Water has a simple molecular structure composed of one oxygen atom covalently bonded to two hydrogen atoms. This gives water a polar molecular structure with partial positive charges near the hydrogen atoms and a partial negative charge near the oxygen atom. This polarity allows water to dissolve ionic compounds through electrostatic interactions between the ions and polar water molecules but not non-polar covalent compounds which do not interact strongly with water. Heavy water, composed of deuterium and oxygen, is produced through electrolysis or distillation of ordinary water. It has similar but not identical chemical properties as ordinary water and is used as a neutron moderator and to produce deuterium.

1. Water

2. The polarity of water:
Water has a simple molecular structure. It is composed of one oxygen
atom and two hydrogen atoms. Each hydrogen atom is covalently
bonded to the oxygen via a shared pair of electrons. Oxygen also has
two unshared pairs of electrons. Thus there are 4 pairs of electrons
surrounding the oxygen atom, two pairs involved in covalent bonds
with hydrogen, and two unshared pairs on the opposite side of the
oxygen atom. Oxygen is an "electronegative" or electron "loving" atom
compared with hydrogen.

3. Water is a "polar" molecule, meaning that there is an uneven
distribution of electron density. Water has a partial negative charge ()
near the oxygen atom due the unshared pairs of electrons, and partial
positive charges( ) near the hydrogen atoms.
An electrostatic attraction between the partial positive charge near the
hydrogen atoms and the partial negative charge near the oxygen results
in the formation of a hydrogen bond.
The ability of ions and other molecules to dissolve in water is due to
polarity. For example, in the illustration below sodium chloride is
shown in its crystalline form and dissolved in water

4. Solvent property of water:
In general, water is good solvent for ionic compounds, but a poor one
for covalent compounds.
Solubility of ionic compounds:
When an ionic compound like NaCl is put in a polar solvent like water,
the electrostatic force of attraction existing between the ions in the
crystal lattice is considerably weekend and rapture by water due to its
high dielectric constant.
The ions of a salt are held together by ionic forces as defined by
Coulomb’s Law.

6. The large dielectric constant of water means that the force between
the ions in a salt is very much reduced permitting the ions to separate.
These separated ions become surrounded by the oppositely charged
ends of the water dipoles and become hydrated. Thus the Na+ and Cl-
ions will be torn apart from the crystal lattice. The negative ion is
attracted by the positive pole of the solvent molecule and the positive
ion by the negative pole of the solvent molecule.

8. Thus the water molecules pull the ions of the crystal apart and the
electrostatic force of attraction existing between the ions of the crystal
are cut off. Further the ions are surrounded by the water molecules
which act as a sheath (or envelope) around the ions and prevent the
recombination of the ions. The ions thus may freely in the solution are
said to be hydrated.

9. • Insolubility of some ionic compounds:
Some ionic compounds like AgCl, BaSO4 etc are insoluble in water. It is
because there are strong attractive forces in their crystal lattice and the
dielectric constant of water is unable to rapture them.

10. Solubility of some polar covalent compounds:
Some polar organic compounds like alcohols, sugar and dissolve in
water because they can strongly interact with water molecule through
the formation of hydrogen bonds.

12. • Insolubility of non-polar covalent compounds: several non-polar
covalent compounds like benzene, CCl4, methane etc. are not soluble
in water. It is because of the fact that these molecules don’t strongly
interact with the water molecules

13. Heavy water and uses:
Preparation of heavy water:
Heavy water is prepared either by prolonged electrolysis or by
fractional distillation of ordinary water.
Properties of Heavy Water:
Physical properties:
Heavy water is colourless, tasteless and odourless liquid.
It has all higher values for physical constants than the corresponding
values of ordinary water.

14. Physical Properties of Water and Heavy Water at
298 K

15. Chemical Properties of Heavy Water:
Although heavy water is chemically similar to ordinary water, chemical
reactions of heavy water are slower than those of ordinary water.

16. The following are the important uses of heavy
water:
As a neutron moderator: Heavy water is extensively used as a moderator in
nuclear reactions since it slows down the fast moving neutrons and thus
helps in controlling the nuclear reactions.
For the preparation of deuterium:
Heavy water produces deuterium on electrolysis or by its decomposition
with metals.
As a tracer compound:
Heavy water is used as a tracer compound for studying various reaction
mechanisms. For example. When hypophosporus acid (H3PO2) is treated with
D2O, only one H-atom is found to exchange with deuterium. This suggests
that H3PO2 has only one ionisable H-atom.