Liquid water

All forms of life based on this alone element, shared by all living things.
The water is perfect, it helps to dissolve the materials and chemicals. The water is home to elements that will serve as catalysts for the production of chemicals necessary for life. It is water that we have this fantastic and wonderful diversity of life on earth.
Living organisms are ordered construction of complex molecules that react chemically together.
Liquid water is perfect to trigger chemical reactions.
In a liquid, the atoms can move easily, while in a solid, the structures are fixed and the molecules can not react chemically.
In a gas, the reverse, the molecules are too far apart, and chemical reactions are rare.
It is in liquid water, that life has chosen to evolve, developing molecules more and more complex.

The water molecule is composed of one oxygen and two hydrogen atoms, electrical charges are hydrogen links maintain the molecules together.
This weak liaison makes water a very special element. Hydrogen is produced early in the history of the Universe, it is the first atom to constitute itself, oxygen is an element that appears later, when thermonuclear fusion reactions within stars.

* If our planet had no relief, liquid water would cover its entire surface to a depth of 3 km.
Apparently, that's a lot of water, but the layer of 3 km is only 0.023% of the radius of the Earth. This is equivalent to the thickness of the skin of an apple compared to the apple.
Image: NASA

All atoms are composed of a nucleus (protons and neutrons), carrying a positive electric charge, around which the electrons carry a negative electrical charge. But an atom is globally neutral.
It is due to electrons that atoms will "stick" them. The smallest of all atoms is the hydrogen atom, it has only one electron. In the water molecule, the oxygen atom captures electrons more strongly than the hydrogen atom. The oxygen atom is therefore negatively charged and hydrogen positively.
This explains that the water molecule is a molecule "polar". This polarity is that the water molecules stick together, the positive side of a attracting the negative side of another.
Such a link between electromagnetic molecules called a hydrogen bond.
This polarization also allows water molecules to stick to many other molecules.
Thanks to its polarity, water allow molecules to meet within the liquid, and to react chemically, hence the importance of liquid water in the development of life. Water is a powerful accelerator of chemical reactions. Water is the most common universal solvent because it can dissolve a large number of chemical compounds.

* The water molecule (H²O) is made of two hydrogen atoms (atomic number = 1), connected to an oxygen atom (atomic number = 8), which have their electrons in common.
The water molecule has a total of ten electrons which eight were made by the oxygen atom and by two hydrogen atoms. The electron is not shown here, it has no precise location in the atom as a timeless wave, the electron is both everywhere and nowhere, a little here and a little there.

* Diagram of change of state or phase transition of pure water.
At the triple point, the three phases coexist at a specific temperature and pressure.
At the critical point of transition between liquid and gas, water reaches the fluid phase, both gas and liquid.