Water, as we all know, is a colourless and odourless compound. It is one of the most important compounds and is quite essential for the survival of living organisms. Around 65% of the human body is composed of water.
The chemical formula of water is H2O. It consists of two elements, namely hydrogen and oxygen. Two molecules of hydrogen combine with one molecule of oxygen to form water. It exists in three forms, i.e. solid, liquid, and gas (or vapour). It has the important property of dissolving many other substances, which makes it a universal solvent.
When observed in small quantities, it appears to be colourless, but it emits a bluish colour due to the absorption of sunlight. The water that we see in water bodies has a distinct hue owing to the aquatic vegetation and organic matter.
Structure of Water
It has a bent molecular structure. It is a simple molecule in which one oxygen atom is bonded to two different hydrogen atoms. Because of the high electronegativity (i.e. the tendency of an atom to attract the bonding electrons towards itself) of the oxygen atom, the bonds present are polar. The shared pair of electrons is attracted to a greater extent towards oxygen atoms than hydrogen atoms, resulting in the formation of dipoles (pair of opposite charges). Partial negative charges are acquired by oxygen, and hydrogen atoms acquire a partial positive charge. The H-O-H bond angle is 104.5 °. The bond angle is slightly smaller than an ideal sp3 hybridized atomic orbital. It has a tetrahedral geometry.
The amphoteric nature of water implies that water can act as both an acid and a base. It means that water can be a proton donor as well as a proton acceptor. Few examples of reactions are illustrated below:- H2O + NH3 ⇨ OH- + NH4+ (acid) (base) (conjugate base) (conjugate acid) H2O + H2S ⇨ H3O+ + HS- (base) (acid) (conjugate acid) (conjugate base)
During auto-protolysis, the ionisation of water takes place where water molecule, i.e. H2O deprotonates ( removal of protons ) in order to form a hydroxide ion OH-. The hydrogen nucleus H+ immediately protonates ( accepts proton) with another water molecule to form a hydronium ion, H3O+. Self-ionisation of water is illustrated below:-
Hard water – It contains salts of Calcium (Ca) and Magnesium (Mg) as hydrogen carbonate, chlorides and sulphates.
Soft water – The water which is free from salts of calcium and magnesium.
The hardness of water can further be classified as:-
Temporary hardness is due to the presence of magnesium bicarbonate Mg(HCO3)2(aq) and calcium bicarbonate Ca(HCO3)2(aq). This type of hardness can be removed by boiling, as boiling promotes the formation of carbonate precipitate from bicarbonates, which can be separated.
Permanent hardness is due to the presence of calcium and magnesium chlorides, sulfates, and nitrates. It cannot be removed by boiling. That's why it is known as permanent hardness. In order to remove permanent hardness, hard water is treated with washing soda (Na2CO3). Fun fact: Soap doesn’t form lather in hard water.
Some important terms associated with water:
Water which is free from all types of minerals and only consists of H+ and OH- ions is called de-mineralised water. It is not at all suitable for drinking purposes because it lacks the ions required for proper body growth.
It is made up of deuterium (an isotope of hydrogen). It is used as a moderator in nuclear reactors. It is also not suitable for drinking purposes as it slows down the metabolic process occurring in the body.