Ask A Scientist Chemistry Archive

name         Mike P.

Question -   My question is about microwaving water.  I've noticed an effect I
don't understand when I heat about 250ml of tap water for about
77 seconds in a Pyrex cup in my microwave.  When I take the cup
out of the microwave it's just to the point of boiling, but it's not.

I then put a fork in the cup of water and the water boils very
vigorously.

I do not understand why.  I would think that with the water below the
level of heat required to produce boiling, the placing of a room
temperature piece of metal in the liquid would quickly reduce the
heat and temperature of the water around the fork and in the
cup of water.  If this is the case, where does the
energy come from to produce the vigorous boiling.

I did a checking experiment by putting the same cup on the open
flame of the kitchen stove.  By taking the water up to and above
the boiling point I did not observe the same event (when the water
started boiling I could see bubbles coming from the ends of the
fork - This I think I can understand as hot sites to start the
boiling).
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Hi, Mike !!!
To start raining, a little bit of dust in air is
important to initiate the water-drop formation.
To start a crystallization process, a "seed"
sometimes is necessary to help the process.
To electrically discharge the atmosphere,
the use of "points" as lightning conductor is useful
Surely that the conditions must
be saturated, just about to happen.
Sometimes, handling a super-saturated solution of a salt,
just a "kick" can precipitate all the excess of
dissolved salt. To detonate TNT, sometimes
just a push is sufficient !!!
To start boiling, it is sometime necessary to
have a "point" to indicate WHERE should the
boiling start. So, your observation about
the microwaving water seems to me so answered.
  Regards
  Alcir Grohmann
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Hi Mike,

This is a very good and common question regarding the phenomenon known as
SUPERHEATING.  WARNING:  SEVERE BURNS CAN RESULT FROM ATTEMPTING THIS
EXPERIMENT.

How can you heat water above 212 °F at 1 atm pressure?  Remember that
evaporation (or boiling in this case) is the primary mechanism of cooling.
If the water never boils and the microwave continues to heat then there will
be superheating until the water explosively discharges its excess energy.
Water boils at 212 °F when the atmospheric pressure is exactly 1 atm (760 mm
Hg).  What is boiling though?  Boiling begins at a temperature when the
vapor pressure of a liquid equals the ambient atmospheric pressure that is
above the pool of liquid.  However, you WILL NOT have boiling water if there
are no sites for the vapor (within the liquid) to nucleate (grow) from.
Good nucleating sites are scratches, irregularities and other imperfections
inside the cup, mug, or in your case the Pyrex.

Your microwaved water was most definitely above the boiling temperature ( ~
210 - 212 °F) for the altitude that you are in.  Your fork was simply
providing an excellent source of these nucleation sites for the superheated
water to release its excess energy.  So even though your relatively cold
fork might have slowed the water molecules that were in contact with it, the
superheated water had enough excess energy to both heat up the fork as well
as boil off the excess energy of the water in the Pyrex to bring it down to
its equilibrium value of T(boiling point).

I nearly got burned by this phenomenon.  I had placed a small amount of
water in a new glass coffee mug to make some tea.  I had put the mug in the
microwave and set it for 3 mins.  I had superheated the water without
knowing it.  I went to grab the mug and as I slid it across the glass
turntable the superheated water had explosively sprayed all over.  So just
like Pavlov's dog salivated when hearing a bell.  I am so conditioned from
that one event that when I take my mug out of the microwave I have to tap it
with a spoon or slide it or perturb it somehow before I get my face anywhere
near it.

Very good question.

-Darin Wagner
========================================================
What is happening is that the water is heated above its boiling point in the
microwave.  Microwaves heat water by being directly absorbed.  The microwave
energy converts to tumbling of the water molecules, which makes the water
hot.  When water is heated on a stove, heat flows into the water from the
hot vessel containing it.

When a liquid boils, molecules of the liquid rapidly pass from the bulk
liquid into vapor bubbles.  The vapor/liquid interface is higher in energy
than either the vapor phase of the liquid phase.  To make a bubble in the
first place, a lot of liquid has to be moved out of the way, because vapor
is much less dense than liquid.  The new vapor must push outward against
both the internal pressure of the liquid and the vapor/liquid interface,
which provides additional force to collapse the bubble.  So, it takes energy
and the right combination of molecular motions to form a bubble that can act
as a nucleus for boiling.

When water is heated in a pot on the stove, the hottest region of the liquid
is that right next to the pot wall.  In addition, the pot walls usually
contain many small scratches that hold small bubbles of air, which can act
as starters for the boiling process.  The heated water doesn't need to
create bubbles from nothing.  With the hottest liquid right next to the
bubbles that start the boiling, conditions are ideal for boiling to start
when water is heated on a stove.

Heating water in a glass or mug with microwaves makes boiling more difficult
for two reasons.  The first is that glasses and glazed ceramic contain fewer
surface scratches than metal pots.  This provides fewer starter bubbles.  If
water is heated on a stove in a smooth stainless steel pot or a glass
(Pyrex) pot, it can sometimes superheat.  Putting something solid into the
water, such as pasta, will cause the water to boil furiously.  This is
because the solid surface carries with it lots of little air bubbles that
can act as nuclei for boiling.  This same effect holds for water heated in a
smooth vessel in a microwave.

The second reason that water is so commonly superheated in microwaves is
that the hottest portion of the liquid is not right next to the pot surface
and its little air bubbles.  When water in the middle of the cup is heated
to above its boiling point, it is very hard for it to create the bubble
necessary for boiling.

In the experiment you conduct, the fork simply provides "nuclei" for the
water to boil in the form of small bubbles of air.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
========================================================
One or two (or maybe both) things may be occurring. You are superheating
the water in the microwave. It is easier to do this in a microwave than on
an open flame because the heating in the microwave is more even (less
convection) than on an open flame. Yes, when you put the fork at room
temperature in the cup you will cool it somewhat, but the larger and faster
effect is that you introduce sites for the formation of water vapor from the
superheated liquid.

This is a common effect that sometimes occurs in the boiling of any liquid.
For that reason, "boiling chips" are put in the distillation flasks used to
evaporate organic solvents.

You may also be providing sites for the evolution of dissolved gases from
the atmosphere. You can illustrate this effect by sprinkling some common
table into a glass of beer -- admittedly a waste of good beer -- but you will
see it froth as the CO2 is provided with sites to form the gas. I think that
the first suggestion is the more likely, however.

Vince Calder
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