How is water unique?

Answer 1

Water is unique in many respects:

1. In most cases, when a body is heated, it expands; when a body is cooled it shrinks. Water expands when it is cooled below about four degrees celsius. The reason is that the water molecule has an unusually strong dipole moment: there is a separation of the negative charge of the oxygen atom from the positive charges of the two hydrogen atoms. This particular geometry produces, for example, the characteristic angle of about 105 degrees that the two atomic bonds of the hydrogen atoms with oxygen form. This characteristic of the water molecule explains the possibility of a new form of interatomic interaction: the hydrogen bond. With decreasing temperatures, water molecules arrange themselves in such a way that the oxygen atom is lined up between one of the hydrogen atoms of the same molecule and the hydrogen atom of the next molecule, creating a highly ordered structure, with a tetrahedral symmetry. So, when water is cooled, molecules are not on top of each other, as in a fluid, but they are placed at the nodes of a lattice, and this takes up more space - so ice is less dense than liquid water. By the way, this also explains why ice is slippery: when you walk on ice, you tend to melt it because at increasing pressures the liquid phase is more stable (because the liquid phase is more dense) and so a thin liquid film of water is produced just above the ice surface.

2. Water is transparent to electromagnetic radiation and this proved fundamental to the development of life on earth. Don't forget that photosynthesis started in the oceans and there massive quantities of oxygen were first produced, thanks to the fact that water is transparent to light.

3. Water is an excellent solvent and this too is fundamental to life as we know it (of course, this is also because water exists in liquid form on earth). The fact that water is a solvent depends critically on an apparently tiny phenomenenon: self-ioniziation.

4. Water has an unusually high specific heat. This is important because water can heavily influence temperatures on earth: it absorbs heat during the hot days and slowly gives it back to the atmosphere, making it warmer.

All these properties are not disconnected among them, but of course they are related: they all depend on the unique properties of the water molecule.