a. Water is a polar covalent compound. Two hydrogen atom found two single covalent bound with oxygen to complete their outermost shell. The electronegativity of hydrogen and oxygen is more. This enables oxygen which is more electronegative than hydrogen to draw shared pairs of electrons towards itself. This results in the formation of slight negative charge over oxygen and slight positive charge over hydrogen. This makes water a di-pole molecule and enables it to dissolve ionic compounds by breaking the electrostatic force of attraction between the molecules.Hence, it is a polar solvent.
b. All the ionic compounds and other polar substances dissolve in polar solvent because polarity of the solvent breaks the electrostatic force of such compounds, resulting in formation of free ions in the aqueous solution.
c. Non polar substances (organic compounds/covalent compounds) dissolve in non polar solvent.
In a simple answer: Water is a good solvent because of its polarity, its slight charges surround other atoms that are charged (Ionic) or partially charged (Polar).
a) In the water molecule two hydrogen atoms are covalently attached to the central oxygen molecule, in a V-shaped structure. The electronegativity of oxygen is far greater than hydrogen, thus both the O-H bonds become polar. In addition to that, there are two non-bonded electron pairs over oxygen, which are directed in a particular orientation in space. This also contributes to make water a highly polar molecule. Generally, the polarity of a solvent can be roughly estimated by its dielectric constant (a non-polar solvent usually has a dielectric constant less than 15). Water is an excellent solvent which has a high dipole moment and high dielectric constant. This makes it easier to separate ions of the solute, overcoming and outweighing the lattice energy value by which the ions (or molecules) are bound together.
b) Highly polar solid substances such as glucose, carboxylic acids, polyhydroxy compounds etc. and ionic solids such as NaCl, K2SO4, NH4Cl, NaOH etc. are freely soluble in water owing to its polar nature. Moreover, water dissolves certain substances due to its excellent hydrogen bonding capabilities.
c) “Like dissolves like” rule of solubility proposes that polar solvents will dissolve polar compounds and non-polar solvents (like oil, petrol, CCl4 etc.) dissolve nonpolar substances. Thus, nonpolar substances like grease or iodine will dissolve best in non-polar solvents.