WaterA water molecule has a structure containing one oxygen atom to two hydrogen atoms. The atoms are arranged in a hydrogen bond, which is a special type of bond that exists between a hydrogen atom bonded to another electronegative atom. A characteristic of water is its expansion upon freezing allowing ice to float on water, a property caused by rearrangement of the water molecules. Ice has a three dimensional structure that contains open spaces. When ice melts the hydrogen bonds break (the ice structure partially disintegrates): in liquid water approximately 15% of the hydrogen bonds are broken. Water has a high specific heat capacity due to the hydrogen bonds absorbing energy as it is heated.

The first recorded statement of a scientific theory regarding the structure of matter is from Thales of Miletus (625 - 547 B.C.) who proposed that all matter is ultimately a form of water.

Robert Boyle (1627-1691) stated in respect of the search by alchemists for Alkahest a hypothetical universal solvent that:

"For Helmont do's more than once Inform his Readers, that both Paracelsus and Himself were Possessors of the famous Liquor, Alkahest, which for its great power in resolving Bodies irresoluble by Vulgar Fires, he somewhere seems to call Ignis Gehennæ. To this Liquor he ascribes, (and that in great part upon his own Experience) such wonders, that if we suppose them all true, I am so much the more a Friend to Knowledge than to Wealth, that I should think the Alkahest a nobler and more desireable Secret than the Philosophers Stone it self."

The hydrogen bonds in water make it the closest modern equivalent of a universal solvent, because it can dissolve more substances than any other known solvent.

John Dalton (1766-1844) represented water with the symbol of two circles one containing a dotTwo papers on "Experiments with Airs," written by Henry Cavendish and printed in 1784 and 1785, contain the great discoveries of the compound nature of water. Antoine Laurent Lavoisier (1743-1794) reproduced the experiment of Cavendish and in so doing demonstrated that hydrogen and oxygen form the basic constituents of water. John Dalton (1766-1844) in A New System of Chemical Philosophy, published in 1808, stated that water, was composed of "1 of oxygen and 1 hydrogen, retained in physical contact by a strong affinity, and supposed to be surrounded by a common atmosphere of heat; its relative weight = 8".