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Name: student
Status: student
Age: 10
Location: N/A
Country: N/A
Date: Around 1999


Question:
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.


Replies:
Dear student,

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 research.

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!

Best regards,
Prof. Robert Topper



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