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I was reading this question and began to ask myself...

Since milk contains calcium, and calcium is a metal (just like potassium), how come putting a cup of milk in the microwave is safe and there are no visible arcs or sparks?

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Calcium is in an ionic form when dissolved in the milk fat and behaves differently to a solid metal. In addition to this the ratio of metal to non metal is a factor - you can actually put foil in the microwave, for example a meat pie that commonly comes in a foil container - this is able to be heated because there is enough water molecules in the pie to absorb the energy from the microwave –  Jason Jan 31 '11 at 5:47
    
If you read the comments on the question you link to, the conclusion seems to be that there's something going on besides simple mineral content. –  Jefromi Jan 31 '11 at 6:10
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When metal is exposed to microwave radiation, an electric potential difference can develop as the microwaves generate electric charge in parts of the metal. Flowing electricity can cause sparks as electrons migrate to places of lower potential. Solid metal is susceptible to this because its electrons are relatively loose, making it a good conductor of electricity.

Calcium (Ca) in milk is bonded with other atoms (mostly as calcium phosphate), so it doesn't behave as a solid metal would: first because its electrons are secured in bonds with other atoms, and second, because the Ca atoms are not aggregated together but intermixed with nonconducting ones.

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Thanks! Just the explanation I was looking for. –  George Edison Jan 31 '11 at 7:27
    
This is the same reason why the iron in meat or green vegetables doesn't cause lightning bolts in the microwave. It's only elemental conductive metals which cause problems. –  Matthew Walton Jan 31 '11 at 10:00
    
I'm by no means a chemist, but wouldn't the calcium ions and phosphate dissociate in solution? Obviously the important thing is that the calcium is ionized, not a piece of metal with a conduction band. I guess you're using "bonded" in the ionic sense, i.e. without meaning that the components are actually physically connected? –  Jefromi Jan 31 '11 at 14:56
    
@jefromi: this is going way back, but I recall that electrons that are part of a complete shell are the most stable and least likely to float away on an electric current... so if the calcium is in solution as part of a salt, it is a Ca2+ ion, and its two missing electrons will have joined a salt partner that was missing two electrons; however I'm not sure if the calcium phosphate in milk is soluble, or part of some larger molecules. –  J. Winchester Jan 31 '11 at 22:09
    
@jonw: That's what I was driving at: the important thing is that the electron configurations are stable, not that the calcium is bonded with anything. (As for the dissociating... ionic compounds are generally soluble, since water is polar. I don't know anything about the chemistry of milk, was just going off of your statement that it was calcium phosphate.) –  Jefromi Jan 31 '11 at 23:18
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