Physics 2000 Einstein's Legacy Microwave Ovens

Water Vapor and Ice

What about microwaves and water vapor?

In both liquid and gaseous water the molecules are free to move around. However, the density of a gas is so much lower than that of a liquid that not much heating can occur. You need neighbors to have damping.

Oh, that's why the air inside the microwave oven doesn't get hot!

So ice should heat quickly because solids have high density.

True, except for one thing: the crystalline structure of solid water locks the molecules in place so they cannot change orientation. No motion means . . .

No friction means no heating. But I can microwave a frozen burrito. What gives?

Well, often the microwave oven doesn't heat frozen food well (as there can be frozen spots inside.) You have to have liquid water for heating to occur.Eventually the outside of the food melts and you can use that water to heat up the rest of the frozen food. The defrost button is specifically designed for this purpose.

I always thought that was just a low power setting.

Not quite. It's not just low power. It pulses the microwaves on and off. There is usually some liquid water at the surface of the ice. The pulse of microwaves heats this water, allowing time for the heat to spread throughout before more heating is done.



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