Thiosulfate

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Thiosulfate is the oxoanion S₂O₃²⁻. Thiosulfates are only stable in neutral or alkaline solutions, but not in acidic solutions, due to decomposition to sulfite and sulfur, the sulfite being reduced to sulfur dioxide:

S₂O₃²⁻(aq) + 2H⁺(aq) → SO₂(g) + S(s) + H₂O

This reaction is usually used to generate a temporary aqueous suspension to demonstrate the diffraction of light in physics. If the light is shone from below, blue light is seen from above, and orange from sideways.

Uses of thiosulfates include the dissolving of silver salts from photographic film. In this context, sodium thiosulfate is often referred to as hypo.