Sodium hydroxide is a white solid, very soluble in water, and its aqueous solution has an astringent flavour and a slippery feel. When sodium hydroxide is exposed to air, it can easily absorb water and gradually dissolve when the surface is wet, this phenomenon is called deliquescence. Its relative density is 2.130, melting point 318.4℃. Boiling point 1390℃. There are two kinds of commercially available caustic soda, solid and liquid: pure solid caustic soda is white, block, flake, rod, granular, brittle; pure liquid caustic soda is a colourless transparent liquid. Solid caustic soda has strong hygroscopicity. Easily soluble in water, exothermic when dissolved, aqueous solution is alkaline, slippery; soluble in ethanol and glycerol; insoluble in acetone, ether. Extremely corrosive, corrosive to fibres, skin, glass, ceramics, etc. React with metal aluminium and zinc, non-metal boron and silicon to release hydrogen; disproportionation reaction with chlorine, bromine, iodine and other halogens; neutralisation with acids to produce salt and water.