Answer. Water molecule has a polar structure, i.e., one end of the molecule is partially positive while the other end is partially negative because of electronegativity difference between oxygen and hydrogen atoms. All other polar substances are soluble in water, because the positive end of the substance is attracted by the negative end (O-) of the water and negative end of the substance is attracted by the positive end (H+) of the water. The electrostatic forces attraction among the ions is overcome by the ion-dipole forces of attraction between ion and water molecules. In this way positive and negative ions of the compounds are pulled apart as shown in figure. Ultimately, these oppositely charged ions are surrounded by water molecules, thus separated and kept in solution. For example, most of the salts like NaCl, KC1, Na2SO4, etc., are soluble in water.