The critical temperature of a liquid is that temperature beyond which the liquid cannot exist, no matter how much pressure is placed on it. The pressure that is needed to cause the gas to condense at the critical temperature is the critical pressure.

On a phase diagram, this point is noted by the ending of the liquid/vapor equilibrium line.

For examples, consider the following compounds: helium, propane and water.

Helium has a critical temperature of -268^oC. Above this temperature, liquid helium cannot exist, no matter how much pressure is placed on it.

Propane, however, has a critical temperature of 97^oC. This means it can exist as a liquid or as a gas below 97^oC, depending on the external pressure. At room pressure, propane is a gas, but it can be easily liquified by placing it in a container and pressurizing it. This is how propane is frequently sold: in 5-25 pound tanks of pressurized, liquid propane at room temperature

Water has a critical temperature of 374^oC. However, at room temperature and pressure, water is a liquid, unlike propane. If we reduce the pressure exerted on a sample of liquid water, we can get it to boil at room temperature.

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