Water is a compound that goes against many trends, and this is obvious when we look at its physical properties. In fact, water should be a gas at RTP, and has a much stronger surface tension and higher viscosity than most other liquids, and the density of its solid form is less than that of its liquid form. Why do these breaks in the pattern of physical properties of similar compounds exist.
Water has extremely strong intermolecular forces. There are three types of intermolecular forces :.
- Van Der Waals' Forces : a temporary dipole that occurs in a molecule, attracting another molecule of the same compound to itself, due to one of the molecules having an electron repel the electron in the other molecule, creating a positive-negative pole and attraction. These forces are the weakest of the three.
- Permanent Dipole Forces : polar molecules causing induced attraction due to the electronegativity of one of their atoms compared to the other. This creates a permanent positive and negative pole in the molecule, which attracts another such molecule. These forces are stronger than Van der Waals' forces, but still relatively weak.
- H-Bonding : occurs only in molecules containing both hydrogen and one of the following elements, bonded together : { N, O, F } if these atoms have a free pair of electrons. Ten times stronger than Van der Waals' forces, they occur when the electron density changes around the atoms : the electronegative atoms gain electron density whereas the hydrogen looses it, in such a way that it is almost donating an electron. This causes an immense permanent dipole that will attract molecules with a similar dipole. Although H-bonding is still only approximately one tenth of the strength of a covalent bond, it is the strongest of these forces by far.
Water stands out even from the H-bonding compounds ( H2O, HF, NH3 ) because of its structure and ability to create two hydrogen bonds per molecule, as opposed to only one by HF and NH3.