The seperation of Nitrogen from oxygen using fractional distillation of air is possible because

Step 1: Analyze the question We are asked why fractional distillation can separate nitrogen from oxygen in liquefied air. The key phrase is "fractional distillation," which is a separation method based on a specific physical property.

Step 2: The concept Fractional distillation separates the components of a liquid mixture according to their boiling points. When the mixture is warmed slowly, the component with the lower boiling point vaporizes first and rises up the fractionating column. The component with the higher boiling point stays liquid longer.

Step 3: Apply it to air Air is first cooled and compressed until it becomes a liquid. The liquid air is then allowed to warm up slowly:

• Nitrogen boils at about -196 °C (77 K), so it boils off first.
• Oxygen boils at about -183 °C (90 K), so it remains in the liquid and boils off later.

Because their boiling points differ, the two gases can be collected separately.

Step 4: Check each option

• A. Nitrogen is less dense than oxygen. Density differences are not what fractional distillation uses. Density matters in other methods, such as separation by settling or centrifugation.
• B. Oxygen is more reactive than nitrogen. Reactivity is a chemical property, but distillation is a physical separation method.
• C. Difference in their boiling points. This is correct. It is the principle behind fractional distillation.
• D. They belong to the same period. Both are in Period 2, but this has nothing to do with how they are separated.

Answer: C

Exam tip: Whenever you see "fractional distillation," think boiling points. Examples include separating crude oil into its fractions, ethanol from water, and gases from liquid air. Also remember that simple distillation separates a liquid from a dissolved solid, while fractional distillation separates liquids that have different boiling points.