Water is weird! A few odd things about the wet stuff

Water is the stuff of life. 70% of the Earth's surface is covered with it and our bodies are mostly composed of it: our brains alone are 75% water. We can last for a month or more without food but less than a week without water. Man has always been facinated with it: seeing insects able to walk on water using its surface tension, fish surviving in liquid water at the bottom of a frozen pond, the awesome power of ocean waves or falling water.

Water is a simple molecule consisting of one Oxygen ion and two Hydrogen ions, hence its formula H2O. It is a polar molecule and so tends to form hydrogen bonds between individual molecules. When water is a solid (ice), these additional intermolecular interactions give rise to an ordered tetrahedral shape. As heat is applied to ice, the number of ordered tetrahedral structures is reduced causing a more disordered arrangement in which the molecules are more densely packed at 4oC. Hence weird fact number 1: water at 4oC is ~9% more dense than ice so ice floats. But... as more heat is applied to water, it causes the water molecules to vibrate more so they move apart and density drops again, so that water starts to follow the same rule other liquids: it is less dense than its solid state.

Going on to weird fact number 2: it takes a lot of energy to heat up water compared to other liquids. The reason is that stable ordered tetrahedral structure: much of the extra heat energy is needed to convert it into the disordered structures, rather than make the molecules vibrate more. Most energy is required between 0oC and 35oC when it steadily removes regions of ordered tetrahedrons from the water. Above 35oC, there are so few tetrahedrons left that water can behave like a regular liquid.

Weird fact number 3: water is unusually hard to compress. The strong attraction between disordered water molecules keeps them closely packed already, more so than other liquids, so there is not much to compress when pressure is raised.

Weird fact number 4: water molecues are more able to diffuse at higher pressures. High pressure produces more disordered structures which are more mobile than the ordered tetrahedral molecules.

Weird fact number 5: water has an unusually high melting/freezing point (0oC) and an unusually high boiling point (100oC). All due to those intermolecular interactions binding the water molecules together.

Weird fact number 6: liquid water exists at very low temperatures and freezes when the temperature is raised. We all know that water solidifies at 0oC, but it is possible to supercool water at normal pressure and maintain it as a liquid down to about -42oC. This is not just an experimental phenomenon, it's found in nature when frogs and fish can survive long winter.

And finally my favourite weird fact number 8: once its evaporates, a water molecule spends around 10 days in the air before coming back...

Remember it's just water now, not magic!