If you’ve ever made homemade ice cream you know that by pouring salt on ordinary ice you cause it to melt, and as a result you produce an even colder temperature than the ice alone could create.
Any idea how that works?
When salt is added to ice, the salt inhibits the H2O molecules from staying in the lattice formation. This reaction is exothermic in nature causing the ice to melt. The solution then has a higher temperature. Note that if the ice is below zero celsius and salt is not added to the extent that the enthalpy of the system is increased to such a point that the temperature is raised to above zero degrees celcius, the solution can still be in a liquid phase because the solution has a lower freezing point than that of pure water. In effect, the solution does not have a colder temperature than that of the ice used in the reaction.
I just went down to my kitchen and took some ice out of the freezer. With a spoon, I broke it up into smaller chunks. I let it stand for 3-5 minutes in a steel pan. There was a thin film of water on the bottom (inside) and liquid water had condensed on the bottom (outside) of the pan. I assume that everything was at 0 celcius at this point.
I then added salt to the mixture and begain to stir. There was both solid ice and liquid water inside the bowl. Within 15 seconds, the water on the outside of the pan froze.
My conclusion is that the salt DOES create a colder temperature than the ice used in the mixture. Changing water from solid to liquid absorbs heat (read: lowers the temperature). By FORCING ice to melt, it lowers the temperature of the solution below 0C.
You can read the second part of this column
[http://www.straightdope.com/columns/990730.html]Plus: why do you use salt to help melt ice in the winter but freeze homemade ice cream in the summer? (30-Jul-1999)
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This is from a site about ice cream:
The concept of melting ice with salt is not new to anyone in this latitude. Indeed, our roads, driveways, and sidewalks are kept bare in the winter by such a
process. As salt is applied to ice, the ice crystal sructure is broken due to the depressed freezing point of the resulting brine solution. As the salt continues to
dissolve more ice melts to accommodate this concentrated salt solution with its very low melting point. At the same time, both the heat of solution of the
dissolving salt, and the latent heat of fusion of the melting ice are adsorbed from the ice itself, thereby lowering the temperature of the salt, ice and brine mixture.
The temperature of this mixture can be controlled by the amount and ratio of salt and ice present. The lowest temperature which can be achieved with a sodium
chloride brine is -21oC, at a concentration of 23% salt. Higher concentrations result in salt crystallization.
Well, certainly Bizerta wins the Dr. Science prize for household chemestry.