Ask A Scientist Chemistry Archive

name         Anand M.
status       student
age          12

Question -   Hello
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 
ethanol.
---------------
Anand,

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 
Internet.

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.

Regards,
ProfHoff 623
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Anand,

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 
lower temperature?

Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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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:
http://newton.dep.anl.gov/askasci/chem99/chem99241.htm
     The "reason" why boiling points are always elevated is true involves 
the subject of  "thermodynamics" -- which is the area that deals with how 
heat behaves.

Vince Calder
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