Q.An amorphous solid (X) burns in air to form a gas (Y) which turns lime water milky. The gas (Y) decolourises aqueous solution of acidified KMnO4. Gas (Y) reacts with oxygen to give another gas (Z) which is responsible for acid rain. X, Y and Z are

Ordinary (rhombic/monoclinic) sulphur exists as S8 molecules — octatomic puckered rings.

At ordinary temperatures the most stable allotropes of sulphur (rhombic and monoclinic) are built from S8 molecules, each a crown-shaped (puckered) ring of eight sulphur atoms. Hence the sulphur molecule is …

The clues fit sulphur: S burns to SO2; SO2 turns lime water milky and decolourises acidified KMnO4 (reducing agent); SO2 + O2 -> SO3, which forms H2SO4 (acid rain). Correct option (a).

Decoding the clues:

• X is an amorphous solid that burns in air -> amorphous sulphur. S + O2 -> SO2.
• Y turns lime water milky: SO2 + Ca(OH)2 -> CaSO3 (white/milky) + H2O.
• Y decolourises acidified KMnO4: SO2 acts as a reducing agent, reducing purple MnO4- to colourless Mn2+.
• Y + O2 -> Z: 2SO2 + O2 -> 2SO3; SO3 + H2O -> H2SO4, responsible for acid rain. …

S8 is an elemental (free) form of sulphur; a free element always has oxidation state 0.

S8 is a molecule made entirely of sulphur atoms (a crown-shaped ring of eight S atoms). Since the molecule consists of only one kind of atom bonded to identical atoms, there is no electronegativity difference and no charge separation …