Skip to content

Answer the following. (Science 2023 31/5/1 Q28)

questionbuddy.in/q/cbse-10-science-2023-31-5-1-q28

ScienceCBSE Class 10 · Board exam 2023, set 31/5/1 · 3 marks · Short answer

CBSE Class 10 Science 2023: question 28

Set in the CBSE Class 10 board exam of 2023, paper 31/5/1, section C, question 28, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q28 · 3 marks · Short answerAcids, Bases and Salts

Answer the following.

  1. (a)

    Sometimes the pH of our mouth gets lower than 5·5. Why ?

    0 marks
  2. (b)

    A basic salt 'X' is obtained by heating baking soda followed by crystallisation. Identify 'X' and state its two industrial uses.

    0 marks
  3. (c)

    Why do copper sulphate crystals turn white on heating ?

    0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 31/5/1.

Hints

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

Hint 1

Who lives in the mouth and feeds on leftover sugar?

Hint 2

Heating NaHCO₃ gives Na₂CO₃. What does recrystallisation add to it?

Hint 3

What gives copper sulphate crystals their blue colour?

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) Bacteria in the mouth break down sugar and food particles left after eating and produce acids. These acids lower the pH of the mouth below 5.5, which can cause tooth decay.

    1 mark

  2. (b) Heating baking soda gives sodium carbonate. Recrystallising it gives 'X' = washing soda, (sodium carbonate decahydrate).

    ½ mark

  3. Industrial use 1: in the glass, soap and paper industries.

    ½ mark

  4. Industrial use 2: in the manufacture of sodium compounds such as borax. (Also accepted: removing permanent hardness of water.)

    ½ mark

  5. (c) On heating, blue copper sulphate crystals lose their water of crystallisation and become white anhydrous copper sulphate:

    ½ mark

Answer: (a) Acids produced by bacteria degrading sugar/food in the mouth; (b) X = washing soda Na₂CO₃·10H₂O, used in glass/soap/paper industries and manufacture of borax; (c) loss of water of crystallisation

Where the marks are

  • Bacteria degrade sugar/food particles and produce acid (1 mark)
  • Identifies X as washing soda, Na₂CO₃·10H₂O (½ mark)
  • Two industrial uses (½ + ½ mark)
  • Water of crystallisation is removed on heating (½ 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 31/5/1: 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.