Excerpt from Solution and Electrolysis

When we dissolve sodium chloride in water an absorp tion of heat is observed. That is to say, if both salt and water when separate are at the temperature of the air, after the solution is formed its temperature is lower. On the other hand caustic potash gives an evolution of heat on dissolving and the temperature rises. During solution there are usually changes in volume. In all but rare cases contraction occurs, and the volume of the resultant solution is less than the sum of the volumes of the solvent and the substance dissolved, or solvend.

The boiling point of a salt solution is higher than that of pure water; and when it is remembered that a liquid boils when the pressure of its vapour is equal to the atmospheric pressure acting on it, we see at once that this statement is equivalent to saying that the vapour pressure of water is reduced by the dissolved salt. The steam which comes off however is the steam of pure water and will be found to assume the temperature at which pure water boils. Thermometers are graduated by marking on them the places at which the mercury stands at the freezing point and boiling point of water. It will be now seen why it is necessary during the latter operation to put the instrument in the steam and not in the water, which may contain impurities and be consequently boiling at a temperature slightly above 100� C. - its normal boiling point. Closely connected with this lowering of the vapour pressure is the lowering of the freezing point also pro duced by the substance in solution. Thus salt water does not freeze at a temperature low enough to solidify fresh water. Here again it is important to observe that the ice frozen out is the ice of pure water. Sometimes, par ticularly if the process of freezing has been rapid, particles of solid salt are shut in by the ice, and therefore redissolve when it is melted, but they are quite distinctly separated from the solid ice and never crystallize out in combination with it.

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