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Today a friend showed me something we could not explain:

She had a pan which she used for years to heat water. On the bottom it had a thick layer of calcium carbonate. She forgot the pan on the hot plate. So all the water evaporated.

Then the calcium carbonate layer at the bottom got really black. Is this possible if it was pure $\ce{CaCO3}$ or must there have been other organic compounds to explain the black colour?

May I have an explanation what happened chemically as well?

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  • $\begingroup$ Black is due to organic, no doubt. $\endgroup$ – Ivan Neretin Oct 16 '15 at 23:16
  • $\begingroup$ So it is impossible that is was the CaCO3? $\endgroup$ – Michael Oct 17 '15 at 7:36
  • $\begingroup$ Why, it's totally possible that it was mostly $\ce{CaCO3}$. But black is due to something organic (probably just a tiny amount of it). $\endgroup$ – Ivan Neretin Oct 17 '15 at 7:40
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Like the comments have said, black residue is usually from organic material that has scorched.

If the pan somehow got to $840 ^{\circ}$C, then the calcium carbonate would decompose into calcium oxide and carbon dioxide, neither of which would lead to a black residue.

$$\ce{CaCO3 -> CaO + CO2}$$

Organic materials are mostly made of hydrocarbons. When you burn the organic matter, you're left with pretty much all carbon. This occurs when there is not enough oxygen to completely burn all of the carbon - when there is enough oxygen, some of the carbon combines with it to form $\ce{CO2}$ and $\ce{CO}$ and the hydrogen combines with oxygen to form $\ce{H2O}$. When there is a lack of oxygen, some carbon is left over, and this is what we see as the black substance.

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