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The s-Block Elements

Multiple Choice Questions

101.

Which of the following carbonates decomposes at lowest temperature?

MgCO₃
CaCO₃
BaCO₃
SrCO₃

102.
A white-coloured inorganic compound, on heating, gives a gas which turns lime water milky and residue is left which is yellow when hot and turns white on cooling. The compound is

Pb(NO₃)₂

PbCO₃

BaCO₃

ZnCO₃

ZnCO₃

Since the obtained gas turns lime water milky, it must be CO₂ and the anion present in the compound must be CO $_{3}^{2 -}$ .

The residue left after the liberation of CO₂ is yellow when hot and turns white on cooling, so it is of ZnO and the cation present in the compound is Zn²⁺. Hence, the white coloured inorganic compound is

${Zn}^{+} + {CO}_{3}^{2 -} \to {ZnCO}_{3} and the reactions are as follows : {ZnCO}_{3} \overset{∆}{\to} ZnO + {CO}_{2} {CO}_{2} + \underset{lime water}{Ca (OH)_{2}} \to \underset{calcium carbonate (milky)}{{CaCO}_{3}} + H_{2} O$