# Color of precipitate formed on reaction of acetylene with Tollen's Reagent

I am asked to find the color of the precipitate formed by the passage of acetylene gas into an aqueous solution of ammoniacal silver nitrate.

I thought ammoniacal silver nitrate was Tollen's reagent, but acetylene is neither a ketone nor an aldehyde so I don't know how to figure out the color of the precipitate.

You are correct that ammoniacal silver nitrate is Tollen's reagent. Tollen's reagent is most notable for its reaction with aldehydes to produce a silver mirror, but it also reacts with several other types of compounds, notably alpha-hydroxy ketones and terminal alkynes.

With terminal alkynes, the alkyne reacts as a acid to form a silver acetylide which precipitates. In the case of acetylene, both hydrogens can react: $$\ce{HC#CH + 2AgNO3 + 2NH4OH -> AgC#CAg + 2NH4NO3 + 2H2O}$$

The colour of the precipitate is variously described as yellow, gray-white, or white. The actual colour you observe is probably strongly dependent on the purity of the sample and the size of the precipitated particles.

• Please, note!!!! Silver acetilenide formed is a sensitive, even if relatively weak explosive !!! Do NOT produce any significant quantities of it and do not dry it, as drying increases its sensitivity dramatically! – permeakra Jul 12 '15 at 8:21