Warning/Heating Water in Microwave
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 he has done numerous times before). Iím 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 microwave
off, he removed the cup from the oven. As he looked into the cup and he noticed that the water was
not boiling, but at that instant, the water in the cup "blew up" into his face. The cup
remained intact until he threw it out of his hand. His whole face is blistered with 1st and 2nd degree
burns, which may leave scarring, and he may have lost partial sight in his left eye.
While at the hospital, the doctor stated that this is a common occurrence and water (alone) should
never be heated in a microwave oven. If water is heated in this manner, something (such as a wooden
stir stick, tea bag, etc) must be placed in the cup to diffuse the energy. It is however, a much safer
choice to boil the water in a teakettle.
A similar thing happened to me a year ago
with my microwave oven. I heated a cup of water in a clear measuring cup for 2 minutes and as the
oven dinged, I reached to open the door, and POOF! The water vaporized. The water never did boil.
I tried it several times since and have not been able to repeat it.
The teabag does not "diffuse"
the energy; it provides nucleation points for boiling. Undisturbed and clean water can be heated beyond
212 degrees without vaporizing -- creating a state called incipient boiling. Any disturbance causes
the excess energy to convert liquid to steam, creating an "explosion.Ē
From: Stromberg, Eric (ER)
Here's an explanation that came from our chemists at
Dow Chemical. Chemists are very aware of this. Usually due to a HIGHLY POLISHED Glazed or very clean
glass surface) and a VERY Narrow opening or tall and thin container. The water pressure and lack of
irregular surface for gas bubble formation (Boiling) allows for the temperature of the fluid to exceed
the boiling point.†
Upon jarring or adding an irregular surface
(such as inserting a spoon) a rapid release of water vapor (gas/steam = boiling) occurs and it starts
at the bottom of the cup. This now occurs instantly and violently and will, in a high and narrow container,
shoot up. Chemists add Boiling Beads to flasks to prevent this from occurring.)
In cases such as the one mentioned, usually the problem
is:† NEW, Highly glazed cups that do not have a scratch or "points' for gas bubble formation
AND Tall and Narrow containers. (Have you ever noticed how some cups burn you lips and others do not?
The ones with highly colored gazing and are highly polished are the dangerous cups!)
Reduce the risk by using wider, older and
NON-HIGHLY Glazed cups with defects inside...use a Brillo pad to roughen the internal surface, or
as suggested, prior to micro waving, insert a glass rod, stir stick or non metallic high temperature
compatible item into the cup to allow gas formation (boiling) to occur. Also, before removing cup
from microwave, insert a spoon and stir.
Again! Be cautious of using HIGHLY GLAZED,
decorative or new cups for heating water. Note that soup, stews, and other liquids that have objects
in them (gas formation points) do not have this problem.
From: MSI, George
Wells, engineer, and tea connoisseur
Yes, itís true. However, DO NOT use a teabag
to diffuse the energy. A tea bag can cause a violent eruption. A through explanation of the physics
involved is beyond what I can afford to devote the time in e-mail. I will say, however, that microwave
energy is absorbed by water more efficiently than most substances. In fact, it is because of the presence
of water that most things cook in a microwave oven.
Water heated in a teakettle absorbs energy
much more slowly and, therefore, has greater convection. Because of the convection (water is hotter
at the top) there is less surface tension and downward pressure to overcome. Water heated in a microwave
oven overcomes the surface tension at the slightest disturbance. The act of removing the cup from
the oven causes the water with less energy to move out of the way of the water that is trying to release
energy, often resulting in a violent eruption of the water. The problem with a tea bag is that it
introduces air, which creates more surface area for the water. The eruption caused by a tea bag is
generally much more violent than it would be with a spoon. A metal spoon, in addition to introducing
almost no air, has the added advantage of immediately absorbing energy
What about milk? Milk has solids and fats,
which stimulate convection. Milk, therefore, will boil rapidly rather than erupt the way water does,
BUT, it will go from a steady state to full boil within a matter of only a couple of seconds.
P.S. tea is best when boiled and steeped
loose in the cup, then strained.
From: Louis A. Bloomfield, Professor
of Physics, The University of Virginia, http://rabi.phys.virginia.edu/HTW/
Yes, this sort of accident can happen.
The water superheated and then boiled violently when disturbed. Here's how it work:
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.
However, 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. However, 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 a metal spoon or almost 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.