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Boiling Point Elevation
By John Brennan
Liquids that contain
dissolved substances have increased boiling points.
This effect is called
boiling point elevation, and it's an example of a colligative property -- a
property that depends on the number of solute and solvent molecules but not the
identity of the solute.
Boiling point
elevation does not have many uses in science, although there are a few
practical uses you've probably encountered in everyday life.
Antifreeze
Ethylene glycol or
antifreeze helps prevent the water in your vehicle's radiator from freezing
through freezing-point depression.
But you may not have
realized that it also elevates the boiling point of the fluid as well.
By raising the
boiling point, it helps to protect against boil-overs.
Many brands of
antifreeze will list both the amount of boil-over protection and the freeze-up
protection offered.
Cooking
Adding salt to water
before or while heating it will increase its boiling point, so the water will
actually be hotter than it would otherwise be when it comes to a boil.
The amount of this
increase, however, is quite negligible at low salt concentrations; adding a
couple grams of salt to 10 cups of water, for example, would only yield a
boiling point elevation of about 0.015 degrees Celsius, which will not affect
your cooking significantly. Nonetheless, cooking is one use of boiling point
elevation.
It's also important
to note that -- contrary to myth -- adding salt to water will not make it boil
faster.
Quite the contrary,
in fact, it will take slightly longer to boil, since its boiling point has now
been elevated.
Measurement of Molar
Mass
Boiling point
elevation depends on the identity of the solvent and the concentration of
solute particles, but not the identity of the solute.
Consequently, just
like freezing point depression, boiling point elevation can be used to
determine the molar mass of a solute.
If the solution is an
electrolyte -- one containing a substance like sodium chloride, for example,
which splits up when it dissolves -- this procedure becomes somewhat more
complicated, because the number of particles produced by dissociation of the
solute must also be taken into account.
Chemists nowadays
generally use techniques like mass spectrometry to determine the molar mass of
compounds, but boiling point elevation and freezing point depression are still
viable alternatives.
Sugar Refining
Once a sugarcane crop
has been harvested and the cane juice extracted, it must be refined to produce
crystalline sugar for consumption.
At some stages during
the process, the cane juice or syrup is boiled, and the temperature at which it
boils will depend on the sugar concentration.
In fact, the boiling
point elevation offers a way to monitor the level of saturation of the
solution, which is an important consideration for crystallization.
Based in
San Diego, John Brennan has been writing about science and the
environment since 2006. His articles have appeared in "Plenty,"
"San Diego Reader," "Santa Barbara Independent" and
"East Bay Monthly." Brennan holds a Bachelor of Science in biology
from the University of California, San Diego.f the Boiling Point Elevation
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