In chapter 2: Water and Hydrogen Bonds, I learnt that water is an important component in cells, being the most abundant component and accounting for 60% to 90% of the mass of the cells. Some of the most important properties of water arises from its angled shaped, the intermolecular bonds it can form and its hydrogen bonding characteristics.
The general shape of a water molecule is V-shape. This is due to the H-O-H bond angle in free water molecule being 104.5° which is formed from the strong repulsion between the lone electron pairs pushing the covalent bond orbitals closer. Within the water molecules, there is an uneven distribution of charge occurs within each O-H bond in which the oxygen is more electronegative than hydrogen, therefore oxygen have higher tendency to attract electrons. Hence, the oxygen becomes partially negative while the hydrogen becomes partially positive. This uneven distribution of charge within a bond is known as a dipole and the bond is said to be polar. The angled arrangement of the polar O-H bonds of water creates a permanent dipole for water molecules. The polarity of water molecules causes it to attract one another and the attraction between one positive hydrogen atoms and the negative electron pairs produces a hydrogen bond. However, compared to other covalent bonds, hydrogen bonds are weak interactions as only +20 kj mol-1 is required to disrupt hydrogens between water molecules.
Besides that, water molecules are considered unusual because they can form four O-H-O aligned hydrogen bonds with four other water molecules when is solid phase (ice). Each of the hydrogen bonds point to the oxygen atom of an adjacent water molecule, forming a tetrahedron. Freezing causes the ‘net’ to become rigid tetrahedral lattice. When melted, some of the lattice is released and water molecules move closer together. This makes liquid water denser than solid, therefore ice floats.
Liquid water molecules also usually can form two to three bonds at any given moment and this contributes to water’s two additional properties which are specific heat and heat of vaporization in which large amount of heat is required to raise the temperature of water because of the multiple hydrogen bonds that must be broken in order to release water molecules. These properties help in minimizing temperature fluctuations within cells. It is a universal solvent in biological systems due to its polarity. The polarity in water molecules is caused by an uneven charge distribution within each O-H bond, Due to the uneven charge distribution, water becomes ionic compound with covalent bond. Sodium chloride (NaCl) is an ionic compound, where Na is positively charge and Cl is negatively charge. Therefore, it dissolves readily in water. When NaCl is added into water (H2O), the negative charge side of water molecule is attracted to the Na and the positive charge side of water is attracted to Cl. Because the covalent bond of H2O is stronger compared to the ionic bond of NaCl, the ionic bond is pull apart and eventually break, causing the Na ion and Cl ion to be surrounded by water molecules and dissolving the salt resulting in homogenous solution.
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