Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium hydroxide : NaOH
hydrochloric acid : HCl
sodium cyanide : NaCN
Class:
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elem=C,N
organic
Summary:
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Hydrogen cyanide (or cyanide salt) slowly changes to a dark brown/black
compound, when it is allowed to stand. At least this happens in strongly acidic
media.
Description:
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Add some solid NaCN to some concentrated hydrochloric acid (appr. 30% by
weight): The NaCN does not dissolve. As soon as the solid touches the liquid,
some hizzing noise can be heard, but this stops immediately again. Probably
the fact, that the NaCN does not dissolve, is due to the insolubility of
NaCl in concentrated hydrochloric acid. The liquid becomes colorless and
some white solid remains at the bottom of the test tube.
Tightly stopper the test tube and put aside.
Wait for two days: The solid has become brown, the liquid still is colorless.
Wait another two days: The solid has become dark brown. The lower part of
the liquid is brown, somewhat like ordinary tea, not a red/brown color.
The liquid is clear.
Shake the liquid: Now all of the liquid is brown and clear. The solid seems
to dissolve a little. Dark brown material dissolves in the liquid and the
color of the liquid becomes like that of very strong tea.
Wait another week: The liquid has become very dark brown, the solid seems to
be black.
Decant part of the liquid in excess solution of NaOH: The resulting alkaline
liquid still is brown and clear. Its color does not change very much, except
of course the dilution of the color.
Shake the remaining part of the dark brown liquid with the black solid in it:
The solid does not dissolve.
Decant the dark brown liquid and add some water: The black solid still does
not dissolve (or it dissolves very slowly). The water becomes light brown,
and remains clear. The shade of brown is more like that of chocolate instead
of tea.
Again decant the water and add new water and shake well: The dark brown stuff
does not dissolve, it is broken apart in many smaller particles and the
result of the shaking is a suspension of brown solid matter in a liquid.
Remark: The brown material is due to polymerization of hydrogen cyanide.
Azulmic acid may be formed in this experiment, but more research on this may be
needed.
BEWARE: THIS IS A DANGEROUS EXPERIMENT. HCN is formed!!! Also during the
storage of the test tube, some HCN may be formed and released, if it is not
stoppered very well. If you are not comfortable using NaCN or HCN or do not
have the right place to perform this experiment, then don't do it!