Question

Why is water a compound but air is a mixture?

Answer 1

Hint: We must always keep in mind that if we want to know the answer to “WHY”, we first need to get the answer to “WHAT”. To know why water is a compound and air is a mixture, we first need to know what is a compound and what is a mixture.

Complete step by step solution:
-So first we need to be clear about the definition of the terms mentioned in the question. There are three types of matter classification- element, compound and mixture.
-Elements are a group of atoms of the same kind only. They can be metal or non-metal but they need to be a combination of the same type of atoms only. Eg, hydrogen ${{H}_{2}}$ , oxygen ${{O}_{2}}$ , etc. They are mostly formed by sharing of electrons.
-Compounds are the substances which are made up by combining atoms or molecules of different types but in a definite proportion. They remain the same every time and everywhere. The ratio in which the different atoms or molecules are mixed never changes irrespective of time, temperature, pressure, place, height, etc.
Eg, $HCl$ is a compound which is made up of two different atoms, a hydrogen atom and chlorine atom and both are mixed in the same ratio. The composition remains unaltered everywhere and every time.
-Mixtures are the substances in which different types of atoms/ molecules/elements/compounds are mixed and the ratio of their mixing is not fixed. It may vary from place to place or from time to time. We cannot study the true composition of a mixture. Eg, when we mix salt, sugar and lime in water to make lemonade, the lemonade thus formed is a mixture as it contains different compounds in different ratios.
-In our question, if we talk about water, we know that its chemical formula is ${{H}_{2}}O$. In water, two moles of hydrogen atom combine with one mole of oxygen atom only. This composition does not change. So water is a compound.
-If we take the example of air, we can observe that air contains many types of gases like nitrogen, oxygen, water vapour, elements and many more substances. Everywhere, the composition of air changes. The composition of air is different when we take the sample near the forest and it is different if we take the sample near an industry. Not only this, composition changes even for the same place but at different times. Thus, the air is a mixture.
Therefore, water is considered as a compound and air as a mixture.

Note: The nature of a compound or a mixture does not depend on the state of matter. We cannot say that water can convert from compound to mixture if we change its state from solid, liquid or gas. Also, if we add something to the water, then the resulting solution will definitely be a mixture but still, individually, water will always be a compound.