MadSci Network: Chemistry

Re: How does salt (a solid) react with ice (a solid) in melting?

Area: Chemistry
Posted By: Richard Oldroyd, Post-doc/Fellow Davy-Faraday Research Laboratory
Date: Fri Jun 27 04:19:39 1997
Area of science: Chemistry
ID: 867143162.Ch

Salt, when dissolved in water, lowers the water's melting point and therefore causes it to freeze at a lower temperature. When both salt and water exist as separate solids, however, there must be some mechanism whereby the solids interact.

The best way of thinking about this problem is to imagine an interaction between two surfaces - a salt surface and an ice surface. When the two surfaces come together, some of the surface water molecules will be attracted to the NaCl ions at the surface of the salt crystal. I shall try and illustrate this to the best of my abilities using ASCII characters...

Step 1

Na-Cl-Na-Cl-Na-Cl-Na-Cl    Salt surface

O--O--O--O--O--O--O--O     Ice surface
H2 H2 H2 H2 H2 H2 H2 H2

Step 2

Na-Cl        Na-Cl-Na
     \     /

     /H2 H2\
O--O        O--O--O--O
H2 H2       H2 H2 H2 H2

Step 3

Na-Cl        Na-Cl-Na

      Na(H2O)  Cl(H2O)

O--O         O--O--O--O
H2 H2        H2 H2 H2 H2

This interaction causes the salt to gradually dissolve in the ice, thus lowering the melting point and causing the ice to melt. The water so formed then allows more salt to dissolve which enhances the process.

I hasten to add here that what I have just described is an extremely simplistic model of the overall process. There are several other factors involved, e.g. some heat is generated when the salt dissolves which also contributes to the melting of the ice.

One thing that should also be remembered is that solids can react together, it is just that the reactions between them are very slow. This stems from the fact that only the surfaces in contact with each other can begin to react, and there is often a large "lattice energy" barrier to overcome, i.e. energy is required to break up the crystalline lattices of the reacting solids.

I hope this answers your question appropriately.

All the best


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