Question

Which of the following is not a pure substance?
A. Gold
B. Glucose
C. Pure Milk
D. Water

Answer 1

Hint: Analyze the chemical composition of each of the substances given in options. For a substance to be pure it should not be a mixture, colloid or emulsion. It should occur naturally in nature without any refining process.

Complete answer:
Let us look at each substance individually. First is gold. Gold is an element in the periodic table. It is found in the group $11$ and period $6$ and has atomic number $79$ . It is one of the among the many d-block elements. Gold is a precious metal. It is bright, dense, soft, malleable and ductile metal and is slightly reddish yellow in color. Gold is found as pure metal in nature and pure gold is referred as 24-karat gold. So gold is a pure substance.
Glucose, which is commonly known as simple sugar has six carbon atoms attached to an aldehyde group. Its chemical formula is $C_6{H}_{12}{O}_6$. It is a sugar occurring naturally with fixed composition. It is found in milk, fruits and wheat in pure form. So glucose is a pure substance.
Water as we all know has the chemical formula $H_{2}O$ and is found in abundant quantities in nature in reservoirs throughout the earth. So water is a naturally occurring substance.
Now we are left with pure milk. Though the name is pure milk but it is not completely pure. Milk is a colloidal solution. It is an emulsion of milk fat in water. Milk comprises fat which is in the form of ester and carbohydrates as sugar. Therefore milk is not a pure substance.

Therefore, option (C) is the correct answer.

Note:
Milk is very nutritious in composition. One glass of milk can consist of the nutrients of a whole diet. Milk also contains important nutrients like riboflavin and vitamin B12. But it also contains acidic animal protein.