Heating Ice in Microwaves: How Does It Work?

[Moe]

Hi everybody,

I recently took a piece of meat from the freezer and put it in the microwave in order to defrost it. Then I realized that this actually shouldn't work. I don't see how you can heat ice in a microwave. So how does this work? I've tried it on ice cream (don't repeat that experiment, it made a huge mess) and the ice cream melted. I even tried it with an ice cube, and sure enough, it turned into water. Can someone explain to me how and why that works?

Cheers,
Moe

 

[arivero]

The microwave works at molecular level, dipole moment of the water molecule. So its ability to deposit energy does not depend of the state, solid or liquid. Of course for solid water some of the energy will be used against the crystal structure.

It should fail with other substances having different absortion of the microwave frequency. For instance one could try to put at the same time a glass of water and same quantity of alcohol, and see what happens. Or a glass of water and a box of glycerine, perhaps. I would not risk to put the alcohol alone, it could be as bad as running the microwave empty.

 

[Tom McCurdy]

what is fun

What is really cool is if you put a cdr disk in the mircowave for a few seconds... it will be electrofiying experience.

 

[Moe]

arivero: The dipoles in an ice crystal can't turn into the EM field like water dipoles. Ice has a permittivity of about 4, water around 80. You shouldn't be able to heat ice in a microwave. I recall one of my first physics professors who made this nifty demonstration. He had frozen raspberries in ice cubes and put them in a microwave. Afterwards, the raspberries were hot, but the ice cube was still intact.

 

[arivero]

Moe, I will try at home this night. You are telling that ice should let to pass the EM field without absorbing it, right? Perhaps it depends of orientation... I do not know how birrefringent is ice for the 2.4 GHz frequency, but it even could be.

Besides, ice cream is a sort of gel, so perhaps the microcrystal structure has not the same solid phase than usual ice.

hmm.

 

[arivero]

Another possibility is that your teacher tricked you:
http://www.zyra.org.uk/microw.htm proposes

Carbon dioxide has no dipole moment except for a tiny dipole during a bending vibrational state and an asymmetric stretching (but not symmetric stretching) vibrational state. Again, vibrational states are in the infrared spectrum and therefore very reduced at low temperatures. Therefore one cannot expect a microwave oven to heat dry ice.

but ok, you are right about permitivities. Fused water on the surface, of course, can heat the ice.

 

[LURCH]

but ok, you are right about permitivities. Fused water on the surface, of course, can heat the ice.

This would be my guess. The instant you remove an ice cube from the freezer, some liquid water begins to form around the surface. I think this liquid water began heating as soon as the microwave was turned around, converting more of the ice cube to liquid, promoting more heating, etc.

 

[Moe]

Heh. I just found out that some microwaves actually come with a heating stage. When you set them to defrost, it uses the conventional heating as well as the microwaves. The heater melts some of the ice, and the water is then heated by the microwaves. It will then of course melt more ice, and so on.
The water on the surface is a good idea. Impurities in the ice could be a reason as well.

 

[Nereid]

So if you're at Vostok (in Antarctica), and outside, with a microwave, and the air temp is -50C, putting snow into the (in thermal equilibrium, with power off) microwave, and turning it on, will result in ... nothing much at all happening??

 

[Integral]

I have tried to melt icecubes in a microwave. It was indeed a very slow process, only when a surface layer of water developed did the ice cube begin to melt. Clearly the microwave did not have the same effect on a ice cube then it did a equal mass of water.

 

[Entropy]

Yeah the mircowaves created by the magnetron only (well only suppose to) hit the water molecules. I learned that on the myth busters, funny show. Just don't heat up distiled water! It will explode!

 

[Moe]

Nereid: I dunno, if it was perfect snow without any impurities, and the microwaves wre not heating the air inside the microwave... not much would happen. Except for the fact that all the energy has to go somewhere. What happens if you turn a microwave on without anything in it?

 

[Gara]

wouldnt the dish get hot or the material on the inside?

 

[arivero]

Let me confirm Integral's results. I hang the ice in a small net of wires (a support to grill pizzas) so the produced water is removed by gravity. Put also a dish to get the water. The ice melted slowly.compared to similar blocks floating in the dish.

For a second experience, I put in the support a cilyndrical sheet of ice, about 10-30 cubic centimetres, and a glass of water, about 75-125 cubic centimetres. I put 800 watts in the microwave. When the ice finally melt completely, the water in the glass was already boiling.

I am in doubt about if the lack of absorption is due to coherence -ice being an ordered crystal- or to solidness. Is it possible to manufacture amorphous ice?

On a related theme, ice is a very interesting material. It is birrefringent, so it can generate circular polarisation from linear one. But the lambda/4 sheet is about a pair of meters! I have wondered sometimes if, in the frozen Earth hypothesis, it could be of some influence for the origin of chirality in living forms.

 

[tavi_boada]

How 'bout this simple explanation. Microwaves heat up the water vapor in the oven, which in turn yields the energy to the ice by direct contact and melts it!

 

[arivero]

perhaps a bit more elaborate. Water vapor adheres to the ice changing to liquid water. Just as it happens with a very cold glass or bottle out from the fridge. Of course this joins to water fusing from the ice cap.

Distinctive experiment could be to put the microwave inside the freezer, so only the water from air is available... but you should need a transparent or doorless fridge sort of the one in stores to sell cold beer bootles or, better, ice cream boxes. Also a blank experiment with a glass of water, so one gets sure that microwave power is enough to overweight freezer power. Sorry I have not access to such a freezer.

 

[RAD4921]

The Big Easy

Ariver is correct but a simpler expaination as to how microwave ovens work (also called radar ranges) is that the mIcrowaves (electromagnetic waves) cause water molecules to rub together creating friction. Substances such as ceramics and glass that contain little or no water do not heat as well in a microwave ovens.

DISTILLED WATER WILL NOT EXPLODE IN A MICROWAVE OVEN!

 

[Entropy]

No not in a microwave. But if distiled water is brought to the boiling point of water and you put something in it like a spoon or sugar or whatever, it violently start to boil, or in other words explode.

 

[flash33773]

From my understanding, since ice is crystalized there is no way that microwaves can move the molecules (to create friction/heat) because they are "arranged". But if there was some free standing water on the ice then it could heat that up and cause the ice to melt from the heat of that. But if you moved a ice cube directly from the freezer to the microwave, it won't melt. And ice cream will melt because it is technically not frozen.

 

[Moe]

The "rubbing" explanation is not that good. Heat is molecule movement, it is not generated by friction between those molecules. If it were, the molecules would slow down.

 

[krab]

Ariver is correct but a simpler expaination as to how microwave ovens work (also called radar ranges) is that the mIcrowaves (electromagnetic waves) cause water molecules to rub together creating friction.

That may sound like a simpler explanation, but it is incorrect in the sense of leading one further away from understanding the phenomenon. Friction is a macroscopic phenomenon that can be understood at the molecular level as: bonds form between atoms and are broken, resulting in thermal excitation. That's not what's heating your food. Instead, the microwaves cause the molecules to jiggle directly, and amount of jiggle = temperature.

 

[Integral]

It has been my understanding that microwaves are of appropriate wavelength to interact with the atomic bonds of a water molecule, thus it induces vibrations within a water molecule. Upon collision with other water molecules this vibrational energy is transferred to kinetic energy of motion. Thus increasing the temperature of water.

I can see 3 main factors which would inhibit the microwave heating of ice.

1. Bond lengths in ice are different from those in water, thus there is less interaction with the microwaves.
2. The crystal structure of ice inhibits the vibration of individual molecules, their neighbor must also move, more mass to move means less movement.
3. Different regions of the ice cube will see different phases of the microwave so the attempt vibrations of one region will oppose the attempted vibration of other regions of the ice.