Description of experiment
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Needed compounds:
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potassium iodide : KI
thallous nitrate : TlNO3
Class:
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elem=I,Tl
precipitation
Summary:
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Thallous iodide is insoluble in water and is a beautiful brightly colored
compound, with a color, depending on temperature.
Description:
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Prepare a solution of thallous nitrate and prepare a solution of potassium
iodide, such that there is excess potassium iodide. Mix the two solutions: An
ochre/yellow precipitate is formed, with tinges of brown as well. Most of the
precipitate is fairly compact, but part of it is very finely divided.
Heat the liquid with the solid material suspended in it, while swirling the
liquid in its test tube: Initially, nothing seems to happen, but when the
liquid becomes hot (near boiling), then the precipitate becomes even more
compact and the liquid becomes totally clear and colorless like water, with a
bright yellow solid at the bottom. When the liquid is shaken and then allowed
to come to rest again, then the precipitate very quickly settles at the bottom
(just a few seconds) and a perfectly clear liquid is
Decant the colorless liquid from the yellow solid, rinse two times with
distilled water and allow the yellow solid to dry: The dry solid remains bright
yellow.
Heat the bright yellow solid in a dry test tube: The solid turns
orange/red/brown when it is hot. The solid must be heated quite strongly before
the change of color occurs (appr. 200 C). When the solid is allowed to cool
down, then the color remains orange/red/brown. One day later, however, the
solid is briht yellow again, as if it had not been heated.