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If you have a bowl of ice that's melting, so the ambient temperature is just above 0 °C, what happens to the temperature of the water when you add salt?

I know that the freezing point of salt water is less than 0 °C (it goes down to around -21 °C for a fully saturated solution), but that's not what this question is about. There's no change in the ambient temperature.

Does adding the salt create a reaction that adds or removes heat? or does the temperature just remain the same?

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1 Answer 1

up vote 7 down vote accepted

In the absence of salt, the ice and water at 0C are in equilibrium, so unless you add or remove heat nothing changes. However when you add salt it reduces the freezing point of the water. This means the ice and salt water are no longer in equilibrium, and the result is that the ice starts to melt.

Melting the ice requires heat. Specifically it requires the latent heat of fusion, and this heat has to come from somewhere. The heat comes from reducing the temperature of the water.

So when you add salt some of the ice melts and this cools the water. The temperature of the salt water reduces until the ice and salt water reach equilibrium again, which will be at a temperature of less than 0C. Add more salt and more ice melts and the temperature reduces further.

This is not a reaction, or at least not in the sense of a chemical reaction. It's just a redistribution of heat between the latent heat of fusion of the ice and the specific heat of the water.

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Of course. I'd forgotten about the latent heat of fusion. – ChrisF Nov 19 '12 at 13:47
@ChrisF for reference, a salt-water solution, or any that 'solidifies at a lower temperature when in composition with each other' is called a eutectic system. – Pureferret Dec 13 '12 at 23:16

protected by Qmechanic Oct 27 at 23:56

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