Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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selenium : Se
sulphuric acid : H2SO4
Class:
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elem=Se
precipitation
redox
Summary:
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Selenium dissolves in very hot concentrated sulphuric acid, forming a dark
green liquid. On dilution, the red allotrope of selenium is precipitated.
Description:
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Add a small amount of selenium (black vitreous allotrope, finely powdered) to
some concentrated sulphuric acid (96 .. 98%): The solid does not dissolve. When
shaken, the solid selenium settles at the bottom and the liquid seems to become
a little bit yellow.
Heat, until the sulphuric acid is a mobile liquid like water and faint white
fumes can be observed above the liquid (BEWARE: this hot acid is EXTREMELY
corrosive, instant destruction of skin): The selenium dissolves and the liquid
becomes dark green and clear.
Let liquid cool down: The liquid remains dark green and clear and becomes more
viscous again.
Pour liquid in its ten-fold amount of water: The acid dissolves, heat is
produced and a brick-red fine precipitate is formed. This brick-red precipitate
is elemental selenium in its red allotropic form.