[Fish200398]

acetylene gas + ammoniac silver nitrate what color is the precipitate
the answer is white, why? i googled it, is the product is silver acetylide ? but it looks grey not white?

and acetylene gas + ammoniac copper 1 chloride, the answer is red. why? how to know what color is precipitate?

[chenbeier]

You have to see, to know or to learn it.
The clean silveracetylid is white, but by producing of it some metal silver also precipitate so crued product is grey.
The copperacetylid is red like copper- I- oxide. Also here it can precipitate some copper metal  what also give the red copper colour.

PS: Dont perform these  Experiments. Very dangerous.

[MNIO]

your reaction is
  1 H-C≡C-H + 2 Ag[NH3]2NO3 ---> Ag-C≡C-Ag + ___

the Ag-C≡C-Ag is your silver acetylide and ppt's out as a grey to white solid depending on purity and particle size. 

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fyi.. silvery acetylide is a heat and shock sensitive explosive.  I would not recommend drying the product, or even running the reaction without extreme caution.

[Fish200398]

do chemist just memorize all this color stuff? this compound will be what color, etc

[chenbeier]

They do, if worked with it.