Which of the following is NOT a pure substance? 

Explanation: 

Pure substance: A pure substance is a substance which is

• Homogeneous in chemical composition, and
• Invariable in chemical aggregation through its mass.

Examples: water, oxygen, nitrogen, atmospheric air, the mixture of steam-water, combustion product of fuel, vapours of ammonia, etc.

Note: 

• The mixture of air (gas phase) and liquid air is not a pure substance. This is due to different relative proportions of nitrogen and oxygen in the air (gas phase) and liquid air in equilibrium.
• Another example is humid air, which is not a pure substance.

Thus, option (1) is the correct answer.