Why does potassium become colourless when reacting with water?

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Offline pipster

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Apparently, when reacting potassium with water, after the potassium has whizzed around on the surface with its lilac flame it then sinks to the bottom, turns completely colourless and then explodes.

Since potassium metal is silver, and potassium liquid is also silvery how do we explain the change to colourless?

I wondered if we are seeing a bubble of hydrogen liquid appear (colourless?) which is then the driving force for the explosion, or whether it is something to do with the change in metal orbital overlap and band gaps(!) in the liquid state.
Just as graphite is black because of the overlap between valence and conduction bands so that electrons of any wavelength can be promoted (i.e. all visible light is absorbed), a liquid without this solid state structure and regularity might have a poorer overlap, larger band gap and so no electrons can be promoted and light is reflected as in insulators.
Am I anywhere near the right lines? Can anyone explain?

Pippa (MSci student)
« Last Edit: 16/06/2008 12:23:19 by chris »


Offline lightarrow

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Colourless is intended as "white" or as "transparent"?
If the thread weren't so old, the OP could have answered. In case it means white, the answer is easy: potassium hydroxyde which forms through the reaction; if it means transparent, a possible answer is that a gas bubble wrapped around the piece of metal, because of water/gas/water refraction, bends light so that to mask the object inside. Just a speculation however.