Copper formed by oxide reduction. Glass tube with copper oxide (black, CuO) being reduced by methane gas (CH4) passing over as it is heated. The copper metal (orange-brown) forms as the methane reacts with the oxygen in the oxide to form water (H2O) and carbon monoxide (CO). The same results are seen if ammonia or hydrogen gas is used instead of methane. The reaction equation using methane is: CH4 + 3 CuO ---> CO + 2 H2O + 3 Cu