Question

A white amorphous solid, which on heating gives a residue which is yellow when hot but it turns white on cooling, and gives a colourless, odourless gas, which um limewater milky​

Answer 1

Answer:

Zinc carbonate (A) is the white amorphous solid which on heating gives colourless gas (B) which is carbon dioxide and the residue (C) which is zinc oxide.

ZnCO

3

Δ

B

CO

2

+

C

ZnO

Carbon dioxide gas turns lime water milky and zinc oxide is yellow when hot and white when cold.

Zinc oxide dissolves in dilute HCl to form zinc chloride which reacts with potassium ferrocyanide to form white ppt.

Zinc carbonate dissolves in dil HCl to liberate carbon dioxide gas (B).

ZnCO

3

+2HCl→ZnCl

2

+H

2

O+CO

2

↑

Ammonium hydroxide neutralizes excess HCl.

Zn(II) ions react with hydrogen sulphide to give a white ppt D which is ZnS.

Zn

2+

+S

2−

→ZnS↑

Zn(II) ions react with NaOH to form white precipitate E which is zinc hydroxide.

Zn

2+

+2NaOH→

white ppt, E

Zn(OH)

2

This ppt dissolves in excess NaOH

white ppt, E

Zn(OH)

2

+2NaOH→

sodium zincate

Na

2

ZnO

2

+2H

2

O