Ice Melting Principles
Date: Around 1999
Hi. I am in school and have many interests. (My parents
asked me not to give out information over the internet) I am writing a
pretty good research paper but I need some information. I have checked
out several books from the library and they all tell me the same thing
but they do not tell me why. Maybe they do and I just don't understand
why. The purpose of my paper is to gain an understanding of how and why
the compound (H2O), interacts differently with the compounds (NaCl) salt
and (C12H22O11) sugar. This is my question: Salt water freezes at a
lower temperature than water but why? What is it about salt's chemical
that causes the ice to melt? Why does salt make ice melt? And what
reaction does sugar have on ice? Will sugar make ice melt? I need to
cite my work, so if you can give me that information, I would also
appreciate if you can give me the sources of the information, but it
cannot be older than 1990. I cannot use anything without a source to
cite. I ask my parents to check their email everyday to check and see if
you have answered me. Thank you very much.
You have asked many questions, and they do not have very simple
answers, but I will try my best.
First of all, I am afraid I cannot give you sources because it
is not proper to cite sources that you have not yourself read.
So what I am telling you is just basically information from
a supposed expert in this area; I am a physical chemist, and
phase transitions (like melting) is my special area of
Anything that is dissolved in liquid water will tend to
lower the frezing point of the solution as compared to the
freezing point of pure water. There are a number of reasons
for this. A basic reason is that in order for a molecule
of liquid water to freeze onto the surface of an ice crystal,
it must physically run into the ice crystal. However, there
are solute molecules (say, sugar) sometimes in the way,
and so the water molecule sometimes runs into the sugar molecule
instead of running into the ice crystal. So you have to cool
things down more in order to get ice to form. I have
oversimplified things a lot, but this is the basic idea.
Now, sugar and salt dissolve differently. For every molecule
of sugar you dissolve, there is one molecule in solution. But
not so for salt! When you dissolve salt, it changes into ions:
NaCl ----> Na+ + Cl-
so for every molecule of NaCl you dissolve, you make two things
in solution. This makes the solution as if there were twice
as many particles, effectively. So the freezing point changes
twice as much if the number of solute particles is the only
important thing! Of course there are other factors involved,
like the charges on the ions above, and the fact that they
are atoms and not molecules...but these only affect how large
the effect is, not whether it happens.
To learn more, go to a local college's library with a parent
and look up a college general chemistry textbook. I like
Kotz and Treichel's "Chemistry and Chemical Reactivity" but
there are many good books out there.
I hope this helps!
Prof. Robert Topper
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Update: June 2012