I sure most here understands how salt & ices works, but it surprise me at how many people just thinks salt make ice colder. .... wrong .... Salt makes ice melt faster and ice melts at 32 deg F. The heat to melt the ice come from the ice cream (& outside air if not wrapped with a towel), thus it freezes. The salt water & ice will not be any colder than 32 deg F. So if you are reusing the same salt & ice, make sure as much fresh ice & salt is added that can be; and add as the ice melts.
That is the molecular explanation of what happens when ice, water and salt is held at the same temperature at equilibrium (0 deg C; 32 deg F). ie: no heat added & no heat extracted. .... But that is not the situation while making ice cream. The temperature of the ice cream mixture must be lowered by extracting the heat (btu). This is done by melting the ice; heat out (from ice cream mix) = heat in (melting of ice) .... (yes, we are neglecting the heat from the air) Salt lowers the freezing point of water; it upsets the equilibrium state of water & ice at 0 dec C (32 deg F) and cause ice to melt faster at 32 deg and/or if more (hard) ice is added the temp of the solution could be lowered and the ice still melt. Here the ice, water & salt is not at equilibrium.
The rapid removal of heat from the ice cream mix is the freezing of the mix. .... (heat out; to lower mix to freezing) = (quantity of ice needed to melt; heat in) ... OH ... there are many web sites attempting to explain what happens with ice, water and salt while making ice cream to are wrong. I read one wrote by a Phd (of what I don't know) that stated ice got colder when the ice absorbed the heat from the mix. ... (BS)..
This a basic Chemistry 101 lab problem that every one who had to take the class had to do.
post edited by edwmax - 2012/07/25 12:50:54