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Is it Safe to Heat Water in a Microwave Oven?

This week's experiment comes from all of the e-mails that I have been receiving about the danger of heating water in a microwave oven. Although it is very uncommon, under certain conditions, you can superheat the water to a point where stirring or adding sugar can cause it to almost explode into steam. To investigate this, you will need:

  • a clean glass
  • some sugar
  • a fresh container of carbonated soda
  • water

It was a challenge to come up with a way to demonstrate what is happening without risking anyone getting burned. I had to settle for a demonstration that is close to what happens instead of recreating the actual event. Fill the glass about half full of soda. Don't use any ice. Pour it gently, trying to get as little foam as possible. Let the glass sit for a moment to let all of the foam go away.

Once the foam is gone, add a pinch of sugar to the soda. Yes, I know that it already has PLENTY of sugar, but a little more won't hurt. Notice what happens. The soda foams and bubbles until the sugar dissolves. Sprinkle in some more sugar and it bubbles some more. When you are done adding sugar, you can drink the soda, so it does not go to waste.

Why did the sugar make the soda foam? Even though the soda is a liquid, it contains a gas called carbon dioxide. This gas is dissolved in the soda. While the soda is sealed in a container, the gas stays under pressure and stays dissolved. When you open the container, the gas begins to come out of the liquid and you get bubbles.

These bubbles need a place to start. One of the best places for the bubble to start is another bubble. Even a very tiny bubble can serve as a nucleus or starting point for the carbon dioxide to come out of the liquid. When you pour the soda into the glass, tiny rough spots on the glass trap tiny bubbles and help produce the foam. Sometimes you can see a string of bubbles rising from one of these rough spots. The grains of sugar do the same thing when you sprinkle them in. So would grains of salt, pepper, sand, etc.

What does all of this have to do with heating water in the microwave oven? When water boils, the bubbles also tend to grow around a "nucleation point" or a rough spot that traps tiny bubbles. Watch a pan of water as it begins to simmer and you will see that streams of bubbles seem to come from the same spots. If you looked at one of these spots under magnification, you would probably find a rough place and some trapped air bubbles.

If you heat some pure water in a very smooth container, it is possible that the water can get hot enough to boil, but not have any tiny bubbles to act as a nucleus. In that case, there would be a point were the water was hot enough to boil, but did not. If at that point, you sprinkled sugar into the cup, added some bullion, or stirred the water, it could cause a sudden rush of boiling foam, which could give a nasty burn. While this is not at all common, it is something that could happen.

To prevent this, keep your face away from containers of water as you remove them from the microwave. Wait a few seconds before you add bullion or other substances to the water. Don't heat water for long periods of time. Consult the instructions for boiling water that came with your microwave (You do still have the manual don't you?). Avoid boiling water in glass containers, as they tend to be smoother. Use a ceramic coffee cup or some other container. Some extreme sites even suggest using some sandpaper to make a small rough spot on the inside of coffee cups, etc., before you heat water in them, to produce nucleation sites. I don't know that I would take it that far.

I guess it all boils down to the fact that this can happen, but it is not common. It seems to happen only when pure water is heated for a long time in a very clean, smooth container. Even then, it must be stirred or have something added to it while it is still superheated in order to flash into steam. Be careful, use common sense and you should not have to worry. Now you don't have to hide in the corner every time someone heats a cup of coffee. Isn't that nice?