I honestly don't know about what happens when the first solid water and solid CaCl2 meet. What you need to think of is all the CaCl2 he had to add to just 50 CC of water to get a 25 degree rise. All that saly would have melted 100 times more water do to the lowering of the freezing point where as that much heat 50 CC X 25 degree rise at 1 cal per degree =1250 cal. That isn't much heat when you think of all the area of snow , the weight of the ice , the latent heat of fusion of the water which is 80 cal /gram (1gram of water is 1 CC) . The exothermic heat is just such a drop in the bucket that its effect in melting real ice amounts on a roof or road , etc is negligible. I will give you that there may be some roll of the heat of reaction in initiallizing the process as I'm just not at all sure but my gut reaction is that even that is not true or if true again almost insignificant beyond just getting the first few molecules of water to go liquid.
But anyway , I think we all now have enough Physical Chemistry knowledge of the salt/water reactions and Phase equilibrium to last us a lifetime.
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