MadSci Network: Physics

Re: When salt is added to ice why is the liquid colder than freezing?

Area: Physics
Posted By: Michael McMurray, Post-doc/Fellow Insulating Materials Group
Date: Thu Dec 12 23:03:16 1996
Message ID: 850024394.Ph

I believe your are right on both accounts. The heat of formation is the measurement of the change in enthalpy during a reaction. Because there is a change in enthalpy the freezing point of water decrease with the amount of solute added, in your case salt. This is why the solution is still liquid below the freezing point of pure water(0C,32F). As you probably know this is because the hydrogen bonding that occurs between hydrogen and oxygen atoms of the water, with the salt making it more difficult for the water molecules to bond together to become a solid.

The reaction that occurred when you added the salt to the water is a exothermic reaction. So heat was given off when you added the salt. This is partially the cause of the melting of the ice. This means that due to this reaction the temperature of the solution would increase not decrease.

32F is the freezing point of pure water, this is not a temperature where ice forms and then the temperature remains constant. I believe in your case that the temperature of the ice was lower than 32F before you added the salt.

Therefore, you measured a temperature below 32F because: The temperature of the ice was initially lower than 32F. Adding salt to water breaks the hydrogen bonding in water therefore lowering the freezing point, so the solution did not freeze.

Thanks for your question
Mike McMurray

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