A white solid X is heated strongly in a dry test tube. It evolves a colourless, odourless gas that turns lime water milky, and the residue is yellow while hot but white on cooling. X is:

Answer: B

Zinc carbonate decomposes:

ZnCO X_{3} ZnO + CO X_{2}

, and

ZnO

is yellow when hot and white when cold (thermochromism).

CaO

and

MgO

are white at all temperatures, and

Na X_{2} CO X_{3}

does not decompose in a Bunsen flame. So X is

ZnCO X_{3}