melting ice

[JPelchat]

hello, i was wondering how salt melts ice, particularly NaCl.

I'm curious how the bonds between the Na and Cl interact with H2O to lower the freezing temperature of the water. Also i would like to know how the enthalpy changes throughout this reaction. In my view of the process i had thought ionic compounds such as salt had relatively weak bonds thus how could it release enough energy to brake the H2O bonds? also where would the energy come from to brake the NaCl bonds? and the would be exothermic correct?

[elementcollector1]

The salt and water form a eutectic mixture with a melting point below 0 degrees C.

[weiming1998]

Salt, or sodium chloride, is an ionic compound. That means that salt actually exists as Na+ and Cl- ions stacked together. The Na+ and the Cl- ions are bonded together with ionic bonds.

As salt crystals dissolve in water, these bonds break, releasing Na+ and Cl- ions. Breaking bonds requires energy, so this is endothermic. But it doesn't stop here. The Na+ ion doesn't exist as free ions, as the negatively charged ends of water is attracted to and bonds to it, forming the coordination complex [Na(H2O)6}+, amongst other things. Bond-forming is exothermic, so this releases energy. I'm not sure about what happens to the Cl- ion, as it is a rather inert anion. The energy released and the energy absorbed balances each other out, resulting in a (very slight) release of heat when salt dissolves in water.

As for why a salt-water mix freezes at a lower temperature, that is because in order for water to freeze, the molecules of water must crystallize in a certain structure. Salt interferes with the crystallisation of water, resulting in a lower freezing point. Other substances like antifreeze or various alcohols also interferes with the crystallisation of water and lowers the freezing point.