Water-reactive substances are those that spontaneously undergo a chemical reaction with water, as they are highly reducing in nature. Notable examples include alkali metals, sodium through caesium, and alkaline earth metals, magnesium through barium. Some water-reactive substances are also pyrophoric, like organometallics and sulphuric acid, and should be kept away from moisture. The use of acid-resistant gloves and face shield are required and should be handled in fumehoods. Water-reactive substances are classified as R2 under the UN classification system and as Hazard 4.3 by the United States Department of Transportation. All chemicals that react vigorously with water or liberate toxic gas when in contact with water are recognised for their hazardous nature in the 'Approved Supply List,' or the list of substances covered by the international legislation on major hazards many of which are commonly used in manufacturing processeses.
Alkali metals
The alkali metals are the most reactive metals in the periodic table - they all react vigorously or even explosively with cold water, resulting in the displacement of hydrogen. The Group 1 metal is oxidised to its metal ions, and water is reduced to hydrogen gas and hydroxide ion, giving a general equation of: The Group 1 metals or alkali metals become more reactive in higher periods of the periodic table.
The alkaline earth metals are the second most reactive metals in the periodic table, and, like the Group 1 metals, have increasing reactivity in the higher periods. Beryllium is the only alkaline earth metal that does not react with water or steam, even if metal is heated to red heat. Additionally, beryllium has a resistant outer oxide layer that lowers its reactivity at lower temperatures. Magnesium shows insignificant reaction with water, but burns vigorously with steam or water vapor to produce white magnesium oxide and hydrogen gas:
A metal reacting with cold water will produce metal hydroxide. However, if a metal reacts with steam, like magnesium, metal oxide is produced as a result of metal hydroxides splitting upon heating. The hydroxides of calcium, strontium and barium are only slightly water-soluble but produce sufficient hydroxide ions to make the environmentbasic, giving a general equation of:
Reactivity series of metals
If metals react with cold water, hydroxides are produced.
