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Boiling Point
The boiling point of pure water under normal atmospheric pressure is 100 degrees Celsius or 212 degrees Fahrenheit. Unless it is put under increased pressure, pure liquid water can never be hotter than this boiling point. Once water reaches the boiling point, additional heat energy is used to change the phase of the water from liquid to gas. The gaseous water, or steam, is hotter than the boiling point, but the actual liquid does not exceed 100 degrees Celsius.
Boiling Point Elevation
The phenomenon of boiling point elevation is not limited to table salt. It will occur with nearly any solute or substance. When the same amount of heat is applied to two samples of equal volume, one of pure water and one of water with salt, sugar or another solute, the pure water will boil first.
Degree of Boiling Point Elevation
Every 58 grams of salt per kilogram of water will increase the boiling point by half a degree Celsius. In U.S. measurements, roughly one ounce of salt per pound of water increases the boiling point by one degree Fahrenheit. Once the water is saturated, meaning no more salt can be dissolved, the boiling point does not increase any further.
Adding Salt to Hot Water
When salt is added to nearly boiling water, the water often boils very suddenly. This phenomenon leads many people to believe that salt decreases the boiling point of water. Actually, the salt particles simply provide a site for the rapid phase change of already hot water into steam. The same thing would happen with non-soluble solids such as very fine sand.