Which of these is a pure substance and why? A Milk, because every portion of the matter is the same, and its components can only be separated chemically. B Coffee, because every portion of the matter is the same, and its components can only be separated chemically. C Salt water, because every portion of the matter is the same, and its components can only be separated chemically. D Carbon dioxide, because every portion of the matter is the same, and its components can only be separated chemically.