While the ubiquitous nature of water might lead many to consider it ordinary, it is in fact in many ways a miracle liquid. Well before the advent of modern science, the very first Greek philosopher Thales (624 – 546 BC) described water as the cause of all things.
Over two thousand years later, although science has been able to explain many of water’s basic physical properties, there still remains much that remains to be understood.
Water is a tetrahedral structure that has two hydrogen atoms bound to a central oxygen atom. This is done through the sharing of a pair of electrons, forming a covalent chemical bond. The two O—H bonds are 0.096 nm in length and lie at an angle of 104.5 degrees to each other.
Water has a dipole, which means a separation of partial electrical charge along the molecule. This leads to the formation of hydrogen bonds, with each oxygen atom in a single molecule able to form an H-bond with two hydrogen atoms.This gives water its cohesiveness.
The network of water molecules is irregular in its liquid phase, with distorted H-bonds. Upon freezing, the H-bonds form water into a regular lattice with greater space between the molecules than present in the liquid state. This results in ice being less dense than water.
The anomalous physical properties of water result from the ways in which individual water molecules interact with each other through hydrogen bonds. These have profound effects on the size and shape of living organisms.
There are many remarkable properties of water, including its high boiling point and its character as the "universal solvent".
Due to its strong hydrogen bonds, water boils at a relatively higher temperature than would be expected for its molecular weight. Comparing molecular weights, ammonia with a molecular weight of around 17 boils at -33 degrees Celsius, while water with a molecular weight of 18 boils at 100 degrees Celsius.
Water is often referred to as a universal solvent because many substances dissolve in it. This property of water stems from the partial charge that develops across water molecules, which allows it to surround charged particles and “pull” them into solution. While this works for charged particles, it also means that nonpolar molecules like oil do not dissolve in water.
Water's density is dependent on temperature. A common example given for this property is the way lakes and ponds freeze from the top down due to solid water being less dense than liquid water. This is crucial to the survival of fish, who can stay at the bottom during the winter season without being frozen solid.
Water has a high surface tension. The high surface tension of water allows small, denser objects to stay on top of the water surface.