When X is heated at 373 K, it loses water molecules and gets converted to Y. Y is used to keep things dry.
(i) Identify X and Y.
(ii) How can X be reobtained from Y ?
Generated by claude-sonnet-4-6 · 2026-06-15 06:45 · grounding rag
Model Answer
(i) X is Gypsum (CaSO₄·2H₂O) and Y is Plaster of Paris (CaSO₄·½H₂O).
Reaction: CaSO₄·2H₂O $\xrightarrow{373\text{ K}}$ CaSO₄·½H₂O + 1½H₂O
(ii) X (Gypsum) can be reobtained from Y (Plaster of Paris) by adding water to it. Plaster of Paris absorbs water and sets into a hard solid mass, converting back to gypsum.
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
Explanation
- Examiners expect you to correctly name both X and Y with their chemical formulae.
- The key fact is that heating gypsum at 373 K removes ¾ of the water of crystallisation to form Plaster of Paris (not complete dehydration).
- For part (ii), the reverse reaction — Plaster of Paris absorbing water to reform gypsum — is the key concept. Writing the chemical equation earns full marks.
- Since no source passage directly covers this topic, this answer is based on standard CBSE Class 10 Science Chapter 2 (Acids, Bases and Salts) content.