Which of the following materials fall in the category of a “pure substance”?

(a) Milk

(b) Iron 

(c) Mercury 

(d) Wood

(e) Air

(b) and (c) only

The correct answer is option 2. (b) and (c) only.

A pure substance consists of a single type of particles. In other words, a substance is a pure single form of matter.

(b) Iron and (c) Mercury contain only one type of particles, hence, they are pure.

Scientifically Impure: (a) Milk, (d) Wood and (e) Air.

Milk is actually a mixture of water, fat, proteins etc.

Wood is a mixture as it is formed of combinations of the elements Carbon, Oxygen, Hydrogen, Nitrogen, and a small percentage of other elements. 

Air is a mixture of gas in gas. Air is a homogeneous mixture of a number of gases. Its two main constituents are: oxygen (21%) and nitrogen (78%).