how does salt affect the freezing point of water
If it's pure water then it freezes at zero degrees. Water with salt in it is a different substance with a different freezing point.
Lets think about it this way so what happens is that when you add salt to water it will increase the number of particles in solution. Normally with water you have H2O with some H+ and OH- built in but mostly we are talking H2O...so now when we add more molecules it takes it longer for water to be able to freeze because water when frozen likes to combine together to make a lattice structure, but we add some other molecules it interferes with it and so you need to lower the temperature more for that to happen to account for the more particles in solution. Hopefully that explains the concept. For future reference, ask such questions in the chemistry section, here: http://openstudy.com/study#/groups/Chemistry
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