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      Boiling Point Elevation

The meaning of a boiling point and the effect of salts can be illustrated by two cooking phenomena. Boiling water is a simple way of attaining a reproducible (constant) high temperature, which at sea level (1 atm pressure) is 100° C.

The situation is slightly different at high altitude. At 8000 feet in Aspen, Colorado, atmospheric pressure is 560 mm Hg rather than 760 mm. Water needs to be heated only to 92° C before its vapor pressure equals 560 mm, and the turbulent bubbling away of vapor that we call boiling begins. Indeed, 92° C is as hot as an open pan of water can be heated in Aspen.

If more heat is supplied, the temperature remains at 92° C, and the liquid simply boils away faster. Among the practical consequences of this are cold coffee, and hard-boiled eggs that take forever to cook. At the other extreme, in a sealed pressure cooker that can take an overpressure of 3 atm (or total pressure of 4 atm), one can raise the temperature to 134° C, thereby making cooking much faster.

The second cooking phenomenon illustrates the influence of salts on boiling point. If a pot of water is brought to a boil, and salt is added, boiling immediately stops. The added ions lower the escaping tendency of water molecules. Only at a higher temperature will the vapor pressure again reach atmospheric pressure, and boiling recommence.

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