Why the density of liquid decreases and density of vapour increases as we approach towards the critical state/point

Question

Pressure and temperature have opposite effects on density. As temperature increases at constant pressure density decreases and as pressure increases at constant temperature the density increases (and vice versa). The change in density for liquids is smaller compared to gases.

But when we talk about the density change when both pressure and temperature increases, how can we say that the density of liquid decreases as we go towards critical point and the density of the gases increases as we go towards the critical point. Since pressure and temperature have opposite effects on density.

Answer 1

For $a,b \gt 0$:

$$\frac{\Delta a}{a} \gt \frac{\Delta b}{b} \implies \Delta (\frac ab) \gt 0$$

$$\frac{\Delta a}{a} \lt \frac{\Delta b}{b} \implies \Delta (\frac ab) \lt 0$$

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If there are 2 mutually opposite effects, the net outcome depends on what effect is stronger.

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Imagine the phase diagram of the compound, figuratively going along both sides of the boiling curve toward the critical point.

For the liquid, the density decrease due thermal motion and formation of nano-cavities is much higher than the density increase due pressure compression. Therefore, the net density change is negative.

For the vapour, the density increase due pressure compression is much higher than the density decrease due gas thermal expansion. Therefore, the net density change is positive.