Which substance is most difficult to liquefy?

The most difficult substance to liquefy among the options presented is parahydrogen, which has a boiling point of -252.9°C. The ability to liquefy a gas is significantly influenced by its boiling point and the intermolecular forces present within the substance.

Parahydrogen, which is a molecular form of hydrogen, exhibits very weak intermolecular forces due to its nonpolar nature. This characteristic results in a notably low boiling point, meaning that the temperature must be lowered significantly to condense it into a liquid state. Moreover, hydrogen molecules are small and light, which contributes to their high kinetic energy at ordinary temperatures, making it harder to achieve liquefaction compared to the other substances listed.

In contrast, substances like carbon dioxide, nitrogen, and argon can be liquefied more easily because their boiling points, while still low, are higher than that of parahydrogen. Additionally, these gases have comparatively stronger intermolecular forces, which assist in their liquefaction under appropriate pressure and temperature conditions. Therefore, the requirement for extreme cooling to liquefy parahydrogen emphasizes why it is the most challenging among the provided options.