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It sounds like a classic urban myth: A guy puts a cup of water in the microwave to make coffee, takes the heated liquid out, drops in some Folgers crystals, and boiling water explodes out of the cup, burning his hands and arms. It’s no myth, says Anuradha Prakash, spokesperson for the Institute of Food Technologists and professor and program director of food science at Chapman University in Orange, California.
When clean water is microwaved in a very smooth container (like a really clean mug), it can superheat—meaning that it reaches a temperature above boiling without bubbling. Bubbles release heat from the water, but for bubbles to form, there need to be “nucleation sites.” These are either impurities in the liquid or imperfections on the surface of the container.
If you’ve superheated the water, then whatever you add to it, like instant coffee or sugar, creates nucleation sites for bubbles to form and expand. “The high temperature causes the bubbles to expand quickly, in some cases causing mild explosions,” says Prakash. Microwaving distilled water is riskier since it contains no impurities, although Prakash says explosions can occur in tap water too if the dissolved minerals are not large enough to act as nucleation sites for the bubbles.
Generally, microwaves will heat a cup of water to temperatures that are close to boiling within a minute or two, says Prakash. The FDA advises adding sugar or coffee before heating to “greatly reduce” the risk of burns from superheated water. Prakash says putting a wooden stirrer in your cup is also a good way to mitigate the risk.
Check out this MythBusters clip where they superheat water and drop in a sugar cube:
Responding to ajs42548 question - apologies for the delay - I've been traveling: Superheated water is a real but difficult phenomenon in which the water is in a metastable state. In order for water to boil there must be bubbles into which the water vapor can migrate. In all circumstances, since the air in the oven is cold, the surface is never the hottest place and the water at the surfaces onto...+READ
Responding to ajs42548 question - apologies for the delay - I've been traveling: Superheated water is a real but difficult phenomenon in which the water is in a metastable state. In order for water to boil there must be bubbles into which the water vapor can migrate. In all circumstances, since the air in the oven is cold, the surface is never the hottest place and the water at the surfaces onto which bubbles cling has to reach 100 C in order for the bubble to overcome the inerfacial tension holding it to the container wall. Meanwhile, the microwave energy has to go somewhere and so it keeps heating all the water but the center tends to get hottest unless here are bubbles there (as in aerated water - the cloudy water coming out of the tap) - in which case it will "boil". Bubbles can't form spontaneously - an infinitely small bubble has an infinitely high pressure - so that superheated zone awaits a bubble from somewhere else in order to "boil". As to temperature - in the early 1980s we measured water temperatures from 110 to over 120 C in these surerheated zones.-COLLAPSE
Responding late to ajs42548 (I've been traveling): the concept of superheated water is strange but it truly exists. We first saw it in the early 1980's while measuring temperatures in water while microwaving and saw temperatures on the order of 115 to over 120 C - all in the center of the container. The water was in a metastable condition and awaited a bubble to appear in order to "boil". Look at...+READ
Responding late to ajs42548 (I've been traveling): the concept of superheated water is strange but it truly exists. We first saw it in the early 1980's while measuring temperatures in water while microwaving and saw temperatures on the order of 115 to over 120 C - all in the center of the container. The water was in a metastable condition and awaited a bubble to appear in order to "boil". Look at it this way - bubbles can't appear spontaneously (an infinitely small bubble has an infinitely high pressure). Consequently, water can't "boil" without bubbles into which the water vapor will be released. Since the air in a microwave oven is cold and bubbles tend to cling to surfaces (unless the water is highly aerated - cloudy tap water) which are cooler than the interior, the microwave energy has to go somewhere, manifesting itself as heat, so it keeps on heating the water, just as if the water was under pressure. A tough, but real, concept - hope this helps.-COLLAPSE
Yes, I understand the physics of superheated water exploding, e.g. taking the cover off of a pressure cooker. Unlike a pressure cooker which puts the water under high pressure, thus increasing the boiling point, microwave heated water is under no such increased pressure. I'm not doubting that superheated water exists here, rather that it seems the boiling point of pure water is not necessarily...+READ
Yes, I understand the physics of superheated water exploding, e.g. taking the cover off of a pressure cooker. Unlike a pressure cooker which puts the water under high pressure, thus increasing the boiling point, microwave heated water is under no such increased pressure. I'm not doubting that superheated water exists here, rather that it seems the boiling point of pure water is not necessarily constant. This is the first I've heard of this phenomenon.-COLLAPSE
The boiling point of water at sea level (standard pressure) is 100 degrees centigrade. What happens with the microwaving is "superheating" and then water can reach local temperatures well in excess of 100 C without boiling. Bubbles have to be present to let the boiling occur - so the water explosion occurs when a bubble from somewhere in the container (usually along the walls) or introduced...+READ
The boiling point of water at sea level (standard pressure) is 100 degrees centigrade. What happens with the microwaving is "superheating" and then water can reach local temperatures well in excess of 100 C without boiling. Bubbles have to be present to let the boiling occur - so the water explosion occurs when a bubble from somewhere in the container (usually along the walls) or introduced externally - from instant coffee, for example - reaches the location of the superheated water which then can form steam and the whole thing "explodes". Remember that superheated water is extremely unstable and wants to cool to 100 C at normal pressure.-COLLAPSE
ajs: I don't have the answer to that, but I have left a cup of water (2 cups total) in the microwave for as long as 2 minutes. It got very hot but didn't boil. I've never left water in a microwave longer than that. It would also, I'm sure, depend on the power (wattage) of your Micro. Mine is quite large with hight wattage. Just be careful!
So exactly how high can the temperature of super heated water go? Does this mean that the boiling point of water is variable, assuming the same air pressure?
This also happened to me. I was reheating a cup of coffee in the micro and left it in too long. when I took it out and dropped a teaspoon of sugar in it, it exploaded all over me and my kitchen. There was literally no coffee left in the cup. So I am very VERY careful, now.
Sorry to tell you this, but your explanations are only partially correct and also wrong in some cases. Even Dr Prakash’s explanation is only partially correct. I am a microwave scientist who has worked with microwave ovens and microwave heating for almost 50 years – I also own about 50 microwave ovens and have been an independent consultant for almost 40 years, dealing primarily with microwave...+READ
Sorry to tell you this, but your explanations are only partially correct and also wrong in some cases. Even Dr Prakash’s explanation is only partially correct. I am a microwave scientist who has worked with microwave ovens and microwave heating for almost 50 years – I also own about 50 microwave ovens and have been an independent consultant for almost 40 years, dealing primarily with microwave heating. I believe I published first report on microwave superheating of water in the early 1980s when we measured temperatures as high as 126 ° C (259 ° F) in microwave heated water prior to it boiling (or exploding). I have lectured on the problem numerous times since the mid 1980s.
Superheating does not occur because of lack of nucleation – there are plenty of microscopic air bubbles along the walls & bottom of any container of water. When you heat water in pot on the stove these walls and bottom become very hot and it is this heat that makes these surface-wall bubbles grow until they get large and buoyant enough to overcome the interfacial tension that holds them to the walls and they release from the walls to float to the surface – expanding and coalescing with other bubbles to cause what we call boiling. However, inside a microwave oven the air is cold so the surface is very rarely the hottest place, rather the interior is hotter. So, as water is heated, the interior, if it is free of air bubble, can superheat. Eruption can then occur when a nucleation site arrives in the superheated area – and that can come from air bubbles released form the container wall by moving or disturbing the water (I.e. moving the cup); or by adding instant coffee, a sugar cube, etc. It “explodes) because steam occupies almost 2000 times the volume as water.
As to your comment that tap water doesn’t superheat because it contains minerals, while distilled water doesn’t and so it superheats – that is incorrect. Tap water can also superheat – if it is left to stand for a bit to allow any air in the water, in the form of microscopic bubbles (cloudy tap water) to float to the surface & disappear. As to the minerals, you would then expect soft water to superheat and hard water not – and that’s not true. They both can superheat.-COLLAPSE
I had a similar situation that Meowzebub had. I heated up a cup of coffee and when I added some cold water, the coffee bubbled up an d overflowed the cup.
I heated a mug full of water and tried to squeeze a wedge of lemon over it. but a tiny tsunami flowed out when the first drops of juice hit the water. I was fortunate in that 1. I wasn't holding the mug; 2. the lemon was pretty dry so only a few small drops came out.
I microwaved a small, single-serving bottle of sake after removing the cap. I had never microwaved sake in a glass bottle before, only in ceramic sake containers. I misjudged the amount of time that the sake needed to be heated and the bottle was very hot. I grabbed it with a hot pad and started to turn the bottle sideways to pour some sake into a sake cup.
The sake shot out of the bottle like...+READ
I microwaved a small, single-serving bottle of sake after removing the cap. I had never microwaved sake in a glass bottle before, only in ceramic sake containers. I misjudged the amount of time that the sake needed to be heated and the bottle was very hot. I grabbed it with a hot pad and started to turn the bottle sideways to pour some sake into a sake cup.
The sake shot out of the bottle like a Roman candle, hitting the twelve foot ceiling and barely missing my girlfriend.
Now, theoretically, from what I have read above, this should not have happened because sake contains oodles of sugar, but believe me, man! did it ever happen. By the way, have you ever tried to remove sticky sake from the ceiling, walls, cabinets . . .?-COLLAPSE
Not only can it happen, I've seen it happen*, and you don't have to add anything to the water to make it erupt. Merely disturbing it upon taking it out of the microwave can be sufficient.
*A sister has a habit of boiling water for tea in mugs in her microwave rather than using a tea kettle...I'll stick with the kettle, thanks.
I often make gravy in my microwave, which many have warned me not to do because of the "explosion." It is really quite simple though. When you remove the liquid, whether it be water or gravy, tap the bowl or cup on the counter and get those bubbles to come up to the top. If it is overheated then you will see a flurry of bubbles come up. Once that happens it is safe to insert a spoon.
If I accidentally leave the water in the microwave too long, I stir it before adding anything. Bubbles arise as soon as the spoon goes in, but rapidly subside, and the water does not make a fuss when the teabag or instant coffee hit it.