Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrochloric acid : HCl
cadmium selenide : CdSe
cadmium sulfide : CdS
Class:
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elem=Cd,S,Se
precipitation
Summary:
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Cadmium ions give a precipitate with sulfide at very low concentration
of free sulfide ions. Only at very low pH, cadmium ions and hydrogen
sulfide can coexist in solution at appreciable concentration.
A very similar effect can be seen with combinations of cadmium and
selenide.
Description:
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Sequence 1:
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Add some solid cadmium sulfide (cadmium yellow) to concentrated hydrochloric
acid (30% HCl): The solid dissolves and a gas is evolved. The liquid starts
bubbling. The liquid becomes colorless and (almost) clear. There is a strong
smell of H2S.
Dilute with a lot of water: The liquid becomes turbid and yellow. By raising
the pH (dilution of acid), the H2S and the Cd2+ ions react, forming a
precipitate of cadmium sulfide again.
Sequence 2:
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Add some solid cadmium selenide (cadmium red) to concentrated hydrochloric
acid (30% HCl): The solid slowly dissolves. There is not a visible evolution
of a gas. The liquid becomes clear and colorless. The liquid, however, quickly
turns light yellow/brown.
Dilute with a lot of water: The liquid becomes turbid and red/brown. The
effect is similar to the effect, observed with cadmium sulfide.
Remark: With cadmium selenide there is the added effect that dissolved H2Se
is oxidized very easily. This causes the change of color, even in the
concentrated acid.
BEWARE: Sequence 2 must absolutely be done outside, with the wind coming
from behind. NEVER try to smell the liquid. H2Se is very very nasty and a
serious risk, much more than H2S is.