Tim Stewart, of King's High School, asks :-

Why does salt dissolve?

Jim Simpson, a chemist at the University of Otago, responded.

Although common salt (sodium chloride) will dissolve readily in water, it wont dissolve in many other liquids such as ether, paraffin or chloroform.

To understand why we first need to know something about solid salt. Sodium chloride consists of sodium ions, which are sodium atoms that have lost one electron and are positively charged, and chloride ions, which are chlorine atoms that have gained one electron to become negatively charged. We also know that things that have opposite electrical charges attract one another. In solid sodium chloride, the positively charged sodium ions are each surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions. The attraction between the oppositely charged ions holds the crystal together very well.

A water molecule consists of two hydrogen atoms and one oxygen atom bound together in a V shaped molecule, with the oxygen atom at the centre of the V. Also the oxygen atom is more negative than the hydrogen atoms so that the water molecule has a negative end, at O, and a positive end at each H.

When salt is put in water, the water molecules can break down the nice orderly structure of the salt crystals by arranging themselves around the sodium ions with their negative O atoms pointing towards and attracting the positively charged sodium ions and their positive H atoms surrounding the negatively charged chloride ions. The arrangement of the ions in water leads to a situation where the energy of the salt solution is lower than that of the salt crystals and the salt will dissolve.

Of course if most of the water evaporates the sodium ions and chloride ions are brought close together again and crystals of solid salt will again appear in the solution.