Re: Why do you put salt in the ice when making ice cream?

Hi Jan.  Good question!  After some thought, I came up with this:

The salt does two things when it is mixed with ice.  One thing that 
happens is that it lowers the ice's freezing point (like you mentioned).
This means that if a mixture of salt and water was put in your freezer,
it would have to be colder than 0 degrees Centigrade to freeze.

The other thing that happens is that when the salt is dissolved in water
(mixing the ice with the salt melts a little ice, and eventually you have 
an ice/salt/water slush) the mixture gets colder (this is called an 
>>>endothermic<<< process).  This is the same principle behind a cold-pack
(in a cold pack, a different kind of salt is dissolved in water to make 
things cold...I think it is ammonium chloride).

Now, since we have a way of cooling things off, and a lower freezing point
for the mixture, everything can get a little colder.  Things get cold 
enough to freeze the milk, but it happens slowly.  This is important because 
ice cream is a mixture called a >>>colloid<<<.  In this type of colloid,
air gets trapped in the freezing cream, so that the ice cream gets 
fluffy (otherwise, you'd have a cream popsickle!).

I hope this answers your question.
feel free to email me if you have further questions
(weibel@atlas.chem.utah.edu)


Best Regards,
Mike Weibel