it is a sodium salt of an unstable acid.it is a colourless crystalline solid consisting of 5 molecules of wateras water of crystallization.it is soluble in water.when heated above 223c it decomposes forming sodium sulphate and sodium pentasulphide.

dilute acids decompose it with evolution of so₂and percipitationof sulphur.

na₂s₂o₃+h₂so₄=na₂so₄+so₂+s+h₂o

it is used in the extraction of silver and gold.