Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Superheated Water
Name: Mike P.
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 2001 - 2002

My question is about microwaving water. I've noticed an effect 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).

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.

Alcir Grohmann

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

Click here to return to the Chemistry Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory