Q1. [4]
The teacher while conducting practicals in the laboratory divided the students into three groups and gave them various solutions to find out their pH and classify them into acidic, basic and neutral solutions.
Group A : Lemon juice, vinegar, colourless aerated drink
Group B : Tomato juice, coffee, ginger juice
Group C : Sodium hydroxide, sodium chloride, lime water
Answer the questions based on the following case:
- (a) For the solutions provided, which group is/are likely to have pH value (i) less than 7, and (ii) greater than 7 ? [1]
- (b) List two ways of determining pH of a solution. [1]
- (c) Explain, why the sour substances such as lemon juice are effective in cleaning the tarnished copper vessels. [2]
Previously asked in CBSE board exam
2023 31/2/1 Q37
Generated by claude-sonnet-4-6 · 2026-06-15 06:44 · grounding stimulus
Model Answer
(a)
- (i) Less than 7 (acidic): Group A and Group B — lemon juice, vinegar, aerated drink, tomato juice, coffee, and ginger juice are all acidic.
- (ii) Greater than 7 (basic): Group C (partially) — sodium hydroxide (pH ≈ 14) and lime water (pH > 7) are basic. Sodium chloride is neutral (pH = 7).
(b) Two ways to determine pH:
- Using pH paper (universal indicator paper) — colour change indicates pH.
- Using universal indicator solution — different colours correspond to different pH values.
(c) Copper vessels tarnish due to the formation of copper oxide (CuO) on their surface. Sour substances like lemon juice contain acids (citric acid) which react with copper oxide, dissolving it and restoring the shiny surface of the copper vessel.
Source: Chapter 2 – Acids, Bases and Salts, NCERT Science Class 10
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Explanation
- (a): Acids have pH < 7, bases have pH > 7, neutral = 7. All items in Groups A and B are acidic; NaOH and lime water are basic; NaCl is neutral.
- (b): Examiners expect "pH paper/universal indicator paper" and "universal indicator solution" as the two standard methods. Digital pH meter is also acceptable.
- (c): Key idea: tarnish = metal oxide layer; acid reacts with metal oxide to remove it. Mention the specific acid (citric acid) and the product (copper oxide dissolves). This is the standard explanation expected in board exams.
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