An unknown inorganic compound (X) loses its water of crystallization on heating and its aqueous solution gives the following reactions :

(a) It gives a white turbidity with dilute HCl solution

(b) It decolourises a solution of iodine in potassium iodide

(c) It gives a white precipitate with silver nitrate solution which turns black on standing.

Identify the compound (X)

Complete Solution:

To identify the compound (X), let's analyze the reactions step by step:

Observations and Analysis

1. Loses water of crystallization on heating: This indicates that (X) is a hydrated compound, commonly a salt containing water of crystallization.
2. Reaction (a): White turbidity with dilute HCl: This suggests the presence of a compound that reacts with HCl to form a slightly soluble product. This often happens with compounds that release SO₂ gas, such as thiosulfates or sulfites.
3. Reaction (b): Decolorizes iodine in potassium iodide: This is a typical reaction of a reducing agent. Thiosulfates, for instance, reduce iodine (I₂) to iodide (I⁻), causing decolorization.
4. Reaction (c): White precipitate with silver nitrate, turns black on standing: A white precipitate with silver nitrate suggests the presence of anions like chloride (Cl⁻) or thiosulfate (S₂O₃²⁻). The blackening indicates a reaction leading to the formation of metallic silver (Ag), which can occur with thiosulfate.

Compound Identification

Based on these observations, the compound (X) is likely sodium thiosulfate pentahydrate (Na₂S₂O₃·5H₂O).

Reasoning for Na₂S₂O₃·5H₂O

• It is a hydrated compound, losing its water of crystallization on heating.

• With dilute HCl, it forms SO₂, causing turbidity:

  Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S + H₂O

• It decolorizes iodine in potassium iodide due to its reducing action:

  Na₂S₂O₃ + I₂ → NaI + Na₂S₄O₆

• It reacts with silver nitrate to form silver thiosulfate, which decomposes to black metallic silver:

  2AgNO₃ + Na₂S₂O₃ → Ag₂S₂O₃ + 2NaNO₃ → 2Ag + S + O₂

Final Answer

The compound (X) is identified as sodium thiosulfate pentahydrate (Na₂S₂O₃·5H₂O).