Why Does Ice Get Cold When You Add Salt?

Describes why ice gets colder when salt is added to it. It also includes an equation to find the temperature difference.

Ice and Salt

            Ice becomes much colder when sodium chloride is added on it. This occurs because the freezing point temperature of ice is decreased. This phenomenon is called freezing-point depression. The freezing point decreases due to the increase of entropy in the system. When the ice is frozen the change of entropy is low, but when the sodium chloride is added more molecules are as well. Therefore, the increase of entropy makes the water molecules harder to assemble into a solid. Regular ice will melt at 0° C by absorbing the surrounding energy from the environment making everything around it cold. Since we lowered the freezing point, more energy must be absorbed from the surroundings thus making the ice colder than before. We can calculate the how many degrees the salt will effect the freezing point of water by using the following equation:

ΔTf = Kf · mB

 ΔTf = freezing point depression        Kf = cyroscopic constant       mB = molality

Where,

ΔTf = f (pure solvent) – Tf(solution)

Kf ­= RTm2MH­f

R = .008314 kJ/K · mol     Tm = 273.15 K     M = .01802 kg/mol     ΔH­f = 6.02 kJ/mol

Kf­ = 1.86 K · kg/mol = 1.86°C/(kg/mol)     mB = 2 · mol(solute)/kg(solvent)

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