The gases that cannot be liquefied by increasing their pressure alone are called permanent gases. They are gases that have a critical temperature below room temperature.

Here are some examples of permanent gases:

* Hydrogen (H2)

* Helium (He)

* Nitrogen (N2)

* Oxygen (O2)

* Neon (Ne)

* Argon (Ar)

Why can't they be liquefied by pressure alone?

To liquefy a gas, you need to decrease its kinetic energy and increase its intermolecular forces. Increasing pressure alone only increases the intermolecular forces. To decrease kinetic energy, you also need to lower the temperature below the critical temperature. The critical temperature is the temperature above which a gas cannot be liquefied, no matter how much pressure is applied.

Liquefying permanent gases:

To liquefy permanent gases, you need to cool them below their critical temperature and then apply pressure. This is typically done using techniques like:

* Linde process: This process involves compressing the gas and then cooling it by letting it expand.

* Claude process: This process is similar to the Linde process, but it uses a more efficient cooling system.

Let me know if you have any other questions!