Pressure and temperature changes with a single-component system qualitative discussion

2 Pressure and temperature changes with a single-component system qualitative discussion 

Dry ice is carbon dioxide (CO2) in its solid phase. We call it dry because it is wholly liquid-free at such solid CO2 looks similar to normal ice (solid water), but it melts without leaving a puddle. We say it sublimes, i.e. undergoes a phase change involving direct conversion from solid to gas, without liquid forming as an intermediate phase. CC 2(i) can only be formed at extreme pressures. 

Solid CO2 is slightly denser than water, so it sinks when placed in a bucket of water. The water is likely to have a temperature of 20 °C or so at room temperature, while typically the dry ice has a maximum temperature of ca —78 °C (195 K). The stable phase at the temperature of the water is therefore gaseous CO2. We should understand that the C02(S) is thermodynamically unstable, causing the phase transition CC 2(s) - C02(g) on immersion in the water. 

Incidentally, the water in the bucket is essential for generating the effect of theatrical smoke because the large volumes of CC 2(g) entrap minute particles of water (which forms a colloid see Chapter 10.2). This colloidal water is visible because it creates the same atmospheric condition known as fog, which is opaque. 

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