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Author Topic: Heating Water in Microwave  (Read 12965 times)

Offline Donnah

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Heating Water in Microwave
« on: 05/07/2003 00:41:09 »
A cup of water heated in the microwave may appear not to be boiling, but can "explode" up and out of the cup while it is being removed from the microwave, due to the buildup of energy.  If water is heated in this manner, something such as a wooden stir stick or a tea bag should be placed in the cup to diffuse the energy.
     
This "explosion" is caused by a phenomenon known as super heating. It can occur any time water is heated and will particularly occur if the vessel that the water is heated in is new.  The water heats faster than the vapor bubbles can form. If the cup is very new then it is unlikely to have small surface scratches inside it that provide a place for the bubbles to form. As the bubbles cannot form and release some of the heat, the liquid does not boil, and continues to heat well past its boiling point. Then, if the liquid is bumped or jarred, it is enough of a shock to cause the bubbles to rapidly form and expel the hot liquid. The rapid formation of bubbles is also why a carbonated beverage spews when opened after having been shaken.
« Last Edit: 05/07/2003 00:43:01 by Donnah »


 

Offline Quantumcat

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Re: Heating Water in Microwave
« Reply #1 on: 05/07/2003 08:03:25 »
If the water is heated, why does the heat energy not allow it to turn into a gas as per normal ... ?

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Offline chris

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Re: Heating Water in Microwave
« Reply #2 on: 05/07/2003 10:51:26 »
This is a really good point - I've seen this being demonstrated but I'm not clear why it happens. My suspicion is that the microwaves produce focal heating within the liquid, rather than at the bottom as would happen in a saucepan, which in some way produces superheated regions. When a nucleation surface - such as a spoon - is introduced the superheated regions vapourise and the water boils. Perhaps the pressure of the surrounding colder water keeps the small superheated regions from boiling until you mix the liquid up and the average temperature then exceeds the boiling point of water and hey-presto it explodes...?

Come on Quantum, you're good at physics, help us out !

Chris
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Offline Quantumcat

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Re: Heating Water in Microwave
« Reply #3 on: 05/07/2003 13:38:15 »
But the regions that are superheated should turn to gas, be much less dense than the cold water, and be able to get through? If one set of water molecules are vibrating due to the microwaves, they should bump into others and therefore spread the heat. Shouldn't they? This must be wrong, since Donnah said it doesn't happen...

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Offline Ians Daddy

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Re: Heating Water in Microwave
« Reply #4 on: 05/07/2003 17:39:34 »
You can take a crayon and microwave it on high for hours and it won't affect it at all. This is because microwaves only heat water molecules. Crayons are wax which is an oil base. When microwaving a cup of water, the dry air above the water is cooler and creates a seal as heat rises and cold falls. It's true that an older, more marred cup or an object in the cup would create an avenue for the pressure to be released. It's like the magic trick where you put a playing card on a glass of water and turn it upside down. It creates a sort of vacuum and will not drop the card or spill. The colder air above the spot-heated water creates a sort of vacuum as well, until disturbed. I'm guessing at this and am not sure if I'm correct, but it makes sense to me.
Just a thought.
 

Offline Donnah

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Re: Heating Water in Microwave
« Reply #5 on: 06/07/2003 17:11:24 »
The above post is actually a condensed version of an email from a friend.  I was rushed when posting and didn't have time to elaborate.  I have heated water in the nuker many times.  Sometimes when I put in a scoop of coffee or chocolate powder the liquid would "explode" and most of the drink would be on the counter.  Now I have a cooler with hot and cold spigots, so it's not an issue in our house, I just didn't want any of you to get burned.
 

Offline Quantumcat

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Re: Heating Water in Microwave
« Reply #6 on: 07/07/2003 00:50:50 »
The microwaves are evenly distributed anyway aren't they, so heat everything at the same rate ... right?


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Offline Ians Daddy

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Re: Heating Water in Microwave
« Reply #7 on: 07/07/2003 05:39:03 »
Not when I nuke a burrito. They always get piping hot on the outside and are sometimes frozen in the middle. I went and got a new microwave with a turn-table in it and don't have this problem as much.
 

Offline chris

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Re: Heating Water in Microwave
« Reply #8 on: 07/07/2003 09:56:46 »
quote:
Originally posted by Quantumcat

The microwaves are evenly distributed anyway aren't they, so heat everything at the same rate ... right?



Not quite. Refer back to the discussion we had on microwaves and cooking about 3 months ago. The microwaves have a wavelength of about 12 cm. Waves have maxima and minima and hence regions of high energy [delivery to the food] and low energy.

Presumably this means that the waves passing through a cup of water might heat one part very well, whilst leaving another patch unheated. If the energy is delivered to the water faster than convection and radiation can redistribute it a region of the water could become extremely hot. If the hotspot is in the middle of the volume of water it cannot form vapour so easily and escape. I think.

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Offline Quantumcat

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Re: Heating Water in Microwave
« Reply #9 on: 11/07/2003 11:17:43 »
If a hotspot couldn't form vapour and get out of the middle of the water because of the weight of the water, you wouldn't get bubbles rising from the hot part at the bottom of a saucepan of water on a hotplate..... If the vapour can indeed not escape, surely there must be another reason. And heat moving from one part of the water to other parts is surely faster than 30 seconds or so of having the water in the microwave .... right? I'm probably going to embarass myself by writing this as it's most likely all wrong, but I don't care, at least i can learn how it actually works instead of just silently doubting it ...

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Offline lightarrow

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Re: Heating Water in Microwave
« Reply #10 on: 25/08/2006 13:30:10 »
Such a sudden boiling of water can happen even heating water in a new glass flask with a flame or other heater. I saw it in an experiment at high school.

So, the paradox: "why doesn't water boils at 100C in that case?"

The reason is: to boil, vapour bubbles have to form from the inside of the liquid, but for this to happen, a bubble have to win the liquid's surface tension (very high for water) because the creation of a new surface inside of a liquid requires energy; furthermore, the smaller the bubble, the more difficult it is for vapour to expand, so, when some bubbles finally form, then their expansions goes faster and faster, almost as a chain reaction, and the liquid boils suddenly.
 

Offline lightarrow

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Re: Heating Water in Microwave
« Reply #11 on: 25/08/2006 17:36:56 »
The above effect can happen better when water is heated slowly, without any movement, in the most homogeneus way is possible, with new containers and pure water, as it has been said. A microwave oven is better suited for this, because it heats all the water at the same time, it's not like a flame which heats only the bottom of the liquid.

In laboratories, when we have to heat liquids, we usually put little pieces of inert material inside the liquid (glass, ceramics ecc.) in order to facilitate the nucleation of vapour bubbles and preventing sudden bursts of vapour, who can easily shatter the glass container.

Even if a bit later, I would like to say: very well! to Quantumcat attitude:
"I'm probably going to embarass myself by writing this as it's most likely all wrong, but I don't care, at least i can learn how it actually works instead of just silently doubting it ..."
Very well Quantumcat!
 

Offline jysk

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Re: Heating Water in Microwave
« Reply #12 on: 25/08/2006 19:41:44 »
quote:
Originally posted by lightarrow



Even if a bit later, I would like to say: very well! to Quantumcat attitude:
"I'm probably going to embarass myself by writing this as it's most likely all wrong, but I don't care, at least i can learn how it actually works instead of just silently doubting it ..."
Very well Quantumcat!




I agree with and admire this attitude.

Mike
 

Offline ukmicky

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Re: Heating Water in Microwave
« Reply #13 on: 25/08/2006 20:19:45 »
So the water molecules turning into steam come together slower than the water heats up.

Michael
« Last Edit: 25/08/2006 20:23:52 by ukmicky »
 

Offline lightarrow

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Re: Heating Water in Microwave
« Reply #14 on: 26/08/2006 08:44:50 »
quote:
Originally posted by ukmicky

So the water molecules turning into steam come together slower than the water heats up.
If you mean that the steam forms after that the water has been heated up, the answer is yes.

The problem is that, without the presence of conditions which help the creation of little bubbles, the water can remain in a "super heated" state, not an equilibrium state of course, even without any more heating at all, indefinitely.

 (In this case of course it shouldn't loose heat with the environment).
« Last Edit: 26/08/2006 08:51:13 by lightarrow »
 

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Re: Heating Water in Microwave
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