Question

Which one of the following is a compound?
(A). The black residue left when sugar is charred
(B). Nitrogen gas
(C ). Air
(D). Carbon dioxide

Answer 1

- Hint: The question is asking you to identify a compound, so you must know the definition of compound to find the final answer.
                  “Compound is a substance formed from two or more elements chemically bonded in a definite proportion.”

Complete step by step answer:
Let's look at all options one by one to reach the solution -
The black residue left when sugar is charred - Charring of sugar means continuously heating the sugar in a dish and evaporating it. When sugar is charred, it forms a black powdery substance, which is charcoal. Charcoal is a form of pure carbon element (C). Hence we can say it is not a compound.
Nitrogen gas - Nitrogen gas $({ N }_{ 2 })$ is a molecule as it is composed of two units of one element that is, Nitrogen. So we can say this can't be termed as a compound.
Air - Air is a homogeneous mixture of different gases present in the environment. A homogeneous mixture is a solid, liquid or gaseous mixture that has the same proportions of its components throughout any given sample. This option also does not satisfy our requirements.
Carbon dioxide - Carbon dioxide $({ CO }_{ 2 })$ contains one unit of carbon as element and two units of oxygen as element. Hence it satisfies a proper definition of compound.
So we can conclude that option D is the correct one.

Note: Always remember, a given species is called a compound only when the elements present in it are different. For example, ozone is made up of three units of oxygen, so it is not a compound; whereas, nitrogen dioxide gas is made up of two units of oxygen and one unit of nitrogen, is a compound.