Liquefaction of Gases — Prelims Strategy

Prelims Strategy

To excel in NEET questions on Liquefaction of Gases, a strong conceptual understanding is key. Firstly, thoroughly memorize the definitions of critical temperature ($T_c$), critical pressure ($P_c$), and critical volume ($V_c$).

Understand their physical significance: $T_c$ is the absolute upper limit for liquefaction. Secondly, grasp the relationship between intermolecular forces and $T_c$: stronger forces mean higher $T_c$ and easier liquefaction.

This is crucial for comparative questions. Thirdly, understand the Joule-Thomson effect: why cooling occurs (work against attractive forces) and the role of inversion temperature ($T_i$), especially for $ext{H}_2$ and $ext{He}$.

Remember that for cooling, $T < T_i$. Fourthly, be able to interpret Andrews' isotherms qualitatively, identifying the gaseous, liquid, and gas-liquid coexistence regions. For numerical problems, which are rare but possible, ensure you know the basic formulas relating $T_c, P_c, V_c$ to van der Waals constants 'a' and 'b'.

Practice identifying trap options, such as confusing $T_c$ with boiling point or assuming all gases always cool upon expansion. Focus on quick recall of key facts and the ability to apply principles to compare different gases.