they cancel out each other's effects to form salt and water along with the release of heat, is called neutralisation.
c) Olfactory indicators: Substances whose smell (odour) changes when mixed with an acidic or basic substance are called olfactory indicators. Example: Onion, vanilla essence.
Q3.
Acid: (i) Tastes sour, (ii) Turns blue litmus paper red.
Base: (i) Tastes bitter, (ii) Feels soapy/slippery to touch and turns red litmus paper blue.
Q4.
Answer: B — Magnesium hydroxide. It is a base (antacid) that neutralises the excess acid in the stomach causing indigestion and heartburn.
Q5.
Natural indicators can be prepared from coloured flowers, leaves, or roots. Example — Red rose extract: Fallen red rose petals are collected, washed, and crushed with a mortar and pestle. They are placed in a glass tumbler, covered with hot water, and left covered for 5–10 minutes till the water turns coloured. The mixture is then filtered, and the filtrate obtained is the red rose extract, which can be used as a natural acid-base indicator (it turns red in acid and green in base).
Q6.
Answer: D — The paste neutralised the acidic substance injected by the ant. An ant's bite injects formic acid (acidic) into the skin; the baking soda paste (a base) neutralises this acid, relieving the pain.
Q7.
Answer: (iii) Vinegar. Since the solution is basic (turns red litmus blue), adding excess of an acidic substance like vinegar would neutralise it and reverse the colour change.
Q8.
Solution A: red litmus turns blue → basic. Solution B: turmeric turns red → basic. Solution C: red rose extract turns green → basic.
Answer: (iv) Basic, basic, and basic
Q9.
Since Mani is blindfolded, she cannot see colour changes, so a visual indicator like litmus, turmeric or red rose extract will not help her. She should use an olfactory indicator, such as onion extract on cloth strips. By smelling the strips before and after adding the unknown solutions, she can tell whether the solution is acidic or basic based on whether the odour changes.
Q10.
Possible combinations for the spray liquid and the writing solution, and the colour obtained:
| Paper treated with (Spray) | Solution used to write message | Colour of Writing Obtained |
|---|---|---|
| Turmeric solution | Soap solution | Red |
| Turmeric solution | Baking soda solution | Red |
| Red rose extract | Soap solution | Green |
| Red rose extract | Lemon juice | Red |
| Blue litmus solution | Lemon juice / Vinegar | Red |
| Red litmus solution | Soap solution / Baking soda solution | Blue |
Q11.
Yes. Orange juice is acidic and nearly colourless when it dries, so the message is invisible. To reveal it, spray or dab a blue litmus solution (or red rose extract) over the paper. Wherever the orange juice was used to write, the blue litmus turns red (or the rose extract turns red), making the hidden message visible.
Q12.
Since red rose extract turns green with liquid X, X must be basic in nature. When excess amla juice (acidic) is added to liquid X, the acid neutralises the base. As more amla juice is added, the acid will start to dominate, and the colour of the mixture will gradually change from green back to red, showing the solution has now become acidic.
Q13.
Nature of solutions based on colour of red rose extract strip:
Fig. 2.14 (No change/pink) → Neutral solution
Fig. 2.15 (Red) → Acidic solution
Q14.
Turmeric paper can identify only basic substances (it turns red). The baking soda solution will turn the turmeric paper red and can be identified. However, turmeric paper cannot tell apart vinegar (acidic) and sugar solution (neutral), since both leave the turmeric paper unchanged (yellow). So, only baking soda solution can be identified with certainty using turmeric paper alone.
Q15.
Completing the flowchart:
Indicator that can be used to test the nature of soil: Litmus paper (blue and red) / natural indicator like red rose extract
The acidic soil can be treated with: Lime
The basic soil can be treated with: Organic matter (manure/composted leaves)
Q16.
(Question text not visible/legible in the uploaded page scan — please re-check the original textbook page for Q16.)
Q17.
| Indicator | Red litmus | Blue litmus | Turmeric |
|---|---|---|---|
| Change | No change | Turned red | No change |
Answer: The liquid is acidic in nature. The blue litmus paper turning red confirms this, since acids turn blue litmus red. This is consistent with the red litmus paper showing no change (it stays red as acids do not affect it) and the turmeric paper showing no change (since turmeric only changes colour, turning red, in the presence of a base — not an acid).
Q18.
Since grape juice mixed with red rose extract gives a red tint, the grape juice is acidic. When baking soda (a base) is added to this mixture, it will neutralise the acid present in the grape juice. As enough baking soda is added, the colour of the mixture will gradually change from red to green, showing that the solution has become basic; along with this, heat will also be released (a neutralisation reaction), and gas bubbles (carbon dioxide) may also be seen forming.
🤿 Dive Deeper
Vinegar is an acid. When it reacts with eggshell or marble (which contains calcium carbonate), it produces carbon dioxide gas, which is seen as bubbling. Soap solution is basic in nature — it does not react with calcium carbonate to produce a gas, so no bubbles are formed when soap solution is poured on eggshell or marble.
🔎 Exploratory Projects
These are hands-on activities to be performed by students at home/school (creating rangoli using acidic/basic substances and natural indicators; testing water samples from rain, taps, and rivers; and testing local soil samples for acidic, basic, or neutral nature) — no fixed textbook answer is required, as results depend on the actual samples tested by each student.
