Why is water considered a pure substance, while milk is not?

Answer : The water is made up of only one type of molecule and milk is made up of different type of molecules.

Explanation :

Pure substance : It is defined as a substance that is made by the combination of only one type of atom or only one type of molecule.

The pure substance can not be separated by simple physical methods.

Heterogeneous mixtures : It is a mixture that appears non-uniformly throughout the solution and the particle size or shapes are also different.

Homogeneous mixtures : It is a mixture that appears uniformly throughout the solution and the particle size or shapes are not different.

As water is considered as a pure substance because it is made up of only one type of molecule that means it is made up of two hydrogen atoms bonded to a single oxygen atom.

While on the other hand, milk is considered as a mixture because various substances (fats, proteins, water, lipids etc..) are present in milk. So, it is not a pure substance.