Chemistry

The Chemistry of Water



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Water is the most important, and most abundant compound on earth. It is essential for our life on this planet. Distilled water has a boiling point of 100 degrees celsius. It has a freezing point of 0 degrees celsius. The reason why water has a so high boiling point has to do with hydrogen bonding between the molecules of water.

 Water is a polar compound which can make hydrogen bondings between the oxygen atom of one molecule and the hydrogen atom of another water molecule. The type of hydrogen bonding in water is intermolecular hydrogen bonding, in contrast to intramolecular hydrogen bonding which exists in the same molecule as happens with ortho-nitro-phenol for example.

Dissolving substances in water can change the boiling and freezing points of water. The boiling point of impure water can be higher than distilled water. While the freezing point of impure water can be lower than distilled water. This is called in chemistry freezing point depression.

 This phenomenon occurs with other solvents as well and not just restricted to water. Water has the chemical structure H2O. This structure is not linear as one would expect from a tri-atomic molecule such as HgCl2 which is linear. The reason why water is not linear is that the central oxygen atom in the molecule has extra two pairs of electrons which are not available for bonding.

The linear geometry of water is energetically higher than the angular geometry. This is so because the repulsion between the electrons is minimal in the angular geometry. The theory in chemistry that lies behind this explanation is called: the electron pair repulsion model, which states that any molecule would choose the geometry of its orientation so that there is minimum repulsion between the bonding and non bonding electrons.

Water can behave as a Bronsted acid or a Bronsted base, namely it can give a proton ( a positive hydrogen ion) or it can give a hydroxyl -OH group when it dissociates. It can also behave as a lewis base because of the lone pairs available on the oxygen atom. Water is a neutral compound which can dissociate to give to proton and a hydroxyl group.

The dissociation is not complete but is an equilibrium between the dissociated form and intact water. The PH of distilled water is 7 and is considered as neutral. Namely neither acidic nor basic. Water can function as a lewis base. However this base is not strong because of the electronegativity of the oxygen atom which holds the electrons firmly close to its nucleus.

Water, through its lone pairs can function as a nucleophile as well. This means it has an affinity for positively charged carbon atoms. An example of the use of water as a nucleophile in organic reactions is the hydrolysis of esters using water. The polar and stable properties of water makes it an excellent solvent for many reactions.

 Water is being used nowadays as a solvent or reagent in green chemistry. Green chemistry is the area of chemistry that uses nonhazardous compounds to the environment such as water and other compounds. Chemical compounds are classified as hydrophilic or hydrophobic depending on whether they like water or dislike it.

Hydrophilic compounds like water and can dissolve in it. On the other hand, hydrophobic compounds dislike water and cannot dissolve in it. An example of such compounds is lipids or oil. Water has a high dielectric constant of 81.7. This allows it to be a good solvent for polar compounds.

Water can be used as a secondary source of energy by converting it into hydrogen and oxygen molecules. Hydrogen, the product of water decomposition is being used as the source of energy. Water can react with alkali metals to give the hydroxides of these metals. Water is being used as a ligand in organo-transition-metals chemistry.

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