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I know that ice gets colder when salt is added. But how big is the effect if it is already −30 °C?

Can it cool it down more or does it just work with −1 °C cold ice?

Another question: If I have a water cooling circuit, does adding salt help it cool down?

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    $\begingroup$ (1) At -30°C, common table salt ($\ce{NaCl}$) wouldn't do anything to ice; $\ce{CaCl2}$ probably would. (2) No. $\endgroup$ Apr 8 '16 at 9:38
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If you add salt to icy water, it's the water that gets colder. Pure water under atmospheric pressures will be $0~\mathrm{^\circ C}$ - even when the ice melting in it is colder (for example if it's just out of the fridge). When you then add salt to that water, it lowers the freezing point of your water-salt solution, all the way down to $-21~\mathrm{^\circ C}$ once it contains $5.2\ \mathrm{mol}$ of NaCl per kg of water. That's the maximum amount of salt this solution can contain at that temperature. (Find the thermophysical data at NIST (page 30).

Now, if you add salt to liquid water (containing no ice), you will actually decrease its temperature slightly because of the energy required to solvate the salt in the water. This won't help you in a 'water cooling circuit' because you also lower the heat capacity.

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  • $\begingroup$ thanks for the asnwers it helped me a lot. i wont use salt water then. $\endgroup$
    – laundmo
    Apr 15 '16 at 7:00

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