A reddish-pink substance on.heating gives off a vapor which condenses on the sides of the test tube and the substance turns blue. If on cooling water is added to the residue, it turns to its original color. The substance is :

lodine crystals

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Copper sulphate crystals

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Cobalt chloride crystals

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Zinc oxide

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Solution

The correct option is C Cobalt chloride crystals
Option (C) is correct.
The substance is cobalt chloride crystals.It is a property of $C o C l_{2}$ .
Cobalt sulphide dissolves in aqua regia in the same manner as nickel sulphide. When the aqueous solution of the residue obtained after treatment with aquaregia is treated with a strong solution of potassium nitrite after neutralisation with ammonium hydroxide and the solution is acidified with dil. acetic acid, a yellow precipitate of the complex of cobalt named potassium hexanitritocobaltate (III) is formed.
$C o S + H N O_{3} + 3 H C l ⟶ C o C l_{2} + N O C l + S + 2 H_{2} O$
$C o C l_{2} + 7 K N O_{2} + 2 C H_{3} C O O H ⟶ K_{3} [C o (N O_{2})_{6}] + 2 K C l + 2 C H_{3} C O O K + N O + H_{2} O$

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