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Chemical properties of copper
Copper is very stable at room temperature and in dry air. In humid air or in the presence of carbon dioxide, a loose layer of "green rust" will cover the surface of copper. Its main component is basic copper carbonate CuCOs·Cu(OH)2.
When copper is heated to 185~200℃ in air, a loose black copper oxide (CuO) film will form on the surface. When the temperature reaches 700℃, a solid brick-red cuprous oxide (Cu2O) will appear on the surface of copper. Cuprous oxide not only combines firmly with the copper matrix, but also can wet soft glass well.
Oxygen is a harmful impurity in copper, usually in the form of Cu2O.
Copper cannot be annealed in hydrogen because hydrogen has a strong diffusion ability in copper and can easily enter copper. Ordinary copper contains cuprous oxide, which reacts with hydrogen to form water vapor. Water vapor cannot dissolve in copper and is not easy to seep out, so it accumulates inside the copper. Its chemical reaction formula is as follows:
Cu₂O + H₂ =2Cu+HO
The presence of water vapor will split and weaken the bonding between copper grains and cause cracks, causing capillary leakage, which is the so-called "hydrogen disease". In the production of electric light sources, parts are inevitably in contact with hydrogen, such as reducing the oxidized surface, or using coal gas (which contains hydrogen) to melt copper parts and glass. Therefore, to avoid "hydrogen disease", oxygen-free copper should be used. Oxygen-free copper can be burned with hydrogen at a temperature of 600~650℃ or annealed in a vacuum furnace.
Copper is insoluble in non-oxidizing dilute acid and can react well with acidic solutions of nitric acid, concentrated sulfuric acid, chromic anhydride (CrOs) or potassium dichromate (K:Or:O7). It can react with hydrochloric acid and dilute sulfuric acid when heated. Alkali has a weaker reaction with copper.
Copper should avoid contact with sulfur, because sulfur and copper generate black cuprous sulfide Cu₂S, which makes the material brittle.
Non-industrial copper and mercury can quickly form amalgam (i.e., mercury and metal form an alloy), making copper very brittle, so copper cannot be used in mercury-filled lamps.
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