Microwaves are awesome technology. They have allowed people to heat food, kill bacteria (in the lab), and recently they have been used as a tool for curing malaria. The availability of the microwave simple experiments can be performed. Experiments like understanding plasma, lightning balls and using it to measure the speed of light. Superheating is an interesting one related to surface tension of water.
Usually when you observe water boiling in a pot on the stove you will see that the water bubbles go from somewhere in the pot bursting at the surface. For a vapor bubble to expand though the temperature must be high enough to exceed the atmospheric pressure. Below this temperature you will not see the bubbles forming. An exception comes with superheating. This is observed when you place something like water in a container with very smooth sides into the microwave. In this case the liquid is observd not to boil even thought he vapor pressure exceeds the ambient pressure. This is because the surface tension of the water suppresses the growth of the bubble.
Surface tension makes the bubble and ambient pressure act on these small bubbles by pushing down on them. If the bubbles are like balloons you can say the other two forces are like your hands pushing down on them. However, without any nucleation in the container like a scratch on the inside of a cup, adding a stirring device or some material to the water these bubbles will be maintained as microscopic bubbles.
If the cup is superheated by adding something to nucleate the water (like instant coffee) a large bubble forms with exploding hot vapor. Superheating occurs simply because a large bubble is easier to inflate than a small one (like blowing up a balloon the first part is harder when the balloon is more elastic). The excess pressure due to surface tension is inversely proportional to the diameter of the bubble. To overcome the ambient pressure and surface tension on the small bubbles it may require exceeding the boiling point by several degrees Celsius. Once the nucleation occurs the pressure due to the surface tension reduces so it expands explosively like in the clip below.
Extra Note: in the video they mention impurities. Things that induce nucleation of the water might be a better term.
Extra Note 2: I used to love cooking milk in the microwave then finding that awesome bit of skin on top of the milk. I saw on this site that this was answered. The skin is comprised of solid proteins that combine with the milk’s fat molecules, which begin to evaporate as the milk is heated. The proteins, casein and beta, clump together when the liquid reaches a temperature of around 45 to 50 Celsius. As the heating continues, the soft protein layer begins to dry out, which is why the milk forms a skin on the liquid’s surface. This layer of skin forms a hard barrier, causing steam to build up beneath it and increase the liquid’s temperature. When left alone, this often causes the milk to boil over.
Mythbusters are cool but sometimes they burst myths that are self-bursting (just with a pen and a paper). Even worse is when they state false ideas. The worst case I saw it right now in this video that our beloved webmaster posted. By looking at this video you get the impression that it is very rare that tap water will explode after microwave superheating. Well, IT IS NOT. It is very simple to test (at least with the tap water in west Helsinki) and after two really impressive explosions, and the correspondent expenses in kitchen towels to dry your microwave, now I do not dare to heat any water at my microwave. Not even for mild heating. I hope that kids watching this program will not try to reproduce the experiment because they could suffer very severe burning. But if the curiosity is impossible to control for you, you can try to keep a glass of water for long enough inside your microwave. After few attempts you will hear the bum there inside.
ReplyDeleteRoberto
Interesting comment and a word of caution to people trying this at home. Don't do it! In the video they are using water from the US which is probably not so pure to begin with and possibly hard water so the surface tension may vary. If you use water from Finland (land of 1000 lakes) it has relatively softer water and possibly would more likely cause superheating in say a very smooth surface cup from a company for example Ariba, Merrimekko or Italla.
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