Question

Which of the following is NOT a pure substance? A. milk B. oxygen C. water D. carbon dioxide

Answer 1

Among the choices given, the substance that is not considered pure is C. milk. Milk is not pure because it is composed of more than one kind of molecule. Milk is composed of lactic acid, water, etc which is a combination of different molecules to form the solution. Others are only made up of one kind of molecule

Answer 2

Milk is not a pure substance.  It is a mixture of many substances in a solution.

I hope this helps.  Let me know if anything is unclear.