In ancient times, water was a precious liquid, but nobody knew of what it was really made. It took chemists like Lavoisier to discover the composition of water in the 18th century. The water molecule consists of two hydrogen atoms linked to one oxygen atom. It is found in all three natural states, solid, liquid and gaseous. We are familiar with it as ice or snow in the solid state and it is also present in innumerable minerals as natural hydrates.
Water vapour is an important constituent of the atmosphere.
As a liquid, water has unusual thermodynamic properties that determine our physical and biological world. Celsius is based on the same scale, but the Celsius zero corresponds to 273.16 K on the thermodynamic scale. This triple point for pure water is taken as a fixed point on the temperature scale. The change of scale was made without changing the value of the basic degree unit. In addition, the isotopic composition of water, which is variable and depends on the origin, is not that accurately defined.