Water of crystallization degree of hydration can be critical to the resulting chemical properties. For example, anhydrous RhCl is not soluble in water and is relatively useless in organometallic chemistry whereas RhCl•3HO is versatile. Similarly, hydrated AlCl is a poor Lewis acid and thus inactive as a catalyst for Friedel-Crafts reactions. Samples of AlCl must therefore be protected from atmospheric moisture to preclude the formation of hydrates. Crystals of hydrated copper(II) sulfate consist of [Cu(HO)] centers linked to SO ions. Copper is surrounded by six oxygen atoms, provided by two different sulfate groups and four molecules of water. A fifth water resides elsewhere
Water-reactive substances 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: Mg + 2HO ⟶ Mg(OH) + H 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 environment basic, giving a general equation of: M + 2HO ⟶ M(OH) + H Water-reactive substances