omon

hopefully

__________________
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote!" B. Franklin

citanon

That's what I meant by heating coils.

S65

Hah! Finally all these chemistry classes I'm taking can go to good use!

So here's how microwaves work: The frequency they operate at, ~2.4ghz, is the frequency at which water molecules start to vibrate. When you increase the average kinetic energy of a system, say the molecules in a cup of water, you get temperature increase -- that is basically what temperature is. So in your microwave you have essentially superheated water so that it goes above its boiling point at 1 atmosphere pressure. Now if you've ever been in chemistry lab you might notice that in boiling things you often place boiling chips into the flask or whatever you're apparatus to distill/heat liquids is in. This is because the boiling chips are rocks with many jagged locations on them that are called nucleation sites; these allow for the phase change from liquid to gas in the high temperature liquid when it gets to the boiling temperature. Like what everyone else said, in smooth containers where you have few imperfections, you can see this superheating in effect.

What's cool is that the opposite is also true. You can cool liquids below their freezing temperature if the liquid is very pure, if the container has no nucleation sites so that crystals cannot form. Then when you disturb it the whole thing freezes up. Same reasoning there too.

Superheated and supercooled, are keywords if you want to find out more.

dalem

But was I right about the latent heat or not?

I need to know!

-dale

S65

@Dale: Jeez what a quick response!

I hadn't really heard of latent heat before, but looking up the definition quickly seems to give consensus. Latent heat is the energy absorbed or released from the change in a substance's phase.

In the terms I'm more familiar with, that would be the heat of fusion or the heat of vaporization -- phase change energies typically measured in KJ/mol or other units. So latent heat is the energy absorbed from changing say, 1g of liquid water into 1g of steam. Or the release of energy from making 1g of liquid water into 1g of ice. All of this while the temperature remained constant.

So the way you used latent heat would be incorrect because superheating doesn't change the actual heat of vaporization of water, but I think your idea is correct. As for the part about microwave vs. stovetop differences, I would think it's not a matter of choosing one method of delivering energy over the other as much as it is the other operative details. On the stove you may work with wares which have nucleation sites for steam formation. If you have a large quantity of water, then the movement of water from hot to cold up and down the pot or whatever would provide enough disturbance to allow for nucleation I would think. However, in microwaves where superheating occurs, the absence of nucleation sites and disturbance of the vessel in which the water is being heated does have the conditions for superheating. A look online says that microwave ovens with turntables do provide enough disturbance to cut down on superheating.

Taboo

As S65 pointed out, a microwave doesn't just add energy in the form of heat. It emits microwave radiation which is a rather weak electromagnetic wave, but still stronger than a regular heat source. So when you blast a cup of water with high amounts of energy, most of the energy doesn't go to changing water into steam. How boiling works is that at the boiling point, a few water molecules leaves the volume of water as steam every few seconds as long as energy continues to slowly flow into the system, and every time a heated molecule leaves, the system cools (sidenote, that's how sweat keeps your body cool). When everything is superheated at once, there is no possible way for an entire volume of water to evaporate to steam immediately.

Water's cohesive properties keep it within the container, and there's also high surface tension at the top keeping molecules from escaping. It remains contained into the liquid almost like a capacitor, full of charged water molecules. And S65 described what happens when you disturb the water. All the energy is simultaneously released when you break the surface tension. Imagine a mousetrap. The spring is full of energy, ready to snap, but doesn't until a mouse disturbs it.

dalem

Ahhh, I see. I had the right general idea but the wrong terminology. My ego can accept that.

Thanks guys!

-dale