Caustic soda is strongly corrosive and has water absorption (so NaOH is placed open and the quality will increase). It can be used as a desiccant. However, it cannot dry acid gases such as sulfur dioxide, carbon dioxide, nitrogen dioxide and hydrogen chloride, but it can react with CO2. The reaction is thus degraded, (which will react with acid gases) and is easily deliquescent and liquefied in air (the phenomenon of dissolution due to water absorption, which is a physical change); dissolved in water, while releasing a large amount of heat, can make phenolphthalein red, so that The purple litmus test solution turns blue, is a strong alkali, corrodes aluminum substances, does not corrode plastics, and only needs to be placed in the air for a few minutes, it will absorb water and become a liquid poison with a melting point of 318.4 ° C, except dissolved in water. In addition, sodium hydroxide is also soluble in ethanol and glycerin; but it is insoluble in ether, acetone, and liquid ammonia.