To understand this phenomenon we would have to understand how water boils in ordinary pans on the stove. When you heat water in a pan over a stove its temperature slowly rises.
At near its boiling point small bubbles start forming in little cracks in the base of the pan. Now all pans contain certain imperfections. These imperfections contain some air. This air escapes at about 30 degrees less than the boiling point.
At about the boiling point these imperfections become the place where water vapour bubbles form, rise up and escape into the air. These bubbles carry away excess heat.
When you heat water in a glass or a smoother surface these are less chances of imperfections and bubbles take longer to form.
So these imperfections act as seeds around which state change first occurs. THis is similar to the condensation. Condensation usually starts around some seeds. This is why sometimes clouds are seeded to enable rain.
Now what happens when you heat water in a microwave? Usually you use ceramic bowls and these are pretty smooth. The water is heated from all sides. IN the pan over stove the water is heated from the bottom which is near a surface. In a microwave water is heated evenly. The water on the surface or in the middle does not touch any surface and any imperfection. So sometimes water may heat up beyond its boiling point without bubble formation. This is called superheated water. When you take this water out of the microwave it moves and would suddenly boil and explode. THis is because it suddeny is disturbed and forms bubbles all over its body. Or if you put something like a fork inside it, the water meets the imprefections in the fork and explodes. This can be pretty dangerous.
So superheated water is water which has exceeded its boiling point but does not have bubbles that carry away excess heat. It is on the edge of boiling. A small push would trigger boiling in the whole body in a very short time producing an explosion.
Note that this does not always happen. SOmetimes water boils in a microwave quite nicely. It all depends on the kind of vessel you use for heating water.
Technology may produce fancy things but nature always shows you that it is supreme. Take care with your microwave.
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