Question

Which of the following is a compound?
A. Carbon Dioxide
B. Iron
C. Oxygen
D. Air

Answer 1

Hint: To solve this question, we should know about the basic difference between an element, a compound and a mixture. An element is a pure substance. A compound is a substance made by irreversibly mixing two or more elements in a definite proportion. A mixture is formed by mixing elements or compounds together reversibly without a fixed ratio.

Complete step by step answer:
We know that a compound is a substance made up of two or more separate elements mixed together in a definite proportion. A compound is not found in the periodic table but has its own characteristic physical and chemical properties which may be a lot different from the elements it is made up of.
In option A, carbon dioxide is made up of 1 molecule of carbon with 2 molecules of oxygen. It is a gas at room temperature. The atomic mass of carbon dioxide can be found by adding the atomic mass of its constituent molecules, i.e. $12 + (16 \times 2) = 44$amu. The state in which carbon dioxide is present is also different from one of its constituent molecules, i.e. carbon, as it is a solid. So, carbon dioxide fulfils all the conditions of being a compound.
In option B, iron is a pure substance which has its own unique properties and characteristics. It is found as a solid in room temperature. It has a melting point of 1540℃. It can also be found in the periodic table. Iron has an atomic number of 26 and atomic mass of 56 amu. So, iron is an element.
In option C, oxygen is a pure substance, it is found as a gas in our atmosphere. It has an atomic number of 8 and atomic mass of 16 amu. So, oxygen is an element.
In option D, air as we know it consists of a variety of gases in different compositions. It has 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide and a number of different gases like neon, helium, water vapour, dust particles etc. present in trace amounts which differ from place to place. Thus, air is a mixture.

$\therefore $ The compound in the given options is carbon dioxide, i.e. option A.

Note:
Though compounds are made up of different elements in a fixed ratio, they are still regarded as pure substances unlike mixtures. This is because compounds have their own characteristic properties and the constituent elements cannot be separated easily by physical means. In the case of mixtures, the constituent elements maintain their own characteristics and can be easily separated by physical means.