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2. The Critical Phenomena

Table of Contents:

1. Introduction
2. The Critical Phenomena
3. Critical Point Drying
4. Intermediate Stage
5. Transitional Stage

The principle of the experiments which were initially carried out using Carbon Dioxide (CO2) was to measure the change in volume with the application of pressure, of a fixed mass of gas, while maintaining the temperature constant and to repeat this for a range of different temperatures.

The results are best understood by considering the graph obtained from plotting pressure (P) against Volume (V) for the series. This is indicated in Figure 1, the curves obtained are termed Isothermal.


Figure 1.

Considering the 10oC Isothermal at low applied pressure, the CO2 ­ is gaseous (Vapour) and generally exhibits the characteristics of a gas (Boils Law) over the range from 'r' to 's'. From the point 's' a very slight increase in pressure corresponds to a change from the vapour state to the liquid state, which is the phenomenon of saturation, from 's' to 't' the pressure is virtually constant while the volume is decreasing and at 't' the substance is all liquid.From the point 't' the graph becomes almost vertical indicating significant application of pressures for very little change in volume, liquids being virtually incompressible.

The 20oC Isothermal has similar general characteristics, however, there is less difference between points 'v' to 'w' compared to the difference between equivalent points 's' to 't' on the 10oC Isothermal, these points representing the difference in volume occupied, between the substances vapour and as a liquid. This indicates that the densities of the saturated vapour and liquid are approaching each other, also the slight departure from the vertical 'w' shows the compressibility is greater than that at higher pressures. This evidence indicates that the properties of the liquid and gas states of the substance are becoming similar and will ultimately coincide.

This in fact is realised at the 31.1oC Isothermal, which does not show any horizontal discontinuity. The temperature at which this occurs is termed the Critical Temperature and has an associated Critical Pressure and Density and hence for a particular mass of gas, a Critical Volume.

If a liquid was heated in a closed system so that the Critical Pressure could be attained, at the Critical Temperature, any visible meniscus would disappear, the surface tension would be zero and it would not be possible to distinguish between the properties of a liquid or gas. We therefore have continuity of state. Above this temperature the gas cannot be liquefied by the application of pressure and strictly speaking a substance should only be classified as a gas above its Critical Temperature, below this temperature where it could possibly be liquefied by the application of pressure, it is more precisely termed a vapour.

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