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In Thomas Andrew's experiment, consider the dome shaped saturation region. If we increase the pressure at constant volume until we reach the critical point, why does the density of vapours rise and the density of liquid fall? Moreover, why will the meniscus or the separation boundary of two phase slowly become less distinct and finally at critical point disappear? enter image description here

Will vapour pressure increase or decrease as we move vertically up in the P-V plane in the saturation region?

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You should clarify what experiment you talking about, maybe this one: Andrew's experiment – TMS Feb 5 '13 at 14:42
Yes that is the experiment i was referring to. this is a link to another explanatory version of – Satwik Pasani Feb 5 '13 at 17:28
up vote 1 down vote accepted

Regarding your second question, it is intimately linked to the feature of scale invariance of critical phase transitions. Essentially what goes on is that a condensing vapour, within the coexistence region, forms droplets of some particular size (more precisely, a distribution of sizes with some particular scale), which depends on the temperature. As you approach the critical point within the coexistence region, this characteristic size increases without bound, and the result is that you will have droplets of all possible sizes from the very smallest to the largest allowed by the container. This causes the blurring of the liquid/vapour meniscus.

This phenomenon is of a particularly universal character, and is ubiquitous in nature. For an illustration in a weird place look up for instance the theory of percolation.

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