Question

Which substance can not be decomposed by ordinary chemical means?
(1) methane (3) ethanol
(2) mercury (4) ammonia

Answer 1

(2) mercury cannot be decomposed by ordinary methods because it is an element, to "decompose" these you need an accelerated atom smasher to separate these into protons, neutrons, electrons, etc.

Answer 2

Elements can't be decomposed while compounds can be decomposed.

The answer would be (2) mercury because since mercury is an element, it cannot be broken down chemically.