A friend of mine recently sent me the following email, which I'm sure has spread all over the internet by now:
> > Microwaving Water!
> >
> > A 26-year old man decided to have a cup of 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 wanted to bring the water to a boil. When the timer shut the oven
> > off,
> > he removed the cup from the oven. As he lookIed into the cup, he
> > noted that
> > the water was not boiling, but suddenly 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 flown out into his face due to the build up
> > 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 is 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.,
> > (nothing metal).
> >
> > It is however a much safer choice to boil the water in a tea
> > kettle.
> >
> > General Electric's
> > Response:
> >
> > Thanks for contacting us, I will be happy to assist you. The e-mail
> > that you
> > received is correct. Microwaved water and other liquids do not
> > always
> > bubble when they reach the boiling point. They can actually get
> > superheated and not bubble at all. The superheated liquid will
> > bubble up
> > out of the cup when it is moved or when something like a spoon or
> > tea bag is
> > put into it.
> >
> > To prevent this from happening and causing injury, do not heat any
> > liquid
> > for more than two minutes per cup. After heating, let the cup stand
> > in the
> > microwave for thirty seconds! before moving it or adding anything
> > into it
> >
> > Here is what our local science teacher had to say on the matter:
> > 'Thanks for the microwave warning. I have
> > seen this happen before. It is caused by a phenomenon known as
> > super
> > heating. It can occur anytime water is heated and will particularly
> > occur
> > if the vessel that the water is heated in is new, or when heating a
> > small
> > amount of water (less than half a cup).
> >
> > What happens is that the water heats faster than the vapor bubbles
> > can form.
> > If the cup is very new then it is unlikely to have small surface
> > scratches inside it that provide a place for the bubbles to form. As
> > the
> > bubbles cannot form and release some of the heat that has built up,
> > the
> > liquid does not boil, and the liquid continues to heat up well past
> > its
> > boiling point.
> >
> > What then usually happens is that the liquid is bumped or jarred,
> > which is
> > just enough of a shock to cause the bubbles to rapidly form and
> > expel the
> > hot liquid. The rapid formation of bubbles is also why a carbonated
> > beverage
> > spews when opened after having been
> > shaken.'
> >
> > If you pass this on you could very well save someone from a lot of
> > pain and
> > suffering
I knew water could be supercooled, which is really awesome to watch, but
I didn't know it could be superheated as well. That explains the "flash
boiling" effect I sometimes get when I heat a mug of water in a
microwave for about 4 minutes and then drop a tea bag in. I always
thought it was caused by small amounts of air in the teabag suddenly
expanding because of the heat in the water...apparently it's the water,
not the air, that is expanding.
I didn't actually forward this to anybody...
Labels: email, microwave, superheat, water