Boiling water in a microwave

Cecil's Columns/Staff Reports
1

In regards to: http://www.straightdope.com/mailbag/mmicroboil.html

Alright, I’m going to have to completely disagree with the reasoning that the ceramic mugs don’t provide nucleation sites, and thus don’t allow the water to boil.

First, even though a ceramic mug is smooth to the touch, there are going to be enough microscopic imperfections for nucleation sites.

But more importantly, this is not why water in a microwave doesn’t always come to a boil.

A microwave sets up an electric field pointing in one direction, then switches the direction of the field, then switches it back and so on. Water is an electic dipole; it has no net charge, but the distribution of the charge in the molecule is such that one end is slightly positive, and one end is slightly negative.

When the water molecules are subjected to this electric field, they will orient themselves with it. And when the electric field changes, the water molecule’s orientation will change. This gives the molecules kinetic energy, hence they move, hence the temperature of the overall water increases.

The reason that water in a microwave can be superheated is that all of the molecules are aligned the same since they’re all subjected to the same electric field. If you’re heating water by using direct heat, the water is going to be randomly aligned, allowing the bubbles the to form. It has nothing to do with the availability of nucleation sites.

When you introduce another substance into the water that was heated in a microwave, you disrupt the alignment of the molecules (and they are no longer subject to the electric field, so there’s nothing to keep them in line).

Standard disclaimer: I of course may be wrong, but this is the understanding of the phenomenon I got from my physics class.

2

Your explanation is wrong, I’m afraid. A microwave oven uses microwaves (a type of electromagnetic radiation) not an electric field. An electric field would have no net translational effect on water because water has no net chrge.

The microwaves produced by the oven cause water molecules to orient their dipoles to the radiation. This causes no net translational motion. Instead it causes the rotational motion. The water molecules spin. The spinning molecules then begin to transfer their energy to neighboring molecules and the substance heats up.

The motion is very random. In fact it is too random, which is what causes the superheating. In convential heated (i.e. on a stove top) water there is a net motion to pockets of the liquid. These are called convection currents. They are caused by the unequal heating of the water. Water near the bottom (and closest to the heat) is hottest. Hot water is less dense than cold water, so it rises. In response, cooler water from the surface rushes down to take its place. The water on the top cools while the water on the bottom heats and so they switch again. This constant motion to the water means that over a short period of time every water molecule has a chance to come in contact with a nucleation site on the sides and bottom of the vessel. A split second after a molecule gets the energy to boil, it comes in contact with a nucleation site and boils.

In microwaved water there is no net movement. All the water is heated uniformily so there is no difference in density between different parts of the water. There is, therefore, nothing to drive the convective currents. Without these currents, it may take a long time before a water molecule, with sufficient energy to boil, comes in contact with a nucleation site. During this time the molecule can absorb more energy and rises in temperature in response. The water becomes superheated.

3

From my physics book: When an oven is operated, the microwaves produce (in the oven) an electric field that rapidly oscillates back and forth in direction. (Fundamentals of Physics, fifth edition, Halladay, Resnick, Walker, page 570.)

I believe the microwaves set up an electric field, so yes, an electric field is used.

Your explanation of the actual process that occurrs is more accurate.

My point is that the reason given, that the containers used have less nucleation sites, is not why water in a microwave superheats. It rather has to do with the method of heating the water itself.

4

The phenomenon discussed in this thread is referred to, in the scientific community, as dunting. It is often seen when boiling liquids in new lab glassware. Even if the liquid is heated over a bunsen burner (allowing convection currents to form) the lack of nucleation sites causes superheating and dunting (abrupt release of the gaseous phase). To prevent this (as it is very unpleasant if you are heating acid) it is common to place boiling chips in the vessel. These are simply broken pieces of ceramic or glass that provide nucleation sites for bubbles to form.
I’ve also noticed a similar event when heating very viscous liquids. While nucleation is not so much the cause as the inability for small bubbles to rise to the surface. As a result very large bubbles are formed and, when the rise and break the surface, they spew liquid all over. This happens mostly with heavy white or cheese sauces.

Dunting…our word or today. Try to use it three times and make it your own.

5

Two points:

  1. Why would stirring the water PRIOR to putting it in the oven help anything? Once you stop stirring, it will setle down pretty fast, and stop moving before it gets hot enough to boil, wouldn’t it?

  2. If the slight action of stirring beforehand really does help, then I would geuss the the slight movement provided by a spinning turntable (such as is common in many microwave ovens) would also suffice to eliminate this problem.

Comments?

6

Kupek: I believe your physics book is wrong. I’ve searched the web and I cannot find any explanations of how a microwave works that include electric fields. I have found only sites that support my explanation.

From Britannica.com:

From Howstuffworks.com:

From the University of Illinois physics web site:

(in all cases the bolding is mine)

I think your physics book has a typo. It should say electromagnetic field instead of electric field.

7

Dr. Lao: Well, wouldn’t be the first time my physics book got something wrong or fudged it. My professor has gone on several tirades about the book. It probably wasn’t a type, but an on-purpose simplification of what was really happening to give an “example” of an application of electric fields. (We never did anything with electromagnetic fields; we did electric fields, magnetic fields, we did electromagnetic waves, but not electromagnetic fields.)

8

I’m scratching my head on this one. Would not an electric field be a subset of electromagnetic fields?

Regarding the water molecules being dipoles, and thus aligning with the electric (or electromagnetic) field, consider that microwave ovens use at least one of two ways to disperse the EM waves around the chamber. They either use a turning reflector assy (above the plastic cover in the roof) or they use the turntable. Either method will cause the food on the tray (or water in this case) to be subjected to a varying field. Thus the food is heated more evenly, and reduces hot spots. If the water is subjected to moving fields, then how could the alignment of the molecules be static, even if they did all align with the field together? Thus your assertion is shown to be incorrect. The standard explanation of nucleation sites is upheld.

Followup point: if it were the field aligning the molecules that were the culprit, if you turned off the oven and let the cup sit for a second, would not the molecules then start moving randomly, and then quickly boil on its own? But that doesn’t happen. However, drop in a pinch of small granulated substance like salt, sugar, Equal, and it will quickly “dunt”. Ergo, nucleation sites.

9

The problem is that it’s pretty damn hard to do a good experiment with this, since the common method for boiling water on a stove will (besides giving you a cool light show) likely destroy your microwave.

Unless you use Corning Ware. (Isn’t that stuff made for stove top and microwave?) If you’re right, then there should be no difference in how the water boils on the stove top or the microwave. I, unfortunately am in a dorm room right now, and do not have access to either Corning Ware or a stove top. (Do have a microwave though.)

And Dr. Lao, your explanation took into account both nucleation sites and the method that the microwave uses to heat the water. It seemed to me, however, that your explanation was different from the one in the article: it’s not that there are less nucleation sites available, but that in a microwave, water molecules are less likely to be around them long enough to form bubbles. Am I right in assuming you are disagreeing with the article, or am I missing something?

10

The article mentions both the fewer nucleation sites and the lack of convection currents as causes for the superheating of water in a microwave. It is my opinion that the lower number of nucleation sites (if, in fact, that microwavable vessels have fewer nucleation sites) is not particularly important and the dominant effect is the lack of convection currents.

So in that respect, I think the article is wrong. But you don’t have to take my word for it. Cecil addressed this a number of years ago in his column which the article linked to. In it he gives essentially the same explanation I did.

Why does Sweet 'n Low make microwaved water fizz so strangely?

11

I’m sorry SDSTAFF David, but your assertion that the water
is " not going to “blow up” and throw all the water into your face, as described in the e-mail message. At least, it won’t unless you’re sitting there with your face an inch away from the water, staring at it"…
is, and I say this in the most positive manner, as full #&*! as a Christmas turkey. I had a first hand experience
with this a few months ago, and in fact, mailed this info to my mailing list as a warning, long before I heard of it happening to anyone else. To wit: I was heating water to boil in a 16 oz (2cup) glass measuring cup to make tea, and had set the timer for about 5 minutes. As I noticed the timer counting down the last few seconds, I went to the microwave and was preparing to open the door when it did in fact violently erupt , spewing hot water all over the inside of the microwave and emptying the container of all but a couple of ounces. I did notice beforehand that there were NO bubbles as one would expect if the water had started to boil. Had I have opened the door and had the container in my hand, I would have gotten a face full the same as the unfortunate gentleman in the chain letter email. Whether it’s done by electromatic waves, electrical waves, sound waves, micro waves, surf’s up waves or whatever, it can, did and will erupt without warning. Be careful out there folks, it could happen to you.

12

I think this might be a little off topic, but water isn’t the only thing that blows up in or just out of the microwave… When my sister and I discovered that the microwave was an ideal way of melting butter for popcorn, we discovered another unfortunate phenomenon. That is, if you leave the butter in for over about a minute (it can be noted that it DOES boil in the microwave), it will make a few ominous popping sounds before spewing half of the cup’s contents all over the inside of the microwave. A couple experiences with cleaning this taught us to mind the butter well…

13

From the “Warning” in Julie’s submission:
“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 stirrer stick / spoon, tea bag, etc.”

This part sounds like pseudoscience to me. A wooden stirrer stick can’t “diffuse the energy”, since wood is a pretty good thermal insulator, and microwaves won’t affect the solid part of the wood (only the water or oils inside). And you wouldn’t want to put a metal spoon in the cup for the same reason you don’t want to put any solid metal objects in the microwave: they absorb and/or reflect a lot of the microwaves, causing damage to your oven. “Energy diffusion” isn’t the desired effect, in any case. Count me as one of the supporters of bubble nucleation sites (or lack of them) as a cause of regular (or irregular) boiling.

For those of you who want to try to duplicate the volcanic explosion effect at home: a mechanical shock can cause a sudden release of gas or vapor in a bubbling or superheated liquid. For instance, you get maximum distance with a jet of Coke from a can if you (1) start with a warm can, (2) shake up the can, and (3) hit the bottom of the can against a solid surface immediately after opening the top. All the carbonation tries to come out of the liquid at the same time, driving some of the liquid quite far away from the can.

Come to think of it, that’s probably why I never have problems with superheated water. My microwave oven has a rotating table built into it which lurches around the whole time the microwaves are on, providing some stirring action and frequent (mild) mechanical shocks.

14

David B, I’m afraid I agree with TheBori and Mick1823, based on my experience in chemistry class. I had a flask of water super-heat. When it finally did boil, it fountained out of the flask - I think there was about half left in it afterwards. If I’d been close to it, looking at it, there would have been a good chance of getting scalded. That’s when my prof explained about boiling chips, in much the same terms that TheBori uses. (On the other hand, he did mention that super-heating is a back-handed compliment on one’s lab technique, since it means that your flask was extremely clean, with no impurities for the bubbles to form around.)

RandyB, the wooden spoon would likely have enough scratches and small imperfections on it to allow the bubbles to form. I agree that “diffusing the energy” sounds like pseudo-science, but who knows what was originally said, and how the listener understood it.

15

mick1823 wrote:

jti wrote:

Both of you guys:

Suddenly boiling over – even violently – does not mean that the water shot UPWARD in some kind of leathal geyser. How HIGH did the water go when it boiled over?

16

well, it’s been a while now, but I’d say 3-4 inches; the flask was on a stand over the Bunsen burner, so the top of the flask was near the middle of my chest.

17

Thank you, tracer, for explaining my point. Unless you’re sitting there with your face strangely near the cup, it ain’t gonna hit you there. jti, even at 3-4 inches, it wouldn’t hit your face if you’re holding the cup at a normal distance.

18

Well, how about the stories that are going around that the U.S. is NOT reponsible for destroying Hiroshima? The bombs dropped by the U.S. planes actually misfired. In fact, a proto-type microwave was being tested in Hirsohima that balmy day in 1945, and someone was boiling water which exploded, destroying the city.

I know this is true, because I read it online at the “Nazis for Free Speech” home page.

19

HEY KIDS!!

How about a great brain teaser to amuse your friends, and a way cool experiment to demonstrate it right in your own kitchen with common household articles? Read on!

First let’s clear up a few points on this microwave water boiling business. As a brand new member I am daunted to enter a forum so filled with expertise. Some members have a great deal of wisdom to share, if the number of posts is any indicator. I think the “truth” of the matter can be synthesized from the observations already posted. May I therefore humbly submit:

  1. Yes, electric fields are present in the microwave oven, as are magnetic fields. The fields are intrinsic properties of the microwave photons, oscillating at 2.5 Ghz (Billion cycles per second) with a wavelength of 12 centimeters in consumer ovens.
  2. Yes, water molecules are electric dipoles and vibrate in concert with the microwave frequency, However they are in constant chaotic motion through the liquid at a range of energies and frequencies far above the level of microwaves, absorbing the energy bit by bit. The electric field gradient in your oven is far too weak to produce a coherent, ordered movement.
  3. As pointed out already, the most convincing refutation of the microwave field boiling suppression hypothesis is the observation that boiling does not commence as soon as the power stops.
  4. While the field intensities in your oven are therefore too low to significantly suppress the boiling of water I would be surprised if very large static or oscillating fields did not have some effect on the bulk properties of water. The boiling point is set by the balance between the attractive, electromagnetic interactions of the molecules, their average thermal energy, and the pressure. Water is liquid at such comfy temperatures for such a small molecule because of its large electric dipole moment, H2S (hydrogen sulfide) boils far below the freezing point of water, even though it’s heavier. The idea that electric fields could influence this balance does not seem stupid, just too small an influence to matter in this context.
  5. Both reduced convection and fewer nucleation sites are involved, as observed in the experiment to follow. The sluggishness of fluid motion can be seen in water heated by microwaves in a clean glass vessel by looking through it against the light, and a scratched up vessel will boil up sooner with less superheating.
  6. No one has pointed out that while water at the surface is at ambient atmospheric pressure, water below the surface is at a slightly higher pressure, by approximately 3 percent per foot. This means that the boiling point is slightly higher for water in the middle or bottom of the cup, water that is not convecting to the top because of the even heating.
  7. It seems a bit audacious and ungraceful to insist that the scalding story or something like it could not have happened, at least not without the person being an complete idiot. Well superheated water and a large, clean tall cup might be enough, maybe not to hit the ceiling, but a foot or more high, there’s certainly enough energy available. I’ve had water boil up out of a glass cup in the microwave and hit the top of the cavity.

Now the brain teaser. You, dear reader, will surely guess the answer but your naive and uninformed friends will likely be stumped. Just ask them this: How can you make water boil by adding ice to it? After various goofball ideas are proposed and everyone gives up, proceed to the kitchen for the experiment. Wagering on the outcome is optional.
Try this, of course, at your own risk and don’t tell your mom I told you about it—Seriously, if you are a kid don’t try it on your own, get a parent to help since very hot boiling water is involved. I assume most of the rest of the members are adults; you’re on your own.
You’ll need the following items:
1 clean, not seriously scratched Pyrex measuring cup, 2 cups(500mL) capacity
1 microwave oven
1&1/3 cup(330mL) clean water
1 ice cube
1 large spoon
1 Comprehensive homeowners & liability insurance policy
A little dramatic flair
The Procedure:
First, wash the measuring cup out(even if it’s already clean) with a little hand dish washing detergent and rinse well. Fill cup to two thirds capacity with clean, preferably filtered, water, distilled water is not necessary. Now get an ice cube from the freezer and tell everyone that this very cube of ice will make this cup of water boil. This is your opportunity to give a little background, or just jive them with some creative b.s. and raise the wager.
Now place the cup in the microwave and set for five minutes on high. You might want to see how long it takes to get the water hot enough without boiling over by experimenting beforehand. If the water boils at any time, in the oven or after, you’ll have to try again. If the cup is smooth enough you should be able to get it superheated with a little practice, it’s not hard. When the water is just about ready, use the spoon to crack up the ice in your hand. Gently remove the cup from the microwave and place in the sink. With a dramatic “abracadabra” toss in the crushed ice and watch the water boil up vigorously for a second or so. Collect bets. Pretty much anything you toss in will work, salt, dirt, etc. but it’s more fun to see the solid, crystalline state of a material cause the liquid state to turn to gas.
Science is such a kick!

Aloha,
sjl52

20

tracer and david, aren’t you both assuming that the microwave is on the counter? both my mother and mother-in-law have their microwaves set above the counter, at face level. I don’t think it would be difficult for this accident to occur in those circumstances.