Chevreul's salt is a peculiar inorganic compound of copper with chemical formula Cu3(SO3)2, systematically naming it copper(I,II) sulfite. This dense red salt is an example of a mixed-valence compound, containing copper is present in both the +1 and +2 oxidation states.
The chemistry of Chevreul's salt is revealing of the mixed-valence nature of the compound. Addition of Chevreul's Salt to dilute hydrochloric acid converts it to copper(I) chloride, alluding to a +1 charge on the copper atoms. However, Chevreul's salt can also be dissolved in aqueous ammonia to produce the deep blue tetraammine copper(II) complex, which, as is included in the name, contains copper in the +2 oxidation state. Little is well-known about this compound, so as of yet not many other reactions are known.
Chevreul's salt is a dense, deep red compound typically produced in the form of a uniform microcrystalline powder. It is air-stable and insoluble in water but reacts with acids and ammonia.
Despite being a very obscure compound, Chevreul's Salt can be very easily synthesized in a matter of minutes. First, a solution of sodium metabisulfite is added to a solution of copper(II) sulfate, forming a grass-green species of unknown composition. The solution can then simply be heated to boiling for a few minutes, gradually darkening to an opaque black. As the boiling subsides, a deep red precipitate quickly falls out of the now-clear solution. As sulfur dioxide is released during the boiling process, it may be necessary to conduct this procedure outside or in a fume hood.
So far, not enough is known about Chevreul's Salt to have any projects for it, though it is a good candidate for display, being a rarely encountered compound of copper and having an unusual color for its composition. Experiment!
Since not much is known about Chevreul's salt, it should probably be assumed for now that it is not a good idea to allow it entry to one's body. As it is insoluble in water, though, skin contact isn't thought to be an issue.
Credit for these findings goes to sciencemadness's MrHomeScientist. The original write-up can be found here: http://thehomescientist.blogspot.com/2013/10/chevruels-salt.html