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I saw that an ice block melts faster when it is placed in a microwave rather than when it is exposed to flame. Could it be because the flame creates a layer of water vapor between the flame and ice? If not, what is the real reason for this?

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    $\begingroup$ Without quantifiable conditions, this has little meaning. Is the energy rate (e.g. watts) impinging on the ice the same for both sources? What if you apply a hundred mini-flames (same total wattage) at different locations? And so on. $\endgroup$ – Carl Witthoft Mar 7 '15 at 14:40
  • $\begingroup$ Directly over a flame or on a pan over a flame. On a pan, as the ice melts over the plan, it can also vaporize and push the ice up off the surface which slows down the melting. Both the energy going into vaporization and the tendency for the ice not to stay in contact with the hot surface would slow down the melting. A curious effect can happens where a hotter plan melts ice more slowly than a colder one. This happens with dry ice too and I think, Liquid Nitrogen as well. Over a direct flame I think the effect would be much less. $\endgroup$ – userLTK Mar 7 '15 at 16:09
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When you heat An ice cube over a flame then heat is absorbed due to conduction and heat flows one sidedly only however in a micro wave the electromagnetic radiations have a greater penetrating power and they start moving dipoles present in ice because of which there is an increase in their vibrational energy, now because of friction between layers heat is generated which melts the ice. These radiation have an advantage as they can cover a larger volume of ice as when you heat ice by conduction thus ice will melt faster in an oven than when you put it over flame.

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