| MadSci Network: Physics |
This is a tricky question...yes, in principle you can readily heat a saturated aqueous NaCl colution to more than 100 degrees (the vapor pressure of H2O at 100C is 760 mm, and can be lowered by ~175 mm by addition of 3 M NaCl, thus significantly raising the boiling point of H2O). It is the effect of the additional salt on the vapor pressure that allows you to heat a saturated salt solution much higher than 100C. However in an OPEN SYSTEM, you'll have difficulties raising the temperature of water vapor, your source for heating the salt solution, above 100C. Simply put, you would generally boil water from a heat source that is generally higher than the target vapor temperature (100C), such as a bunsen burner source. Once the temperature of liquid water reaches 100C, and the vapor pressure equals 760 mm, you readily convert liquid to gaseous water. Unless the chamber you are using to contain your water is sealed, the vapor will diffuse away and the temperature will never practically exceed 100C by very much. In a CLOSED SYSTEM (a steel vessel), you will be able to convert all the water to steam at 100C, and will continue to heat the system well beyond 100C causing a rise in internal vapor pressure...this is the basis of a steam engine, harnassing the controlled release of pressurized water vapor in the form of mechanical work. In an open system, such as a pot of boiling water, the steam that is produced will be at 760 mm of vapor pressure, corresponding to 100C. If this source of steam is used to heat a solution of salt water, then you will never exceed the 100C temperature required to convert liquid to vapor water. I hope this is clear enough. Regards, Dr. James Kranz
Try the links in the MadSci Library for more information on Physics.