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The teacher while conducting… (Class 10 Science 2023, Q37)

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ScienceCBSE Class 10 · Board exam 2023, set 31/2/1 · 4 marks · Case-based

CBSE Class 10 Science 2023: question 37

Set in the CBSE Class 10 board exam of 2023, paper 31/2/1, section E, question 37, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q37 · 4 marks · Case-basedCompetency-basedAcids, Bases and Salts

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

  1. (a)

    For the solutions provided, which group is/are likely to have pH value (i) less than 7, and (ii) greater than 7 ?

    1 mark
  2. (b)

    List two ways of determining pH of a solution.

    1 mark
  3. (c)

    Explain, why the sour substances such as lemon juice are effective in cleaning the tarnished copper vessels.

    2 marks

  4. OR

    (c)

    "pH has great importance in our daily life." Justify this statement by giving two examples.

    2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 31/2/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Sour-tasting substances are acids. Which group has substances like soap or alkalis?

Hint 2

pH can be found with a special paper or a mixture of indicators.

Hint 3

Tarnish on copper is a basic compound. What happens when an acid meets a base?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (a)(i) Groups A and B have pH less than 7. Lemon juice, vinegar, aerated drink, tomato juice, coffee and ginger juice are all acidic.

    ½ mark

  2. (a)(ii) Group C has pH greater than 7, because sodium hydroxide and lime water are basic. (Sodium chloride is neutral, with pH 7.)

    ½ mark

  3. (b) Two ways to find pH: (1) using pH paper; (2) using universal indicator, then matching the colour with the pH colour chart.

    1 mark

  4. (c) Tarnished copper has a dull layer of basic copper oxide / basic copper carbonate on it, formed by reaction with air and moisture. This layer is basic in nature.

    1 mark

  5. (c) Lemon juice contains acid (citric acid). The acid neutralises the basic layer and forms a salt that dissolves and is washed away. The shiny copper surface is then exposed again.

    1 mark

  6. (c) OR – Example 1: Our stomach produces HCl so that the pH is right for digestion. Excess acid causes indigestion, which is treated with antacids such as milk of magnesia.

    1 mark

  7. (c) OR – Example 2: Tooth decay starts when the pH in the mouth falls below 5.5. Bacteria produce acids from food particles, and these acids corrode the enamel. Using a basic toothpaste prevents this. (Other valid examples: survival of aquatic life becomes difficult when rain lowers the pH of river water; bee stings and nettle stings inject acid, which is relieved by a mild base.)

    1 mark

Answer: (a)(i) Groups A and B; (ii) Group C. (b) pH paper and universal indicator. (c) The acid in lemon juice neutralises the basic copper oxide/carbonate layer and removes it. OR: pH in digestion and in tooth decay (or aquatic life, stings).

Where the marks are

  • Groups A and B: pH < 7 (½ mark)
  • Group C: pH > 7 (½ mark)
  • pH paper and universal indicator (½ + ½ mark)
  • Tarnish is a basic copper oxide/carbonate layer (1 mark)
  • Acidic lemon juice neutralises it to a salt that washes away (1 mark)
  • OR: any two correct examples of the importance of pH in daily life (1 + 1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Acids, Bases and Salts, one at a time, marked against the answer key and marking points.Back to the full paper2023 board paper, set 31/2/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.