Impurities and Boiling Point Depression
Name: Anand M.
Date: Tuesday, March 18, 2003
Thank you for your time. I am doing a science fair project on what
impurities affect the boiling point of water. I am wondering what
impurities can lower the boiling point of water. Salt, Sugar, Lemon
Juice, and Baking soda all make it go higher but none of them make it go
lower. I also can not use a vacuum and do not have the access to
You will need some sort of water-soluble, low-boiling liquid like methyl
alcohol -- it is available at a hardware store. Bear in mind, when an
alcohol-water solution is heated to boiling, the vapor which boils off is
not pure water or pure alcohol. To understand what I mean here, look up
the word, "azeotrope," in a science reference book or search it out on the
Be especially careful if you attempt a proof of these ideas. Use a
solution that has lots of water in it and only a little alcohol.
Otherwise, you are facing a fire hazard. Please do your experiments under
adult supervision. Good luck with your project.
So far, you are doing a good job of making observations and measurements.
The additives you have studied so far do not boil before water does, as
ethanol would. (Could you expect that to affect your measurements?)
Is there any reason there should be something that makes water boil at a
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
ALL normally-behaving substances that are not volatile themselves
MUST increase the boiling points of any pure solvents. The reasons for
this are a bit involved to be explained here - but it's true. So all of
the things you tested will increase the boiling point of water.
Some very volatile and water-soluble liquids, like ethanol or
acetone, that evaporate very quickly can mix with water and "carry" the
water along with them as they evaporate. Chemists frequently use a little
acetone to "dry" glassware after it has been cleaned to remove the drops
of water remaining. However, this is not "boiling" in the sense that the
term "boiling" is used.
You can find some additional discussion on the NEWTON BBS web site at:
The "reason" why boiling points are always elevated is true involves
the subject of "thermodynamics" -- which is the area that deals with how
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Update: June 2012