Acid-base indicators can be used to distinguish between an acid and a base. Universal indicator, a mixture of several indicators, shows different colours at different concentrations of acids and bases, thereby indicating their pH on the pH scale of 0 – 14. The pH of a solution is measured by pH paper, which is a paper impregnated with a universal indicator. Answer the following questions: (a) Solution P is a strong acid while solution Q is a strong base. On the pH scale, where would you place the solutions P and Q? (b) A solution has a pH of 7. Name a compound you would use to (i) increase its pH, and (ii) decrease its pH. (c)(i) When the pH of a solution is decreased from 4 to 2, what effect does it produce on its hydronium ion concentration? State the colour change shown by the pH paper. (c)(ii) A person is feeling pain and irritation in the stomach due to indigestion. What could be the pH of the fluid in the stomach? Write the common name of the medicines people use for remedy. Give the chemical name of "milk of magnesia" often used for this purpose.

ScienceCBSE Class 10 · Board exam 2025, set 31/2/1 · 4 marks · Case-based

CBSE Class 10 Science 2025: question 37

Set in the CBSE Class 10 board exam of 2025, 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

Acid-base indicators can be used to distinguish between an acid and a base. Universal indicator, a mixture of several indicators, shows different colours at different concentrations of acids and bases, thereby indicating their pH on the pH scale of 0 – 14. The pH of a solution is measured by pH paper, which is a paper impregnated with a universal indicator.

Answer the following questions:

(a)
Solution P is a strong acid while solution Q is a strong base. On the pH scale, where would you place the solutions P and Q?
1 mark
(b)
A solution has a pH of 7. Name a compound you would use to (i) increase its pH, and (ii) decrease its pH.
1 mark
(c)(i)
When the pH of a solution is decreased from 4 to 2, what effect does it produce on its hydronium ion concentration? State the colour change shown by the pH paper.
2 marks
OR
(c)(ii)
A person is feeling pain and irritation in the stomach due to indigestion. What could be the pH of the fluid in the stomach? Write the common name of the medicines people use for remedy. Give the chemical name of "milk of magnesia" often used for this purpose.
2 marks

Figure in the paper: A horizontal pH scale line with tick marks and labels from 0 to 14 in steps of 1, titled 'pH Scale'. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/2/1.

Hints

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

Hint 1

Acids have pH below 7 and bases above 7; strong ones lie at the extremes.

Hint 2

Adding a base raises pH and adding an acid lowers it; lower pH means more H⁺.

Hint 3

Stomach pain from indigestion is caused by too much acid; what neutralises it?

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.

(a) P, a strong acid, lies at the low end of the scale, between pH 0 and 4 (around pH 1).
½ mark
(a) Q, a strong base, lies at the high end of the scale, between pH 12 and 14.
½ mark
(b)(i) To increase the pH, add a base such as sodium hydroxide (NaOH).
½ mark
(b)(ii) To decrease the pH, add an acid such as hydrochloric acid (HCl).
½ mark
(c)(i) A lower pH means a more acidic solution, so the hydronium ion ( $H_{3} O^{+}$ ) concentration increases. A fall of 2 pH units means 100 times more.
1 mark
(c)(i) The colour of the pH paper changes from yellowish-orange (at pH 4) to red (at pH 2).
1 mark
(c)(ii) Indigestion is caused by excess acid in the stomach, so the stomach fluid has a low pH, about 1 to 3.
1 mark
(c)(ii) The medicines used are called antacids.
½ mark
(c)(ii) Milk of magnesia is magnesium hydroxide, $M g (O H)_{2}$ .
½ mark

Answer: (a) P: pH 0–4; Q: pH 12–14. (b)(i) NaOH (a base); (ii) HCl (an acid). (c)(i) H₃O⁺ concentration increases; colour changes from orange/yellow to red. (c)(ii) pH about 1–3; antacids; magnesium hydroxide, Mg(OH)₂.

Where the marks are

P at pH 0–4 (½ mark)
Q at pH 12–14 (½ mark)
Base to increase pH (½ mark)
Acid to decrease pH (½ mark)
(c)(i) Hydronium ion concentration increases (1 mark)
(c)(i) Colour changes from orange/yellow to red (1 mark)
(c)(ii) Low pH, 1–3 (1 mark)
(c)(ii) Antacids (½ mark)
(c)(ii) Magnesium hydroxide (½ mark)

Revise the chapter

Chapter: Acids, Bases and Salts (notes, formulas, where marks are lost and every past question)
Topic: pH Scale and Its Importance

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 paper2025 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.

CBSE Class 10 Science 2025: question 37

Hints

Step-by-step solution

Revise the chapter

More questions from Acids, Bases and Salts