Haruka
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Question about colour
If I have a precipitate of CaCO3 (which I have seen on the internet that is a white colour) How can I explain that color...? I mean since the Barium
is a green-yellow colour... so, Why the precipitate is white? =S.. Help please!
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Molecular Manipulations
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Not sure what you mean. What barium? Barium metal is silver, gold when impure. It's compounds are colorless, just like calcium.
Calcium carbonate doesn't contain any barium, and if it were contaminated with barium carbonate, it would still be colorless.
-The manipulator
We are all here on earth to help others; what on earth the others are here for I don't know. -W. H. Auden
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Haruka
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When you do flame tests with barium chloride, the color cast is a green-yellow so if I have precipitate of barium I would expect that color ... but
otherwise the color I get is a white. I don't know if that have something to do with the carbonate ion ...
I leave the reaction:
BaCl2 + K2CO3 ----- BaCO3 (precipitate) + 2KCl
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Amos
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The color that an element's salts produce in a flame test has no relation whatsoever to what color their compounds are going to be. Barium carbonate
is white because it's white, as are nearly all compounds of the metals in the first two groups of the periodic table(or colorless).
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ziqquratu
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When you do a flame test on a sodium-containing compound, the flame colour is yellow, yet common sodium salts (chloride, carbonate, sulfate, and on
and on) are all white or colourless. This also holds true for a variety of other elements - potassium, lithium, strontium and zinc all jump to mind as
generally having colourless salts but giving distinct flame colours.
In the flame test, the metal ion absorbs heat energy, resulting in the excitation of an electron to a non-ground energy level. When that excited
electron returns to the ground state, the absorbed energy is emitted as a photon. Since the energy required to excite an electron varies from one
element to the next, the energy - and thus colour - of the emitted photon likewise differs between elements, which allows the elements to be
differentiated by this method.
In addition, it should be noted that electrons can often be promoted to one of multiple energy levels, and so the colour observed in the flame is the
sum of the emitted photons. Lithium, for example, has four or five visible emission lines - two or three in the violet-blue, one orange and one red.
Sodium has a green and two yellow. Barium has something like 20 lines ranging from green through red! And, of course, whilst some elements have
visible emission lines, many (heck, probably all) elements also have non-visible emission lines (in the UV or IR).
The colours observed in the flame test, however, have nothing to do with the colour of the compounds of a particular element - and that becomes
particularly evident if you consider the transition metals, where compound colour depends on the environment the metal is in as much as on the metal
itself. I've only had a quick glance, but Wikipedia's overview (https://en.wikipedia.org/wiki/Color_of_chemicals) appears to be a reasonable starting point.
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