(a) With the aid of a phase diagram, explain what is meant by a supercritical fluid. Give examples of commercial processes that involve the use of supercritical fluids.

(b) Even though CO₂ is classified as a ‘greenhouse gas’ (see Box 14.7), why is the use of supercritical CO₂ regarded as being environmentally friendly?

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ENVIRONMENT Box 14.7 'Greenhouse gases Carbon dioxide normally constitutes 20.04% by volume of the Earth's atmosphere, from which it is removed and returned according to the carbon cycle: Living green-plant material Plants consumed as food Photosynthesis Metabolism + Wood fires Decay of dead plant material Herbivores and carnivores Respiration CO₂ Combustion Fossil fuels peat, coal, oil, natural gas = CO₂ taken up into seawater Weathering and industrial processes (e.g. see Fig. 11.7) Calcareous (e.g. shells) material is produced CaCO, as limestone, marble and chalk