Explain why water is a compound and not a mixture.

Hint: The compounds are formed by two or more elements which have been bonded through bonds. While in the mixture the elements are not bonded but they have variable composition. The mixture is formed by mixing two or more substances physically.

Complete step by step answer:
The compound is defined as a substance which is made up by two or more elements. Whereas the element is the substance which is made up of one type of particle. These particles can be atoms or molecules. If these particles are molecules then we combine two atoms to form a respective molecule such as nitrogen, hydrogen, oxygen which get further combined or bonded so that the formation of compound takes place. Such as water. for example:
$${\text{2}}{{\text{H}}_{\text{2}}}{\text{ + }}{{\text{O}}_{\text{2}}} \to {\text{2}}{{\text{H}}_{\text{2}}}{\text{O}}$$
In compounds the substances which are present have a definite composition and a fixed structure and it does not tend to be separated by physical means such as filtration or evaporation but get rarely separated by fractional distillation.
Whereas the mixtures are defined as the substances which are made up by the two or more substances but they do not get bonded together and each of the substances in it tends to have its own properties and the ratio of substances is variable not definite. They have a tendency to be separated by evaporation, filtration or distillation.
So from the above discussion it is clear that water is considered as a compound and not a mixture as it is made up of two kinds of elements which have a definite and fixed proportion.

Note: There are different types of compound such as metallic compound, ionic compound and covalent compounds. Whereas mixtures are of two types such as homogeneous and heterogeneous mixtures. The melting point of the compound is defined while that of mixture is not defined.