Question

Define: 

Critical temperature, critical volume and critical pressure.

Answer 1

i. The temperature above which a substance cannot be liquified by increasing pressure is called its critical temperature (T_c). 

ii. The molar volume at critical temperature is called the critical volume (V_c). 

iii. The pressure at the critical temperature is called the critical pressure (P_c).

Note: Critical constants for common gases

i. N₂ and O₂ , have T_c values much below 0 °C and their P_c values are high. Consequently, liquefaction of O₂ and N₂(and air) requires compression and cooling.

ii. The T_c value of CO₂ nearly equals the room temperature; however, its P_c value is very high. Therefore, CO₂ exists as gas under ordinary condition.