What determines whether compounds will dissolve in water?

Answer 1

Water is known as a very polar solvent. That is, the molecules have localized area of concentrated electrical charge. The oxygen atoms are regions of (-) charge and the hydrogen atoms are areas of (+) charge. These areas of localized charge want to interact with neighboring areas of opposite charge. So, oxygen atoms tend to form loose bonds with hydrogen atoms on other nearby water molecules. This lowers the energy of the entire system and therefore promotes stability. For this reason any compound that is able to mimic the tendency of a water molecule to interact with another water molecule will ensure that the compound will indeed enter into association with water (that is, dissolve). The occurrence of groups containing local charge on the solute molecule ensure its ability to dissolve in water. Specifically, polar groups such as OH, NH, CO are associated with the ability to dissolve in water. Certainly the occurrence of net charge (+/-) on a molecule that contains an ionizable group (COOH, NH2, PO4, etc) is a good indicator of water solubility.