Here's why:

* Effusion: This is the process where gas molecules escape through a small hole into a vacuum. Lighter molecules effuse faster than heavier ones. This difference in effusion rates can be used to partially separate a gaseous mixture.

* Condensation: This is the process where a gas changes into a liquid. Different gases have different boiling points. By cooling the mixture, the gas with the higher boiling point will condense first, allowing for separation.

Example:

Imagine a mixture of helium (He) and neon (Ne) gases. Helium is lighter than neon.

1. Effusion: If the mixture is allowed to effuse through a small hole, helium will effuse faster than neon, resulting in a slightly higher concentration of helium in the escaping gas.

2. Condensation: If the mixture is cooled, neon will condense first because it has a higher boiling point than helium. This leaves behind a gas richer in helium.

Note: These methods are often used in combination with other separation techniques for more efficient results.