Chemistry · Ch 4 — Hydrogen
Chemical Properties
Chemical Properties
Water's chemical behaviour depends strongly on what it is reacting with.
Reaction with metals. The most reactive metals, the alkali metals, decompose water even in the cold, liberating hydrogen and leaving an alkaline solution:
The Group 2 metals (except beryllium) react similarly but less violently, and their hydroxides are less soluble than the Group 1 hydroxides:
Some transition metals react only with hot water or steam, forming the metal oxide -- steam passed over red-hot iron, for example:
Lead and copper decompose water only at white heat, while silver, gold, mercury and platinum have no effect on water at all. Among the non-metals in their elemental form, carbon, sulphur and phosphorus normally do not react with water either -- though, as already seen (Section 4.2.2), red-hot or white-hot carbon does react with steam to give water gas.
Reaction with halogens. The halogens react with water to give acidic solutions. Chlorine, for instance, forms a mixture of hydrochloric acid and hypochlorous acid -- the latter responsible for chlorine water's antibacterial action and its use as a bleach:
Fluorine behaves differently, instead liberating oxygen from water:
Reaction with non-metal compounds. Compounds of non-metals similarly react with water to give acidic or alkaline solutions -- carbonate solutions, for example, are slightly alkaline:
Water as an amphoteric oxide. Water can both accept and donate a proton, so it can act as either an acid or a base depending on what it meets. With HCl it accepts a proton (behaving as a base):
With the weak base ammonia, it donates a proton (behaving as an acid):
Water as a solvent and hydrolysing agent. Water dissolves ionic compounds readily, and it also hydrolyses certain covalent compounds:
Water of crystallisation. Many salts crystallising out of aqueous solution trap water molecules in their crystal structure -- water of crystallisation -- which can sit either as a coordinate-bonded ligand on the metal ion or as loosely held interstitial water in the lattice, and sometimes both in the same compound: …
What this figure shows. Ball-and-stick structure of [Cu(H2O)4]SO4.H2O: a central green copper(II) sphere is octahedrally surrounded by four coordinated water molecules (red O, white H) arranged around it, with a yellow sulphur atom (the sulphate group, its red oxygens including the one that coordinates to Cu) shown above and below the copper centre. A fifth, uncoordinated water molecule is drawn off to one side, linked not to the copper but by an intermolecular hydrogen bond to one of the sulphate's oxygens. A colour key at the foot of the figure marks O …