Which substance is most likely to be an element?
A green solid which turns black and produces brown fumes on strong heating
A colorless liquid that boils at 78 oC exactly
A reddish brown powder which burns in oxygen to form a black powder only
A colorless gas that forms a white precipitate in calcium hydroxide solution
The correct option is B A colorless liquid that boils at $$78\ ^oC$$ exactly
A colorless liquid boils at $$78^oC$$ exactly, and that means that it is a pure substance. A pure sample could either be an element or a compound. Answer $$B$$ will not be able to substantiate that the sample is an element. The correct answer is $$C$$. You must understand that if a compound were to undergo combustion, it will yield at least two products. Since that powder burns in oxygen to form a black powder only, it suggests that the powder is an element, which had combined with oxygen to form a basic oxide or something.