# Why Did my Coffee Boil Over? Or, Why did it Stop Boiling? **URL:** \ **Category:** Factual Questions\ **Created:** [September 16, 2014, 5:07pm UTC](https://boards.straightdope.com/t/why-did-my-coffee-boil-over-or-why-did-it-stop-boiling/698663 "2014-09-16T17:07:17Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![TruCelt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/trucelt/32/523_2.png) [@TruCelt](https://boards.straightdope.com/u/TruCelt)\ **Post date:** [September 16, 2014, 5:07pm UTC](https://boards.straightdope.com/t/why-did-my-coffee-boil-over-or-why-did-it-stop-boiling/698663/1 "2014-09-16T17:07:17Z")
A little over a year ago, I entered [this thread](http://boards.straightdope.com/sdmb/showthread.php?t=689946) regarding an overflow in my tea cup. We never did (IMHO) come to a satisfactory conclusion. Now today I’ve had another similar near-catastrophe which may have narrowed the question to something answerable. Today’s little nightmare involved a half-mug of coffee I was reheating in the microwave. Just brewed/filtered coffee, no additives. It boiled over while in the microwave, so I let it sit for just under a minute before taking it out. The coffee had stopped boiling as soon as the microwave turned off. Knowing that I had overheated it in the first place, and having an enormous amount of free space now in the mug, I grabbed an ice cube and gently dropped it in. What resulted was a quite violent overboiling, which brought the crest of the coffee a good inch or more over the lip of the cup. By now I had barely more than a quarter cup still in there. Thank goodness I was holding the mug ever so slightly tipped away from me, so that the hot coffee bathed the inside of the freezer instead of my hand and wrist! So. Now I think that the real question is not so much “Why did it boil over?” but “Why did it stop boiling when the microwave stopped?” In both cases, my false assumption must have been that if the coffee/tea was boiling, and then stopped, it must have dropped below the boiling temperature. But that’s clearly not the case. Why then, would the liquid have stopped boiling, once started, if it was still so very hot? -or- Why has the addition of very cold water or ice caused a hot liquid to boil over so violently? 😕 ---
**Author:** ![Inner\_Stickler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inner_stickler/32/318_2.png) [@Inner\_Stickler](https://boards.straightdope.com/u/Inner_Stickler)\ **Post date:** [September 16, 2014, 5:36pm UTC](https://boards.straightdope.com/t/why-did-my-coffee-boil-over-or-why-did-it-stop-boiling/698663/2 "2014-09-16T17:36:23Z")
I still think in both the tea and coffee instances that it is superheating. I know you don’t find that answer satisfying and I don’t either. (Definitely, one would think a teabag would prevent superheating.) But I don’t see what else it could be. I’m tempted tonight to boil pots of water on the stove and drop ice chunks in them to see what happens. I wouldn’t think they’d react as violently as you describe. ---
**Author:** ![PastTense](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pasttense/32/14550_2.png) [@PastTense](https://boards.straightdope.com/u/PastTense)\ **Post date:** [September 16, 2014, 5:36pm UTC](https://boards.straightdope.com/t/why-did-my-coffee-boil-over-or-why-did-it-stop-boiling/698663/3 "2014-09-16T17:36:37Z")
Read about Superheating: “Superheating can occur when an undisturbed container of water is heated in a microwave oven. When the container is removed, the water still appears to be below the boiling point. However, once the water is disturbed, some of it violently flashes to steam, potentially spraying boiling water out of the container.[3] The boiling can be triggered by jostling the cup, inserting a stirring device, or adding a substance like instant coffee or sugar. The chances of superheating are greater with smooth containers, because scratches or chips can house small pockets of air, which serve as nucleation points. Chances of superheating can increase with repeated heating and cooling cycles of an undisturbed container, like when a forgotten coffee cup is re-heated without being removed from a microwave oven. This is due to heating cycles progressively de-gassing the liquid. There are ways to prevent superheating in a microwave oven, such as putting a popsicle stick in the glass or using a scratched container.” > **[Superheating | Occurrence via microwave oven](https://en.wikipedia.org/wiki/Superheating#Occurrence_via_microwave_oven)** > > Superheating can occur when an undisturbed container of water is heated in a microwave oven. At the time the container is removed, the lack of nucleation sites prevents boiling, leaving the surface calm. However, once the water is disturbed, some of it violently flashes to steam, potentially spraying boiling water out of the container. The boiling can be triggered by jostling the cup, inserting a stirring device, or adding a substance like instant coffee or sugar. The chance of superheating is Another article: > **[Superheating and microwave ovens](https://www.animations.physics.unsw.edu.au/jw/superheating.htm)** > > Superheating and microwave ovens ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [September 16, 2014, 5:40pm UTC](https://boards.straightdope.com/t/why-did-my-coffee-boil-over-or-why-did-it-stop-boiling/698663/4 "2014-09-16T17:40:33Z")
water can superheat in a microwave. it gets above the boiling point, a disturbance, movement or breaking the surface can cause it to violently boil. ---
**Author:** ![TruCelt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/trucelt/32/523_2.png) [@TruCelt](https://boards.straightdope.com/u/TruCelt)\ **Post date:** [September 16, 2014, 8:08pm UTC](https://boards.straightdope.com/t/why-did-my-coffee-boil-over-or-why-did-it-stop-boiling/698663/5 "2014-09-16T20:08:57Z")
> [@PastTense](#): > > Read about Superheating: > "Superheating can occur when an undisturbed container of water is heated in a microwave oven. When the container is removed, the water still appears to be below the boiling point. However, once the water is disturbed, some of it violently flashes to steam, potentially spraying boiling water out of the container. . . > [Superheating - Wikipedia](https://en.wikipedia.org/wiki/Superheating#Occurrence_via_microwave_oven) > > Another article: > [http://www.animations.physics.unsw.edu.au/jw/superheating.htm](http://www.animations.physics.unsw.edu.au/jw/superheating.htm) > [@johnpost](#): > > water can superheat in a microwave. . . Oh fortheluvaPete, here we go again. Both cups were already boiling. They were not undisturbed, nor were they superheating in the sense you are suggesting. Unless you have a link that shows why a cup of liquid would boil, then stop boiling, then magically superheat in the absence of any additional heat source, you haven’t answered the question. Inner Stickler: I certainly wouldn’t expect plain water to achieve the kind of height and boilover I got with these cups. These were full of coffee or tea, and so slightly more viscous, enough to hold the bubbles as they rose. ---
**Author:** ![Ignotus](https://avatars.discourse-cdn.com/v4/letter/i/ecc23a/32.png) [@Ignotus](https://boards.straightdope.com/u/Ignotus)\ **Post date:** [September 16, 2014, 10:07pm UTC](https://boards.straightdope.com/t/why-did-my-coffee-boil-over-or-why-did-it-stop-boiling/698663/6 "2014-09-16T22:07:00Z")
Microwaves are just plain evil. For the love of God, get rid of that shit and learn how to cook! Water is a poor conductor of heat, so strange phenomena are bound to happen when you disturb a non-uniformly, microwave-pre-heated body of water by tossing something into it. While a coffee mug ain’t exactly the Mariana’s Trench, 10 cm of water makes 0,01 atm. of pressure difference. It would equate about a third of a centigrade in boiling point, if I have done my homework correctly. That is enough to account for observed effects. And get rid of that nuke! ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [September 17, 2014, 7:42am UTC](https://boards.straightdope.com/t/why-did-my-coffee-boil-over-or-why-did-it-stop-boiling/698663/7 "2014-09-17T07:42:10Z")
> [@TruCelt](#): > > Oh fortheluvaPete, here we go again. > > Both cups were already boiling. They were not undisturbed, nor were they superheating in the sense you are suggesting. Unless you have a link that shows why a cup of liquid would boil, then stop boiling, then magically superheat in the absence of any additional heat source, you haven’t answered the question. > Inner Stickler: > > I certainly wouldn’t expect plain water to achieve the kind of height and boilover I got with these cups. These were full of coffee or tea, and so slightly more viscous, enough to hold the bubbles as they rose. Which is more likely: a) Superheating is not completely understood, or at least not completely described in easily accessed online sources. Although the initial conditions in your case do not match the limited descriptions online, the results match superheating **exactly** , and do not make sense in any other scenario we have come up with. b) This is a completely new phenomenon, involving liquids around boiling temperature, that just happen to mimic superheating in its result, but is actually … something else. ---
**Author:** ![TruCelt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/trucelt/32/523_2.png) [@TruCelt](https://boards.straightdope.com/u/TruCelt)\ **Post date:** [September 17, 2014, 4:24pm UTC](https://boards.straightdope.com/t/why-did-my-coffee-boil-over-or-why-did-it-stop-boiling/698663/8 "2014-09-17T16:24:44Z")
> [@naita](#): > > Which is more likely: > > a) Superheating is not completely understood, or at least not completely described in easily accessed online sources. Although the initial conditions in your case do not match the limited descriptions online, the results match superheating **exactly** , and do not make sense in any other scenario we have come up with. > > b) This is a completely new phenomenon, involving liquids around boiling temperature, that just happen to mimic superheating in its result, but is actually … something else. OK, so assuming it was hotter than boiling point, why did it stop boiling? And why, during constant movement from the microwave to the water cooler, and despite the presence of the tea bags did those two boil over after the addition of RO water (which couldn’t possibly be providing any nucleation points)? ---
**Author:** ![TroutMan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/troutman/32/6721_2.png) [@TroutMan](https://boards.straightdope.com/u/TroutMan)\ **Post date:** [September 17, 2014, 4:43pm UTC](https://boards.straightdope.com/t/why-did-my-coffee-boil-over-or-why-did-it-stop-boiling/698663/9 "2014-09-17T16:43:35Z")
Microwaves don’t necessarily heat evenly. Mine heats the top part of the liquid in a mug to boiling while the bottom half is warmish at best. I think some of your coffee boiled over and the remainder was superheated. Like **naita** said, the behavior you observed when you dropped in the ice fits superheating exactly. It is much more likely that we haven’t adequately explained the initial boilover than it is that you are observing some new phenomenon. (Unless you carbonate your coffee and make ice out of Mentos.) ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [September 17, 2014, 4:49pm UTC](https://boards.straightdope.com/t/why-did-my-coffee-boil-over-or-why-did-it-stop-boiling/698663/10 "2014-09-17T16:49:32Z")
> [@TruCelt](#): > > OK, so assuming it was hotter than boiling point, why did it stop boiling? And why, during constant movement from the microwave to the water cooler, and despite the presence of the tea bags did those two boil over after the addition of RO water (which couldn’t possibly be providing any nucleation points)? No idea. It seems like the sort of situation that requires more than second hand information about eyeball measurements and perceptions to make sense of. You should only drink hot beverages out of cups with at least four microwave safe temperature sensors from now on, and always record video. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [September 17, 2014, 5:01pm UTC](https://boards.straightdope.com/t/why-did-my-coffee-boil-over-or-why-did-it-stop-boiling/698663/11 "2014-09-17T17:01:15Z")
the boiling could have stopped and more heat gone into the liquid. some materials, ceramic like, can hold a lot of heat. this heat might then have gone into the liquid after the boiling stopped raising the liquids temperature. ---
**Author:** ![Labrador\_Deceiver](https://avatars.discourse-cdn.com/v4/letter/l/7c8e57/32.png) [@Labrador\_Deceiver](https://boards.straightdope.com/u/Labrador_Deceiver)\ **Post date:** [September 17, 2014, 7:37pm UTC](https://boards.straightdope.com/t/why-did-my-coffee-boil-over-or-why-did-it-stop-boiling/698663/12 "2014-09-17T19:37:13Z")
> [@Ignotus](#): > > Microwaves are just plain evil. For the love of God, get rid of that shit and learn how to cook! > > Water is a poor conductor of heat, so strange phenomena are bound to happen when you disturb a non-uniformly, microwave-pre-heated body of water by tossing something into it. > > While a coffee mug ain’t exactly the Mariana’s Trench, 10 cm of water makes 0,01 atm. of pressure difference. It would equate about a third of a centigrade in boiling point, if I have done my homework correctly. That is enough to account for observed effects. > > And get rid of that nuke! Advising the OP to get rid of her microwave is a stupid piece of advice, the kind which is usually dispensed by people who think they know more about cooking than they actually do. A microwave is a very, very useful kitchen tool.