Alumina is one of the main components of the earth crust. Pure alumina is the main raw material for smelting metallic aluminum. Aluminum is easily oxidized by air to produce alumina. When aluminum products are placed in the air, a thin layer of dense alumina protective film is formed on the surface, which protects the aluminum inside from oxidation. Therefore, aluminum products can be used as cooking utensils. Alumina is amphoteric oxide, which can be dissolved in both acid and alkali. Therefore, aluminum products are not resistant to acid and alkali, and the outer oxide film will be damaged when encountering acid or alkali..