Today at work the kettle broke. Being the caffeine addicts that we are, going without our coffee was simply not an option. My friend was surprised when I was tentative about boiling the water in the microwave, so she got an impromptu lesson in super heating and nucleation :-)
When you boil water in a saucepan, you may have noticed that you initially get small bubbles forming at seemingly random points around the pan. These bubbles are forming at small defects in the pan. These tiny defects hold microscopic bubbles that make it slightly easier for the new bubbles to form. This process of forming new bubbles is called nucleation and the small defects are called nucleation sites.
What happens if there are no nucleation sites? If you have a nice new clean mug/glass that you are using in the microwave, then this might well be the case. The short answer is that bubbles don't form. This means that the microwave is continuing to add energy to the water, heating it up, but the water isn't boiling. You end up with superheated water! The water is more than 100°C but still in liquid form in your mug.
Now you take your mug out of the microwave and add your coffee. The coffee granules create nucleation points and suddenly your water is boiling very rapidly!! The massive expansion of the water as it become steam causes the water to seemingly explode out of the mug and all over you (causing nasty scalds). This could even happen just by moving your mug of superheated water, so be careful!
If you are interested in a few numbers, there is a good article on this here:
http://www.phys.unsw.edu.au/~jw/superheating.html
They also have a video:
When you boil water in a saucepan, you may have noticed that you initially get small bubbles forming at seemingly random points around the pan. These bubbles are forming at small defects in the pan. These tiny defects hold microscopic bubbles that make it slightly easier for the new bubbles to form. This process of forming new bubbles is called nucleation and the small defects are called nucleation sites.
What happens if there are no nucleation sites? If you have a nice new clean mug/glass that you are using in the microwave, then this might well be the case. The short answer is that bubbles don't form. This means that the microwave is continuing to add energy to the water, heating it up, but the water isn't boiling. You end up with superheated water! The water is more than 100°C but still in liquid form in your mug.
Now you take your mug out of the microwave and add your coffee. The coffee granules create nucleation points and suddenly your water is boiling very rapidly!! The massive expansion of the water as it become steam causes the water to seemingly explode out of the mug and all over you (causing nasty scalds). This could even happen just by moving your mug of superheated water, so be careful!
If you are interested in a few numbers, there is a good article on this here:
http://www.phys.unsw.edu.au/~jw/superheating.html
They also have a video:
This is the mpeg format video taken straight from the above website (from the physics department at UNSW) - this video is theirs and all rights remain with them. They also have a quicktime format to be found at the above link. !
We did heat our water in the microwave and we didn't get hurt: our mugs are quite old and therefore have many hairline cracks and other such nucleation points and also we didn't quite heat the water to boiling (it only needed to be hot enough to dissolve the instant coffee).
I am please to report that by lunchtime we had a new kettle :-)
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