Soluble Substances and Evaporation
Country: New Zealand
Date: Winter 2011-2012
Do soluble substances evaporate with the water?
As a general rule, no. If the soluble substance is a solid, then its
boiling point is well above that of water, so it cannot possibly
boil off. If the substance is a liquid, it may have a boiling point
that is below that of water and will boil off at a lower temperature
than water. If the boiling point is higher than that of water, than
it will boil off after the water has evaporated.
Some substances, like ethanol for example, form an "azeotrope" with
water. The combination of ethanol and water form a tight
intermolecular connection that makes the two substances boil off at
the same time.
There is also an issue of "entrainment". If you boil something
vigorously, the strong bubbling causes the otherwise separate
substance to be included in the boiling water. If you boil salt
water hard enough, the bubbling and roiling of the water can cause
the salt to climb along the walls of the container. This is not
technically a boiling of the salt, the salt is just being carried by
the strong evaporation rate.
Greg (Roberto Gregorius)
Some do and some do not. It is dependent on how volatile they are. Alcohols will, for example, but salt will not.
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Update: June 2012