How a microwave oven heats food, and why the middle stays cold
Why frozen food boils at the edge and stays icy in the microwave, and what defrost does
Put a frozen meal in the microwave at full power, and the edge can boil while the middle is still solid ice. Ice and liquid water do not take energy from the waves the same way. See why, and what the defrost button does about it.
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1
Miva puts her frozen soup in at full power for 5 minutes. Then she looks in. The soup at the edge boils, but in the middle there is still a block of ice.
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2
In ice, each water molecule holds on to its neighbours in a fixed pattern. The waves pull on them, but they can hardly turn. So the waves pass through ice and leave very little heat behind.
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3
At the edge, a little ice melts first. In liquid water, the molecules are free to turn, so they take the energy of the waves very well.
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4
From then on, the melted edge takes almost all the energy, and the ice in the middle gets almost none. The edge gets hotter and hotter until it boils.
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5
So Miva puts the bowl back in, and this time she presses the defrost button. On many microwaves, defrost does not make weaker waves. The magnetron switches fully on for a few seconds, then off, again and again.
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6
In each pause, the warm soup at the edge passes its heat into the ice, by conduction. The ice melts a little at a time, and the edge never gets the chance to boil.
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7
After dinner, Rono wants to try something. He lifts the turning glass plate out of the microwave and picks up a big bar of chocolate.
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Quick check
One chicken breast.
Show the answer
The thin end. It melts first, and then its liquid water takes most of the energy of the waves, while the thick end is still ice.
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Go further
More about the pauses.