Freezing Point Depression & Phase Change

1. The problem statement, all variables and given/known data
We have been doing freeze point depression of water when salt is added in class. For homework we were given two scenarios. One with an ice cube in water where both the water and the ice cube are at 0 degrees celsius. The second scenario is the same as the above except that salt has been added to the water. In both scenarios the systems are isolated from the surrounding environment.

We have to use ideas about particles and energy transfer to explain what is going on and in each scenario whether there is a net melting, freezing or equilibrium between melting and freezing. In scenario 2 we have to explain why the temperature of the water goes down.

2. Relevant equations


3. The attempt at a solution
I have looked at some stuff online and used my notes. I think that in scenario 1 there is freezing and melting taking place because some particles will have enough energy to melt and some will be moving slow enough to freeze. However, the net result is freezing and melting happen at the same rate so the ice and water stay as they are.

I’m really confused with scenario 2. In class we learnt that salt crystals get in the way of ice freezing. Therefore, this must mean that the rate of freezing goes down. The rate of melting will remain unchanged. Since the rate of freezing has decreased then the there will be a net result of melting. Melting removes heat from the liquid water which is why the water will go below 0 degrees in scenario 2 but where will this energy go? Normally water particles collide with the ice cube and transfer their energy to the ice causing it to melt. Is this what is happening in this situation? Will the ice cube go above 0 degrees despite the water going below 0 degrees. that seems really non-intuitive to me!

Any help is much appreciated!

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