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In 1808, H. Davy in Britain discovered sodium while electrolyzing molten soda and sodium hydroxide. In nature, sodium exists in combined forms and is widely distributed; it accounts for about 2.64% of the Earth’s crust. Common minerals that contain sodium include rock salt (NaCl), feldspar (NaAlSi2O6), Chilean nitrate (NaNO3), and borax (Na2B4O7·10H2O). It is also present in large quantities in seawater. Sodium is an element in Group IA of the third period, with the symbol Na. A silver-white metal with a beautiful luster; it is lighter than water, soft and malleable. At room temperature it has a waxy texture, but it becomes brittle at low temperatures. It is highly reactive chemically, capable of combining directly with non-metals. It oxidizes rapidly in air; when burned, it produces a yellow flame and forms sodium peroxide (Na2O2). It reacts violently with water to produce hydrogen and sodium hydroxide. It forms alloys with various metals, especially lead and mercury. There are more compounds of sodium than of any other metal, mainly including chlorides, borates, carbonates, sulfates, etc. Sodium can be used to produce compounds such as sodium peroxide and tetraethyl lead. Alloys of sodium and potassium (containing 50–80% K) are liquid at room temperature. Sodium can be used as a heat conductor in reactors, and it is a strong reducing agent that can reduce metals such as titanium, zirconium, niobium, and tantalum from their molten halides. The spectrum of sodium vapor, characterized primarily by the yellow D line, can be used as a monochromatic light source or in sodium vapor lamps for lighting. Sodium vapor is released into the upper atmosphere by rockets, creating bright orange-yellow clouds (i.e., sodium clouds). Sodium ions are an essential nutrient for humans and animals, and are primarily consumed in the form of sodium chloride. Sodium is corrosive to the skin and burns when heated; it must be stored in kerosene or paraffin oil, and care should be taken to ensure safety when using it. Sodium can be produced by electrolyzing molten sodium hydroxide or sodium chloride.