Can you melt ice in a microwave oven?

[Alex Franke]

Now that jonmulzer got me thinking about physics again, I was sitting here looking at the ice in my glass of H2O -- thinking about how the water expands when it's frozen because all the water molecules "lock up" into a larger structure.

...And then I thought about how I *think* a microwave oven cooks... by exciting water molecules... and then I wondered, "Well, if the water molecules are all locked up in ice, then how can they be excited?"

So once my glass is empty and I finish some code I'm working on, I'm going to go down stairs and see what happens when I try to microwave an ice cube.

Any thoughts on how it will work? My guess is that it probably won't work very well....

[LCHIEN]

i think it will work quite well.
Ice are just water molecules (not water locked up in ice) at low excitation.
Water molecules resonate at microwave frequenices, the water molecules will get all excited and turn to water and steam (higher excitation states of water than ice).

[conwaygolfer]

microwaved ice

The kitchen is usually off limits to me. Just like I "own" the garage/workshop. But since the wife is not home, I decided to try your experiment. I put 2 icecubes on a paper plate and set the microwave for one minute. At 30 seconds - nothing. At one minute, the plate was hot and the ice had just begun to melt. I decided to stop at that and wipe off the plate. Did not want to leave any evidence. Wife could be home any minute. I only do the repairs and maintenance - she does all the cooking!!!

[Alex Franke]

Okay, here's what I did.

I had to bring a power tool into it somehow, so I got a little plastic cup from a fruit snack and drilled a hole in the bottom so the melted water could drain away. My thought was that any water would heat up and I didn't want that affecting the ice.



I took out two ice cubes. One in a bowl on the counter, and the other as shown. 30 seconds on high and there was no sign of any melting, but the (microwave-safe) plate was getting warm. Another 30 seconds and still no sign of melting.

That's when I put the other cup with some cold tap water in microwave with it. 30 more seconds and there was one drop of water that came off the ice but hadn't even fallen through the hole. The other cup of water was already hot. and the interior of the oven was getting hotter.

30 more seconds and the water was too hot to touch (the picture above), and the ice cube had melted only a bit more, and only pitted at the bottom -- where it was in contact with the water. The rest was wet but not significantly melted. By that time the inside of the oven was very warm -- so was the plate and even the plastic cup that the ice was on.

At that time the ice cube on the counter was a little glassy, too. But not quite as glassy as the one in the microwave.

So I'd have to say that microwave ovens aren't very good at melting ice... or maybe my experiment was faulty. Dunno. Need to talk to a physicist again, I guess. :lol:

[Kristofor]

You're forgetting about the heat of fusion, that's a lot of extra energy...

To raise a kilogram of water from 10 degrees to 30 degrees takes 20 kilocalories.

To raise a kilogram of water from 0 degrees to 20 degrees (same temperature change) takes 100 kilocalories.

This is the same reason (well, heat of vaporization rather than heat of fusion) why it's fairly quick to get your water boiling in the microwave, but takes a long time for it to all boil off, that extra phase change requires lots of extra energy.

The microwave excites the water molecules because the molecule itself is polar, not because they are free to move around in a liquid.

[Alex Franke]

To raise a kilogram of water from 0 degrees to 20 degrees (same temperature change) takes 100 kilocalories.
. . . .
The microwave excites the water molecules because the molecule itself is polar, not because they are free to move around in a liquid.

But microwave ovens don't cook with heat, right? It's just the heat created when the waves excite the water molecules. So it seems to me that it would be more difficult to excite a molecule that's nicely aligned into a crystalline structure than one that's not -- if for no other reason than you have to "break" that structure. Doesn't it arrive at that structure because the molecules fit together better that way? ...that they're naturally aligned and held into that structure?

Or wait a minute... if ice is held together at a molecular level like that, then that would imply that it would take less energy to bring ice from -15 to -5 than it would from -5 to 5. Now that's doesn't make a heck of a lot of sense... now I'm totally confused.

Edit: By that last part, I mean it implies that it would take more energy (-5 to 5) than would otherwise be required if the water were in the same state -- even considering the heat of fusion.

[LCHIEN]

kristofoer is right. If you remember your high school chemistry or natural science there's a heat of conversion. When you heat ice you remove energy and the temperture drops from below freezing to freezing gradually and then stays at freezing until the heat of conversion is totally removed. The it proceeds to increase temperature again.

Your cube was colder than freezing. When you heated it, the mircowaves heat it evenly throughout and it stays frozen until the whole cube reaches freezing then it melts.

When you heat a cube in a pot or a hot surface, the water at the point of contact melts rapidly but not the cube above... this is due to the temperature gradient.

That's why microwaves are good for reheating, they warm the whole thing evenly.

Alex, it always takes 1 calorie of energy to raise one gram of water 1 degree C at 1 atmosphere of pressure and at any temperature. The exceptions are, to go from 0 to +1 C (actually that just to go past 0) and to go from 100 to 101°C (actually its just to go past 100). Each of those steps takes a very specific amount of energy to change phase. 80 calories and 540 calories, respectively.

So since your ice cubes are originally at probably -5C and you want to heat them to liquid water at Room temperature (25C), it would take 85 units of energy just to bring them to melting point, then another 75 to bring it to room temperature. It would seem like it was ice for a very long time.

[gerti]

If one were to put the microwave in the freezer (but don't do that), freeze it, add ice and turn it one, the ice would not melt. At least that is how I learned it.

[RodKirby]

Why do I read these posts

[LCHIEN]

Why do I read these posts

you don't have to . you're excused.

BTW, do microwaves make things colder in Australia?

[leehljp]

Why do I read these posts

'Cause you want something to do until that Vespa arrives!

[cabinetman]

I tried the test. I placed an ice cube on a paper towel on a microwave safe plate.

Set the temp for "high" and the time for 30 seconds. After the dinger dinged, I could tell the molecules in the ice cube got excited. The surface of the ice cube got very glossy and wet.
.

[RodKirby]

'Cause you want something to do until that Vespa arrives!

Spot on, Hank

Learner's permit tomorrow, Vespa delivery tomorrow night or Friday morning

The bad news - rain predicted for the next 4 days
(Be a while before I let it, or me, get wet )

[cgallery]

If microwaves can't melt ice because the water molecules are "locked," then how do they cook frozen dinners?

[docrowan]

The principal that it takes a very large amount of extra energy to make a phase change (either from solid to liquid or liquid to gas) is used extensively in your air conditioner, refrigerator, and heat pump. If you have a swamp cooler or evaporative cooler in your shop, this is the principal it uses also.

I would guess that most frozen foods have a lower freeze point than pure water because of the salt and other natural chemicals in them. The freeze point of a frozen burrito is might be 25 degrees F instead of 32 degrees F. If your freezer is set to 20 degrees F, then the microwave does not have to put in as much energy to get the burrito to it's melting point. At that point, all the energy goes in to making the phase change. Also, there's not as much water in most foods than in an ice cube. Ever tried thawing out chicken stock? It takes forever because it's virtually all water. I'll bet it takes less time than an ice cube because of the salt in it, though.