Description of experiment
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Needed compounds:
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magnesium : Mg
hydrochloric acid : HCl
sodium sulfite : Na2SO3
hydrogen peroxide : H2O2
selenium : Se
sodium sulfide : Na2S . 3H2O
Class:
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elem=Se
redox
Summary:
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Selenium dissolves in a sulfide solution, forming a deep red/brown solution.
Hydrogen peroxide is capable of oxidizing this solution.
Description:
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Add a very small piece of selenium to a solution of sodium sulfide: The solid
slowly dissolves. The liquid becomes deep red/brown and remains clear. It takes
several hours before the small piece of selenium dissolves completely.
Dilute the red/brown liquid with some water and then add some H2O2 (10%) to
this: The liquid becomes turbid and orange/yellow. On top of the liquid some
red solid appears.
Let liquid stand overnight (in an attempt to get the orange/yellow precipitate
as a thin layer on the bottom): The liquid has turned colorless and clear.
Sequence 1a
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Add part of the liquid to an excess amount of a solution of sodium sulfite
in 10% hydrochloric acid: The liquid remains colorless and has a strong smell
of SO2.
Set aside for several days: A brick-red fine precipitate has settled at the
bottom. This probably is the red allotrope of selenium.
Sequence 1b
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Add another part of the clear liquid to an excess amount of 10% HCl: The
liquid remains clear and colorless.
Add a small piece of metallic magnesium: The magnesium dissolves, while
bubbling vigorously and some heat is produced. The liquid becomes turbid and
orange. Apparently, very finely divided selenium is produced.