Why Doesn’t Ice Melt in the Microwave?

(Last Updated On: November 16, 2021)

If you haven’t heard of this food tip before, maybe your life is about to get a lot cooler. Place an ice cube on something you’re reheating in the microwave. Your food will be steamed but the ice cube won’t actually melt. But aren’t microwaves supposed to heat things up? So then doesn’t that make ice melt? Why doesn’t ice melt in the microwave?

Also don’t start putting weird things into the microwave to see what will happen.

How Does a Microwave Work, Then?

Microwaves heat stuff up with–you probably guessed it if you’re checked in–microwave radiation. Which sounds super scary at first, because people don’t really know what radiation is. When you think of “radiation,” you probably think of the kind that damages your DNA and gives you cancer (as well as weird mutations). While you absolutely should not stick your hand in the microwave, microwave radiation is not the same as what you might be thinking of at first blush. The radiation that destroys your DNA is called ionizing radiation. This is some high power stuff, like gamma rays, x-rays, and the higher boundary of ultraviolet rays. Ionizing radiation is powerful enough to basically punch through your DNA like a bullet–photons and other subatomic particles classified as ionizing radiation just move that fast. Radioactive material (like the stuff that makes Geiger counters tick) are radioactive because they give off ionizing radiation. 

On the electromagnetic spectrum (in order of ascending wavelength) we go from radio waves, to microwaves, to infrared, visible light, UV rays, x-rays, and gamma rays. But how does microwave radiation actually heat up your food?

Why Don’t Microwaves Cook Evenly?

Well when a microwave spits its radiation at your food, it’s exciting the molecules a bit as the energy is transferred to them. This is just how heat works. When something is hot, the molecules that make up that thing are just vibrating a lot faster (and a lot more). The relative distance between the molecules is greater than if they were a solid.

Think about it with water. When water is frozen, the molecules are moving less than if it were steam. They’re content to be still–frozen, if you will. But once an ice cube is heated up, the molecules vibrate and eventually the ice turns into liquid water, because the molecules are moving. Once you boil it and turn to steam, the molecules are moving so much they can go wherever they want.

This same thing applies to your food. But there’s a problem with microwaves. They don’t always penetrate very far, and don’t always make it to the center of your bowl. That’s why you can have a boiling lava hot pocket that’s also ice cold on the inside. Hotter parts of the food have to pass on their heat to the cooler parts–which is why it’s common to let some frozen foods sit for a bit after microwaving. 

Water Is Weird

If you’ve read some of the other more science-adjacent posts here (or you’ve taken high school chemistry), you know water is kind of weird in the way its molecules behave. 

So ice. For starters, it actually takes a lot of energy to melt ice because of how water molecules bond with each other. Water molecules move into a pretty orderly lattice when frozen, and further they are held together by hydrogen bonds, some of the strongest intermolecular bonds out there. Ice also doesn’t absorb microwave energy as easily as water. Combining these two effects means ice will more quickly create steam than it does a pool of liquid water. 

Water accepts microwaves better because water molecules are polar. This means they have a positively and negatively charged end, and microwaves exploit this to make them rotate around faster (and thus heat up). When water molecules are frozen into an ice cube, the hydrogen bonds we mentioned earlier make it harder for the molecules to rotate as quickly (and thus heat up). 

Now you know these ice sculptures won’t have as bad a time in the microwave as you first thought.

About the Author:

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Kyler is a content writer at Sporcle living in Seattle, and is currently studying at the University of Washington School of Law. He's been writing for Sporcle since 2019; sometimes the blog is an excellent platform to answer random personal questions he has about the world. Most of his free time is spent drinking black coffee like water.