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The inside of our kettle is lined in a blue-green deposit. Our water is very hard, but what minerals could cause the coloration?

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    $\begingroup$ Can you post a photo of your kettle? It appears what you desribe as "blue-green deposit" could be explained in various ways. $\endgroup$ – tipavi Jul 16 '16 at 17:24
  • $\begingroup$ To specify: Are you describing a turqouise crust (rare in kettles) or a rainbow like patina (common in kettles or pots)? $\endgroup$ – tipavi Jul 16 '16 at 17:31
  • $\begingroup$ @tipavi You're absolutely right in that a photo would help clear this up. It's great that there's discussion, though)) $\endgroup$ – Zubo Jul 16 '16 at 23:20
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If your building's water pipes are made of copper, that could explain the blue-green coloration. Hopefully the inside of your pipe doesn't look like this, though:

Copper Pipe rusting

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  • $\begingroup$ Unlikely that the kettle is made from copper... ;) $\endgroup$ – tipavi Jul 16 '16 at 15:59
  • $\begingroup$ Unlikely that the kettle is a pipe - you might want to amend your answer. $\endgroup$ – Todd Minehardt Jul 16 '16 at 16:19
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    $\begingroup$ The implication would be that the tinge of copper comes from the water coming through the pipes. $\endgroup$ – Jon Custer Jul 16 '16 at 16:36
  • $\begingroup$ Why couldn't the kettle be made of copper? Why couldn't the minerals come from copper piping? I think that's a perfectly logical hypothesis. Also: google.de/… $\endgroup$ – Zubo Jul 16 '16 at 17:05
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    $\begingroup$ Old buildings have water pipes made of copper - I'll amend my answer to be clearer about that @ToddMinehardt $\endgroup$ – IT Tsoi Jul 16 '16 at 17:16
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This is not caused by any mineral in the water, but an oxidation of the kettle's steel itself. Several monolayers of metal oxide are stacked onto each other, basically forming a dielectric mirror.

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  • $\begingroup$ I think that's a possibility, but certainly not the only one! Wouldn't bet on it $\endgroup$ – Zubo Jul 16 '16 at 17:04
  • $\begingroup$ @Zubo: Looking at your comments above I guess we are talking about different phenomena. A photo might help. $\endgroup$ – tipavi Jul 16 '16 at 17:22
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This might be dangerous! StackExchange is not necessarily the place to check this with.

If this is CuSO$_4$, that's actually mildly toxic. It could also be something like the German "Grünspan", a copper rust that makes a patina on copper house roofs that's got a blue-green color. It is a mixture of copper hydroxide, carbonate, acetate, and maybe others. This is all not good if this is your drinking water.

You should get that checked by professionals. However, ammonia solution in excess should produce a deep blue tetraamine complex with copper(II)-ions, you could try that (although if it doesn't work, that's by no means a safety assurance).

Have a look here for an example of Grünspan: Grünspan

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