Question

Which substance is most likely to be an element?

• A. A green solid which turns black and produces brown fumes on strong heating
• B. A colorless liquid that boils at $78{}^{o}C$ exactly
• C. A reddish brown powder which burns in oxygen to form a black powder only
• D. A colorless gas that forms a white precipitate in calcium hydroxide solution

Answer 1

The correct option is B A colorless liquid that boils at $78{}^{o}C$ exactly

A colorless liquid boils at ${78}^{o}C$ 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.