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Please Pass the Science
by dr. scott berk

Simplistic, misleading, and somewhat condescending. . . but never boring!


I recently got the following "Alert" from the safety guy where I work. Now, I admit, it's been a while since I took a physics class, but this has GOT to violate at least a few laws, right? Can this really happen?



I feel that the following is information that any one who uses a microwave oven to heat water should be made aware of. About five days ago my 26-year old son decided to have a cup of instant coffee. He took a cup of water and put it in the microwave to heat it up (something that he had done numerous times before). I am not sure how long he set the timer for but he told me he wanted to bring the water to a boil. When the timer shut the oven off, he removed the cup from the oven. As he looked into the cup he noted that the water was not boiling but instantly the water in the cup "blew up" into his face. The cup remained intact until he threw it out of his hand but all the water had flew out into his face due to the buildup of energy. His whole face is blistered and he has 1st and 2nd degree burns to his face which may leave scarring. He also may have lost partial sight in his left eye.

While at the hospital, the doctor who was attending to him stated that this a fairly common occurrence and water (alone) should never be heated in a microwave oven. If water is heated in this manner, something should be placed in the cup to diffuse the energy such as a wooden stir stick, tea bag, etc. It is however a much safer choice to boil the water in a tea kettle. Please pass this information on to friends and family.

When I first read this, I thought it had "Urban Legend Chain Letter" written all over it, and I'm still pretty sure that's what it is. However, it is theoretically possible for this to occur through a process called superheating, and it violates no known laws of physics. When I scanned the internet, I found a few paragraphs on the subject written by Professor Louis A. Bloomfield, author of the book, "The Physics of Everyday Things". His explanation agrees completely with what I was thinking, so I included it below. These injuries appear to be quite rare, but if you're still very concerned, just throw a grain of sand in with the water to provide a site of nucleation. Chemists do this all the time when boiling things.

--from http://rabi.phys.virginia.edu/HTW//microwave_ovens.html

There is a story circulating by email about a 26 year old man who heated a cup of water in a microwave oven and had it "explode in his face" when he took it out. He suffered serious burns as a result. Is this possible and, if so, how did it happen? -- JJ, Kirksville, Missouri

Yes, this sort of accident can happen. The water superheated and then boiled violently when disturbed. Here's how it works:

Water can always evaporate into dry air, but it normally only does so at its surface. When water molecules leave the surface faster than they return, the quantity of liquid water gradually diminishes. That's ordinary evaporation. However, when water is heated to its boiling temperature, it can begin to evaporate not only from its surface, but also from within. If a steam bubble forms inside the hot water, water molecules can evaporate into that steam bubble and make it grow larger and larger. The high temperature is necessary because the pressure inside the bubble depends on the temperature. At low temperature, the bubble pressure is too low and the surrounding atmospheric pressure smashes it. That's why boiling only occurs at or above water's boiling temperature. Since pressure is involved, boiling temperature depends on air pressure. At high altitude, boiling occurs at lower temperature than at sea level.

But pay attention to the phrase "If a steam bubble forms" in the previous paragraph. That's easier said than done. Forming the initial steam bubble into which water molecules can evaporate is a process known as "nucleation." It requires a good number of water molecules to spontaneously and simultaneously break apart from one another to form a gas. That's a rare event. Even in a cup of water at several degrees above the boiling temperature, you might have to wait minutes before such a rare event occurred. In reality, it usually occurs at a defect in the cup or an impurity in the water--anything that can help those first few water molecules form the seed bubble. When you heat water on the stove, the hot spots at the bottom of the pot or defects in the pot bottom usually assist nucleation so that boiling occurs soon after the boiling temperature is reached. But when you heat pure water in a smooth cup using a microwave oven, there is virtually nothing present to help nucleation occur. The water can heat right past its boiling temperature without boiling. The water then superheats--its temperature rising above its boiling temperature. When you shake the cup or sprinkle something like sugar or salt into it, you initiate nucleation and the water then boils violently.

Fortunately, serious microwave superheating accidents are unusual--this is the first injury I've ever heard about. You could minimize the chance of this sort of problem by deliberately nucleating boiling before removing the cup from the microwave. Inserting almost any food into the water should trigger boiling in superheated water. A pinch of sugar will do the trick, something I've often noticed when I heat tea in the microwave.

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