The elements of heat and of non-metallic chemistry; especially designed for candidates for the matriculation pass examination of the University of London

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1868 Excerpt: ...The reaction is thus expressed:--. 4H20 + 3Fe = Fe304 + H8. Water Iron. Oxide of Hydrogen. (Steam). iron. § 89. Indeed the oxidation of metals, when exposed to the air, is generally due to the decomposition of the water which exists as vapour in the air, and rarely, or only in part, to the direct union of the metal with atmospheric oxygen. § 90. Some metals have such a great affinity for oxygen that they rapidly decompose water, in order to oxidize, even at ordinary temperatures. Thus sodium, if thrown into water, decomposes it and forms oxide of sodium, Na20, which dissolves in the water, while the hydrogen escapes. The so formed oxide of sodium combines then with a fresh equivalent of water, forming hydrate of sodium. Thus:--H20 + Na2 = Na20 + H2. Water. Sodium. Oxide of Hydrogen. Bodium. Na20 + H20 = 2(NaHO). Oxide of Water. Hydrate of sodium. Bodium. If the sodium be introduced into an inverted tube full of water standing over water, the sodium, being lighter Fig. 50. than water, will rise to the top of the tube, and as it decomposes the water (fig. 50) the hydrogen will collect in the upper part of the tube; the hydrate of sodium, NaH0, forming with water a solution heavier than that body, will sink and gradually mix with the water. § 91. Many metals which decompose water, even when cold, are nevertheless not deeply HO attacked or rusted by water, because the coating of metallic oxide at first formed, being insoluble in water, forms a water-proof coating or film of oxide which prevents the further oxidation of the metal, and therefore the further decomposition of the water. Such a metal is zinc (fig. 51); the oxide of zinc being insoluble in water. If the oxide be removed as fast as it is formed, a fresh surface of the metal is kept expo...