I have noticed that when I microwave an ice cube it appears to melt more slowly than I would expect. For example, an equal volume of water starting at 0 deg C would probably be at boiling point before an ice cube that was at -15 deg C had melted. I realize there is enthalpy of fusion to take into account in the melting process but I believe there is more to it than that.

As I understand it a microwave oven works by exciting the water molecules in whatever is being cooked and if memory serves the frequency used is one that causes rotation of the molecule. Since the ice cube is solid I'm assuming the molecules aren't free to rotate and therefore the microwaves have a much reduced effect. In fact I'm wondering if a perfect single crystal of water would respond at all to being microwaved. Does this sound right?

I've been trying to rack my brain for a way of testing this theory but I can't think of a way of getting an perfectly dry ice cube into a microwave to see if anything happens. Even a tiny amount of surface water, caused from interaction with a warm atmosphere, would encourage melting.

  • $\begingroup$ Just spit-balling here...I see at least two issues (a) the rotational modes are suppressed, but they will be coupled to phonon modes in the bulk so there is still a channel for energy to be transferred from the radiation field to the material, but (b) the modes will be shifted by virtue of the molecules being bound so you may be off-resonances. A detailed consideration is beyond me. $\endgroup$ – dmckee --- ex-moderator kitten Jul 23 '11 at 19:11
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    $\begingroup$ I don't think it's a resonance phenomenon at all. Microwaves would probably be in the same part of the spectrum as the rotational bands of water, but the discrete rotational bands you'd see in a gas are not observable in liquids or solids. I actually don't think there's any quantum mechanics required at all, except in the sense that quantum mechanics ultimately determines facts like the nonvanishing dipole moment of the water molecule. $\endgroup$ – user4552 Jul 23 '11 at 19:35
  • $\begingroup$ In the Question you said equal volume of ice and liquid water. But to equal volumes correspond different amounts of mass of H2O. $\endgroup$ – Helder Velez Jul 24 '11 at 8:22

I stumbled on this question rather late - and when the link to the image in @Georg's answer was no longer working I started a little digging of my own. I came upon the following plot (at http://www1.lsbu.ac.uk/water/microwave.html) which explains this very well:

enter image description here

It shows unambiguously that water has a strong absorption peak in the "low GHz" range (right around the microwave) while the absorption peak for solid ice happens at a much lower frequency - about 6 orders of magnitude lower.

The article goes on to explain this by stating that the dipole in the water molecule attempts to align with the changing electric field; when the phase difference of this alignment is at 90 degrees (resonance) the heat transfer is maximized. For liquid water you are near resonance - for ice, you are far away. Quoting from the page (I put key phrases in bold):

The water dipole attempts to continuously reorient in electromagnetic radiation's oscillating electric field (see external applet). Dependent on the frequency the dipole may move in time to the field, lag behind it or remain apparently unaffected. When the dipole lags behind the field then interactions between the dipole and the field leads to an energy loss by heating, the extent of which is dependent on the phase difference of these fields; heating being maximal twice each cycle. The ease of the movement depends on the viscosity and the mobility of the electron clouds. In water these, in turn, depend on the strength and extent of the hydrogen bonded network. In free liquid water this movement occurs at GHz frequencies (microwaves) whereas in more restricted 'bound' water it occurs at MHz frequencies (short radiowaves) and in ice at kHz frequencies (long radio waves).

Incidentally - and I admit, to my surprise - it seems that the resonance peak for liquid water shifts quite a bit with temperature; see this graph from the same source (I don't quite understand what the units are… but the general shape and direction with temperature are evident; note the 2.45 GHz line which corresponds to the typical frequency of the home microwave oven):

enter image description here

At 2.45 GHz, the dielectric absorption decreases as temperature goes up. This suggests that cold water heats more rapidly than hot water, but I haven't attempted to measure this myself. Might be a fun follow-up for somebody. I think that "microwave physics" is an underused topic for school science fair experiments…

  • $\begingroup$ The figure is somewhat unclear, but it illustrates that cold water heats up more slowly than hot water for 2.45 GHz microwaves. $\endgroup$ – KDN Oct 1 '15 at 18:54
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    $\begingroup$ @KDN - you are right; I corrected the last paragraph. $\endgroup$ – Floris Oct 1 '15 at 19:36
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    $\begingroup$ Sorry for making my comment; your initial interpretation was correct. I had seen the arrows as idicating the line at 0C, rather than illustrating the progression from 0 to 100. $\endgroup$ – KDN Oct 7 '15 at 15:18
  • $\begingroup$ @KDN = Ah yes... I accepted the edit except for the salt water bit (it might be true, but I have no data for that.) $\endgroup$ – Floris Oct 7 '15 at 15:36
  • $\begingroup$ The salt water bit comes from the same source all these images are from. $\endgroup$ – KDN Oct 7 '15 at 17:23

The unusual thing is the really high absorption of microwaves by bulk water, whereas the ice behaves more normal like most solids and liquids. In liquid water we have an effect of relaxation of orientational polarisation. The polarisation is achieved not by rotation (not possible in liquid water) but by shift of hydrogen atoms along the hydrogen bonds. This is a kind of Kohlrausch conduction mechanism. This process is extremely fast, so polarisation of water is one of the fastest processes in liquids. There is debate, whether tunneling plays a role to enhance the shift of the protons. The same mechanism is responsible for the extraordinary (about tenfold) mobilities of H+ Ions in water.

Here is a plot of Water spectrum in microwave domain. Note the incredible absorption maximum with k about 3 !


  • $\begingroup$ You refer to "orientational polarisation," but then you claim that rotation is "not possible in liquid water." A molecule can't change its orientation without rotating. I think what you really mean here is that (classically) rotation is damped in much less than a full rotational cycle, or (quantum-mechanically) there are no discrete rotational bands. These statements are completely different than saying that rotation is impossible in general. Rotation through a small angle is still rotation. The WP article on dielectric heating has a nice discussion of all this. $\endgroup$ – user4552 Jul 24 '11 at 16:43
  • $\begingroup$ No not at all. You want to correlate orientation to rotation. This is wrong. As I recommended already, read about ice structure, local order in liquid water, and on hydrogen bonds. You will have to look into physical Chemistry textbooks. This is outside the scope of most physics research and books. $\endgroup$ – Georg Jul 24 '11 at 19:02
  • $\begingroup$ @Georg - the link you gave does not work (any more?). Any chance you could update? This is a very interesting topic. $\endgroup$ – Floris Sep 18 '14 at 11:57
  • $\begingroup$ It seems that the link should be to page 184 of books.google.com/… $\endgroup$ – KDN Oct 7 '15 at 18:22

This is a nice application of $W=F\Delta x$ for mechanical work, or its rotational analogue $W=\tau\Delta\theta$. Molecules in a liquid are free to rotate, so $\Delta\theta$ can be large. If you think of the lattice in an ice crystal as being built out of individual water molecules, then $\Delta\theta$ is limited by the ability of the lattice to deform, and you get a much smaller amount of work done by the electric field.

If you try melting a stick of butter in the microwave, you'll see that it doesn't melt evenly. The melting starts in a certain spot, and then the spot spreads. The idea here is that once you get even a tiny bit of liquid, that liquid becomes efficient at absorbing energy, so the process snowballs. For this reason, I don't think you can test the theory by trying to use ice that has been thoroughly dried. As soon as the tiniest amount of liquid forms, it starts to grow.

Some possible experimental tests:

The explanation is independent of the detailed nature of the substance, so it should be true for all substances that the solid heats more slowly than the liquid. This seems to be verified by the fact that butter acts the same way as water.

An insulating liquid without any electric dipole moment should heat more slowly than one like water that has a dipole moment. I don't know of a safe, easy example, though.

[EDIT] Reading a little more on the web, it turns out that there are four qualitatively different effects by which microwaves can heat matter:

  1. dielectric heating -- This is the effect I described above.

  2. ionic conductivity

  3. electronic conductivity

  4. hysteresis

In most foods, 1 and 2 are of approximately equal strength. 3 can occur in soot particles formed when food burns. In pure water and ice, only 1 is significant.

  • $\begingroup$ ""Molecules in a liquid are free to rotate, "" This is wrong in 99 % of materials. (Rotation is possible when the molecules rotate in the solid as well eg Camphor and similar rather spherical molecules) For ice with the diamond structure and 4 hydrogen bonds per molecule it is totally wrong. The unusual thing is not why the uptake of energy is so low in ice, but why it so exceptionally high in liquid water. Re butter, the start is where some droplet of bulk water is in or at the butter. $\endgroup$ – Georg Jul 23 '11 at 19:38
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    $\begingroup$ @Georg: I'm happy to be corrected if I'm wrong, but I'm not able to follow the logic of your post. Could you explain at greater length? You quoted my statement about liquids, but all your own statements are about solids. $\endgroup$ – user4552 Jul 24 '11 at 2:35
  • $\begingroup$ That is some very lengthy thing . I' recommend to read about lattice of ice and then about structures in liquid water. The density anomality of water is another effect from that locally ordered structures. You mentioned that the rotational frequencies of water in gas phase are not found in liquid, another hint that rotation is not at work. (BTW even when rotation were at work, this would not explain why the absorption is that high.) $\endgroup$ – Georg Jul 24 '11 at 10:08
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    $\begingroup$ @Georg: "You mentioned that the rotational frequencies of water in gas phase are not found in liquid, another hint that rotation is not at work." No, that's incorrect. Discrete rotational bands are not observed in liquid. That doesn't mean that rotation doesn't exist. By analogy, we don't observe discrete energy levels in the emission spectrum of a tungsten lightbulb filament, but that doesn't mean that there are no possible electronic excitations in solid tungsten. It just means that they appear classical because of the large number of particles. $\endgroup$ – user4552 Jul 24 '11 at 16:19
  • $\begingroup$ If rotation in liquid water would "go on", one would expect two broad bands centered at the same frequencies as the rotational gas phase lines were. $\endgroup$ – Georg Jul 24 '11 at 18:57

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