Why do salt dissolve in water?

Answer 1

A1 Because water is a solvent, and as long as the material, in this case salt, has less density than the liquid itself, it should sink. But, other solutes like sugar, are denser and water, and will simply sink.

A2 Strictly, there are many many salts, and common table salt, NaCl is the one we meet most commonly. This dissolves readily in water as you remark.

Copper Sulphate 'bluestone' is another salt. Some salts are essentially insoluble, such as many of the Mercury salts - especially the Mercurous ones.

Answer 2

The table salt NaCl (sodium chloride) is an ionic compound. Water, H2O, is a highly polar compound capable of participating in ion-dipole interactions (one of the intermolecular forces). When salt is added to water the Na+1 Cl-1 ions of NaCl preferentially dissociates in favor of interacting individually with polar water molecules in solution (entropically favorable). Therefore, a 'shell' of water molecules will form around the Na+ and Cl- ions with the partially positive hydrogen facing Cl- ions and partially negative oxygen facing the Na+ ions (opposite charges attract). However, 'shell'-like structures are ordered and entropically unfavorable. Therefore a certain amount of water will only dissolve a certain amount of salt at a certain temperature due to equilibrium effects of the entropically favorable process of ionic dissociation and entropically unfavorable process of solvation.

(Note for previous answerer: water does not ionize when salt is dissolved in it! Water only hydrolyzes in appreciable amounts when a electric current is passed through it. The interaction of water and ionic compounds is mediated through ion-dipole interactions, which a type of intermolecular bonding like dipole-dipole interactions, hydrogen bonding, and van der Waals forces)