ut 1827. Cuprous oxide is reduced by hydrogen, carbon monoxide,
charcoal, or iron, to the metal; it dissolves in hydrochloric acid
forming cuprous chloride, and in other mineral acids to form cupric
salts, with the separation of copper. It dissolves in ammonia, forming
a colourless solution which rapidly oxidizes and turns blue. A
hydrated cuprous oxide, (4Cu2O, H2O), is obtained as a bright yellow
powder, when cuprous chloride is treated with potash or soda. It
rapidly absorbs oxygen, assuming a blue colour. Cuprous oxide
corresponds to the series of cuprous salts, which are mostly white in
colour, insoluble in water, and readily oxidized to cupric salts.

Cupric oxide, CuO, occurs in nature as the mineral melaconite (q.v.),
and can be obtained as a hygroscopic black powder by the gentle
ignition of copper nitrate, carbonate or hydroxide; also by heating
the hydroxide. It oxidizes carbon compounds to carbon dioxide and
water, and therefore finds extensive application in analytical organic
chemistry. It is also employed to colour glass, to which it imparts a
light green colour. Cupric hydroxide, Cu(OH)2, is obtained as a
greenish-blue flocculent precipitate by mixing cold solutions of
potash and a cupric salt. This precipitate always contains more or
less potash, which cannot be entirely removed by washing. A purer
product is obtained by adding ammonium chloride, filtering, and
washing with hot water. Several hydrated oxides, e.g. Cu(OH)2.3CuO,
Cu(OH)2.6H2O, 6CuO.H2O, have been described. Both the oxide
and hydroxide dissolve in ammonia to form a beautiful azure-blue
solution (Schweizer's reagent), which dissolves cellulose, or perhaps,
holds it in suspension as water does starch; accordingly, the solution
rapidly perforates paper or calico. The salts derived from cupric
oxide are generally white when anhydrous, but blue or green when
hydrated.

Copper quadrantoxide, Cu4O, is an olive-green powder formed by mixing
well-cooled solutions of copper sulphate and alkaline stannous
chloride. The trientoxide, Cu3O, is obtained when cupric oxide is
heated to 1500 deg.-2000 deg. C. It forms yellowish-red crystals,
which scratch glass, and are unaffected by all acids except
hydrofluoric; it also dissolves in molten potash. Copper dioxide,
CuO2H2O, is obtained as a yellowish-brown powder, by treating cupric
hydrate with hydrogen peroxide. When moist, it decom