Soda, the common name of sodium thiosulfate, is also called Hypo, which contains five crystal waters (Na2S2O3·5H2O), so it is also called sodium thiosulfate pentahydrate. Soda is a colorless and transparent crystal, easily soluble in water, and its aqueous solution is weakly alkaline.

Baking soda is a white crystal, soluble in water, and its aqueous solution is weakly alkaline. In hot air, it can slowly decompose and release a part of carbon dioxide; When heated to 270℃, carbon dioxide is completely decomposed and released.

These properties of baking soda make it have many important uses in production and life. In fire extinguishers, it is one of the raw materials to produce carbon dioxide; In the food industry, it is a main raw material of baking powder; It is also a commonly used raw material when making cool drinks; Medically, it is a medicine for treating hyperacidity.

Soda has a strong complexing ability and can form a complex with silver bromide. The reaction formula: agbr+2na2s2o3 = = = nabr+na3 [ag (s2o3) 2]. According to this property, it can be used as a fixing agent. During phase washing, excessive baking soda reacts with the silver bromide of the unexposed part of the negative film, and is converted into soluble Na2 [Ag (S2O3) 2], and the AgBr is removed, so that the developed part is fixed.

Soda also has strong reducibility, which can reduce chlorine and other substances.

Na2S2O3+4cl2+5h2o = = = H2SO4+2nac+6hcl, so it can be used as a dechlorination agent for cotton fabric after bleaching. Similarly, iodine stains on fabrics can also be removed with it.

In addition, baking soda is also used for tanning leather, electroplating and extracting silver from ores.