Science · Chemical substances · chapter 2 of 13

Class 10 Science Chapter 2: Acids, Bases and Salts case study question with answers

One case-based question of the kind the board asks in Section E: a passage and a table, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks

In a school laboratory in Kochi, Meera tests six samples with universal indicator, a mixture of indicators that shows a different colour at each pH. She records the pH she reads from the colour chart in the table below.

Sample	pH
Gastric juice	1.2
Lemon juice	2.2
Pure water	7
Blood	7.4
Milk of magnesia	10
Sodium hydroxide solution	14

Part 1 · 1 mark

Which sample has the highest hydrogen ion concentration?

(a) Lemon juice(b) Gastric juice(c) Pure water(d) Sodium hydroxide solution

Show answer

Answer: (b) Gastric juice

The lower the pH, the higher the hydrogen ion concentration; gastric juice has the lowest pH, 1.2.

Part 2 · 1 mark

Which sample would turn colourless phenolphthalein pink?

(a) Lemon juice(b) Pure water(c) Milk of magnesia(d) Gastric juice

Show answer

Answer: (c) Milk of magnesia

Phenolphthalein turns pink only in a basic solution; milk of magnesia, pH 10, is the only basic sample among the options.

Part 3 · 2 marks

A person with indigestion is advised to take milk of magnesia. Using the table, explain how it gives relief.

Answer and marking scheme

Indigestion is caused by excess hydrochloric acid in the stomach; gastric juice has pH 1.2. Milk of magnesia, Mg(OH)₂, has pH 10, so it is a mild base; it neutralises the excess acid and gives relief.

excess stomach acid (HCl, low pH) causes indigestion (1)
milk of magnesia is a base (pH 10) that neutralises the excess acid (1)

Is blood acidic, basic or neutral according to the table? Within what pH range does the human body work, and why does that range matter?

Answer and marking scheme

Blood has pH 7.4, greater than 7, so it is slightly basic. The body works within a narrow pH range of 7.0 to 7.8, and living organisms can survive only within a narrow range of pH change.

blood slightly basic, pH above 7 (1)
body works within 7.0 to 7.8; organisms survive only a narrow pH change (1)

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