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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.

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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.

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    $\begingroup$ 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! $\endgroup$ – permeakra Jul 12 '15 at 8:21

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