Here's what happens. You heat a cup of water using a microwave oven. When you remove the cup from the microwave, the water is very hot, but does not seem to be boiling. But as soon as you add a spoonful of instant coffee powder, the liquid immediately seems to begin boiling again, and may even overflow the cup.

We investigated this phenomenon by first attempting to make it happen.
The coffee was heated in a microwave oven long enough to reach a boil, although it never actually bubbled.
The cup was then quickly removed,
and powdered instant coffee was added.
As soon as the powder touched the water, it seemed to start boiling vigorously, and coffee spilled out onto the counter.

What seemed to happen was that the water started to boil when the powder was added. Could the powder be lowering the boiling point of the water? If so, the water temperature would instantly be above the boiling point, and the water would begin to boil.

This would explain the phenomenon, but it's not a correct explanation.

Here's what really happens.

When you boil water on a stove, you will see bubbles streaming from various places on the bottom of the pot. Some parts of the water are reaching the boiling point before others; tiny scratches inside the pot give bubbles of gas released from the water a place to get started ( process called 'nucleation'). Because the heat is coming from the bottom, convection currents keep the water constantly in motion, so that all of it eventually gets a chance to boil.

In a microwave, however, the water stays relatively still, since it is heated everywhere at once. If you heat water in a smooth cup, there may be no places for bubbles to attach themselves. All of the water gets hotter and hotter, without boiling. In fact, the water temperature can actually get higher than the boiling point without the water boiling ... this is called superheating. As soon as you add instant coffee to this superheated water, you suddenly provide thousands of particles to which bubbles can attach, and instantly all of the water begins boiling.

In fact, if your cup of water has been superheated without boiling, any disturbance, even putting the cup down on the counter, may be enough to trigger instant boiling. The phenomenon can be quite violent; people have been injured by the explosive action of water which would not boil in the microwave, but which did boil when removed and disturbed. Flash boiling can empty the cup, spraying boiling water all over the room, the microwave, or you.

 This phenomenon is one of the most common dangers of using a microwave oven. Glass or smooth ceramic containers are the most likely to superheat water because their surfaces are perfectly smooth. However, defects in the surface, dirt, or other impurities usually help the water boil before it becomes dangerously superheated.

You can reduce your chances of being injured by superheated water by not overcooking water (or any other liquid) in the microwave oven in the first place. You should also wait about a minute or so before removing a cup from the microwave if there is any possibility that you have superheated it. Also, be careful when you first introduce powders, utensils, teabags, or otherwise disturb very hot liquid that has been cooked in a microwave oven. Keep the water away from your face and body until you're sure it's safe, and don't ever hold your face over the top of the container.
It's better to have the liquid boil violently while it's inside the microwave oven than when it's outside on your counter where it can splatter all over you. If you have more than one cup of potentially superheated water, wait several minutes for it to cool down.


Avoiding Microwave Hazards:
  • Put your teabag or instant coffee in the water before heating.
  • Don't put twist ties, or anything else with metal in it, in the microwave.
  • Don't cook things too long.
  • Stir food midway through cooking to distribute the heat. A microwave heats food from the inside out, and only the water in the food gets directly heated. So the container may be cool, but parts of the food may be beyond boiling temperature.
  • Allow the heated material to cool before touching it. Wait a minute or two for every cup of water you've got.
  • Sample food before giving it to a child. Test the temperature in several places.
  • Do not let plastic wrap touch foods during microwaving. Loosen at one corner to allow steam to escape.
  • Lift a lid or plastic wrap away from your face.
  • Keep children away from microwaves when removing hot liquid or food.
  • Don't microwave in styrofoam containers.
  • Don't dry herbs or your laundry in the microwave ... they can ignite.
  • Use a turntable inside the microwave to keep the liquid moving and to ensure even heating.



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