Pure copper turns black because oxidation occurs to produce copper oxide, which is black.
Copper oxide (CuO) is a black oxide of copper, slightly amphoteric and slightly hygroscopic. The relative molecular mass is 79.545, the density is 6.3~6.9 g/cm3, and the melting point is 1326°C. Insoluble in water and ethanol, soluble in acid, ammonium chloride and potassium cyanide solutions, slowly soluble in ammonia solution, and can react with strong alkali. Copper oxide is mainly used to make rayon, ceramics, glazes and enamels, batteries, petroleum desulfurizers, pesticides, and also for hydrogen production, catalysts, green glass, etc.
Copper is a transition element, chemical symbol Cu, English copper, atomic number 29. Pure copper is a soft metal. When cut, the surface is reddish-orange with a metallic luster, and the pure copper is purple-red. It has good ductility, high thermal conductivity and electrical conductivity, so it is the most commonly used material in cables and electrical and electronic components. It can also be used as building materials and can be composed of many kinds of alloys. Copper alloys have excellent mechanical properties and low resistivity, the most important of which are bronze and brass. Additionally, copper is a durable metal that can be recycled multiple times without losing its mechanical properties.