Question

Gases are liquefied under
A.Low temperature and low pressure
B.High pressure and low temperature
C.High temperature and high pressure
D.None of these

Answer 1

Hint: Molecules in gaseous state are very far from each other. They also have high kinetic energy which keeps them in constant motion. The conditions which would reverse this nature of gaseous molecules would be suitable for liquefaction of gases.

Complete answer:
First let us see what liquefaction of gases is. Liquefaction of gases is a process of physical conversion of gases into liquid state. This process is also known as condensation.
Liquefaction of gases can only be achieved under certain special conditions of temperature and pressure. Desired temperature and pressure can be achieved by certain compression and expansion processes.
In a gaseous state, the molecules are scattered very far away compared to liquid states in which molecules are more closely bound to each other. So for achieving this condition we have to apply high pressure on gas so that molecules can contract and come closer.
Once the gas particles are brought closer by applying high pressure it is necessary to keep them intact in this state only. For this the kinetic energy of molecules should be decreased. We know temperature is directly dependent on kinetic energy. So for decreasing kinetic energy we have to decrease temperature. So low temperature is required for liquefaction of gases. From our observation we can see option B matches the condition which we have discussed for liquefaction of gases.

So option (B) is the correct answer.

Note: Ammonia is gas which is very easy to liquefy. This is because of the intermolecular hydrogen bonding present in between ammonia molecules. This hydrogen bonding is possible because nitrogen, which is highly electronegative with a lone pair is bonded to hydrogen atoms.