Answer the following. (a) A compound 'A' is used in soda-lime fire-extinguisher and decomposes on heating to form compound 'B'. Compound 'B' is used for removing permanent hardness of water. Identify 'A' and 'B'. (b) Write chemical formula of Plaster of Paris. At what temperature gypsum is heated to obtain Plaster of Paris?

(a) A = NaHCO_3 (baking soda), B = Na_2CO_3 (washing soda). (b) CaSO_4·1/2H_2O; 373 K (100 °C).

ScienceCBSE Class 10 · Board exam 2026, set 31/4/1 · 2 marks · Very short answer

CBSE Class 10 Science 2026: question 25

Set in the CBSE Class 10 board exam of 2026, paper 31/4/1, section B, question 25, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q25 · 2 marks · Very short answerAcids, Bases and Salts

Answer the following.

(a)
A compound 'A' is used in soda-lime fire-extinguisher and decomposes on heating to form compound 'B'. Compound 'B' is used for removing permanent hardness of water. Identify 'A' and 'B'.
1 mark
(b)
Write chemical formula of Plaster of Paris. At what temperature gypsum is heated to obtain Plaster of Paris?
1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 31/4/1.

Hints

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

Hint 1

Which sodium salt gives off CO $_{2}$ in a fire extinguisher?

Hint 2

Heating that salt gives washing soda. For (b), gypsum loses water at the boiling point of water.

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) A = sodium hydrogen carbonate (baking soda), $N a H C O_{3}$ . It is used in soda-acid fire extinguishers.
½ mark
On heating it gives B = sodium carbonate (washing soda), $N a_{2} C O_{3}$ , which removes permanent hardness of water:
$2 N a H C O_{3} heat N a_{2} C O_{3} + H_{2} O + C O_{2}$
½ mark
(b) Formula of Plaster of Paris: $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ (calcium sulphate hemihydrate).
½ mark
Gypsum ( $C a S O_{4} \cdot 2 H_{2} O$ ) is heated at 373 K (100 °C) to obtain Plaster of Paris.
½ mark

Answer: (a) A = $N a H C O_{3}$ (baking soda), B = $N a_{2} C O_{3}$ (washing soda). (b) $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ ; 373 K (100 °C).

Where the marks are

Identifies A as sodium hydrogen carbonate (½ mark)
Identifies B as sodium carbonate (½ mark)
Writes the formula $C a S O_{4} \cdot \frac{1}{2} H_{2} O$ (½ mark)
States the temperature 373 K (½ mark)

Revise the chapter

Chapter: Acids, Bases and Salts (notes, formulas, where marks are lost and every past question)
Topic: Baking Soda, Washing Soda and Plaster of Paris

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 paper2026 board paper, set 31/4/1: 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 2026: question 25

Hints

Step-by-step solution

Revise the chapter

More questions from Acids, Bases and Salts