Which of the following gases is the most easily liquefied?

Correct answer: (a) NH₃

Explanation

A gas is easier to liquefy when the forces between its molecules are stronger, which gives it a higher critical temperature.

Ammonia (NH₃) molecules are polar and attract each other by hydrogen bonding, so ammonia has a high critical temperature (about 132 °C) and is liquefied easily by pressure alone at room temperature. That is why it is used as a refrigerant. Methane, carbon monoxide and hydrogen have only weak forces between molecules and very low critical temperatures; hydrogen is the hardest to liquefy.