Question

Which of the following is a mixture?
A. Water.
B. Oxygen.
C. Carbon dioxide.
D. Air.

Answer 1

Hint: We must note in this question that a mixture is a material consisting of two or more different substances which are physically mixed. A mixture is the physical combination of two or more substances that hold identities and combine in the form of liquids, suspensions, and colloids.

Complete step-by-step answer:
Mixtures are one product where chemical substances such as elements and compounds are mechanically combined or mixed without chemical bonding or other chemical alteration, so that each material maintains its own chemical properties and makeup. Since there are no chemical changes to their constituents, a mixture's physical properties, such as its melting point, can vary from those of the components.
Let's look one by one at all of the options.
1. Water.
We know that when two or more substances physically combined together is a mixture. In water, however, two atoms of hydrogen combine chemically with one atom of oxygen, creating a new material which has properties other than hydrogen alone or oxygen alone.
Water, though, is not a mixture.
2. Oxygen.
As we have already discussed that a mixture is a combination of two or more substances, and that oxygen is an element denoted as O.
Oxygen, then, is not a mixture.
3. Carbon dioxide.
Carbon dioxide consists of the carbon and oxygen molecules. The chemical composition of carbon and oxygen makes up carbon dioxide. Therefore, carbon dioxide is not a mixture.
4. Air.
Water is a homogenous mixture of nitrogen, oxygen and smaller quantities of other compounds from the gaseous liquids. Air contains 78% of nitrogen, 21% of oxygen, and 1% of other elements such as $CO_2$, water vapour etc. Mixtures are unlike chemical materials, since physical methods such as filtration, freezing, and distillation can isolate the substances in a mixture.
So, we can say air is a mixture.
So option (D) is the correct answer.

Note: If we ask questions of this kind, we should realize that mixtures may be homogeneous or heterogeneous. A mixture where its constituents are uniformly distributed is called homogeneous mixture, such as salt in water. A mixture in which there is no uniform distribution of its constituents is called a heterogeneous mixture, such as sand in water. Based on these properties we can easily classify whether the given element or compound is a mixture or not.