Question
Define: Critical temperature, critical volume and critical pressure.
✓
Answer
$i.$ 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 $(P_c).$
$i. \ N_2$ and $O_2,$ have $T_c$ values much below $0 ^\circ C$ and their $P_c$ values are high. Consequently, liquefaction of $O_2$ and $N_2 ($and air$)$ requires compression and cooling.
$ii.$ The $T_c$ value of $CO_2$ nearly equals the room temperature; however, its $P_c$ value is very high. Therefore, $CO_2$ exists as gas under ordinary condition.
$ii.$ 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 $(P_c).$
| Substance | $T_{(c)}\ K$ | $P_{(c)}\ bar$ | $V_{(c)}\ dm^{(3)}\ mol^{(-1)}$ |
| $H_{(2)}$ | $33.2$ | $12.97$ | $0.065$ |
| $He$ | $5.3$ | $2.29$ | $0.0577$ |
| $N_{(2)}$ | $126$ | $33.9$ | $0.09$ |
| $O_{(2)}$ | $154.3$ | $50.4$ | $0.0744$ |
| $CO_{(2)}$ | $304.1$ | $73.9$ | $0.0956$ |
| $H_{(2)}O$ | $647.1$ | $220.6$ | $0.045$ |
| $NH_{(3)}$ | $405.5$ | $113$ | $0.0723$ |
$ii.$ The $T_c$ value of $CO_2$ nearly equals the room temperature; however, its $P_c$ value is very high. Therefore, $CO_2$ exists as gas under ordinary condition.
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