Why boiling when cold bottle is placed in hot bath?

[BronYrAur]

Ok, I know that this forum is supposed to be for industrial-type questions, but the physics behind this is boggling me.

When I heat my baby's bottle, I first bring a bowl of water to a rolling boil in the microwave. I let the bubbling subside and then place a cold (40 deg F) bottle of milk in the water.

I immediately get a second or two of violent boiling when I put the cold bottle in the hot water. I am confused by this. I could certainly understand it if the temperatures were the other way around. But why do I get boiling when I place a 40 deg F bottle into a 212 deg F bath. The bowl is open to atmosphere so I don't see how there could be any "flash" steam. Any thoughts?

 

[MintJulep]

Probably the cold bottle causes some very localized contraction of the water, resulting in some micro-bubbles that subsequently serve as nucleation points to trigger boiling.

 

[25362]

My thinking: after removing the heat, it appears the water is in a state of superheat. When introducing a spoon or any other (even cold) object, steam bubbles can form because of the created turbulence, or because the addition of surface irregularities that may serve as nucleation sites for bubbles to form.
If my thinking is correct this would frequently happen when the water boiling in the microwave oven is done by using smooth wall containers.

 

[moltenmetal]

Microwaves superheat water- they heat it so gently that you can take it well above its normal boiling point. The same thing happens in round-bottomed laboratory glassware heated with an external heating mantle- the liquid superheats, then "bumps"- boils forming one big bubble which displaces the liquid all over the place and generally makes a mess. You put a few "boiling chips" in there and you get lots of surface to nucleate boiling.

Whether your baby's bottle is cold or not is not the issue. Insert any extra surface into that superheated water and you are likely to nucleate boiling. Just be careful it is not a spoon- your fingers might be too close to the water and you might get an unwelcome suprise.

 

[unclesyd]

Sugar or sweetener are the best at creating havoc.

 

[MintJulep]

Molten,

You are correct that microwaving very easily creates superheated water. However, the OP states that he gets a rolling boil, which to me indicates boiling at saturation.

 

[25362]

Depending on the geometry of the water container and the oven type, it has been reported that radio microwaves quickly and directly heat up the exposed surface resulting in rolling boil, while the remainder is being heated up mostly by conduction and may reach a state of superheat as explained by moltenmetal.

 

[moltenmetal]

Despite bubbling while it's in the microwave, it settles down to a quiescent superheated liquid rather quickly when you shut off the microwaves. As has been mentioned, just dump in something with surface area- sugar, sweetener, anything- which nucleates boiling and BOOM- it bumps, sloshing boiling water all over the place. So something about the combination of glassware and a microwave oven combine to produce water superheated to a considerable degree.

 

[unclesyd]

Our lab use thing characteristic of heating by microwave to facilitate dry of heat sensitive samples. We had a solution of an almost saturated organic salt in water. The drying process consisted of putting the solution in a tall beaker and after a short heating period take the beaker out and the water would boil off. This was accomplished by energy input just from taking the beaker from the oven.

 

[EdStainless]

This is a serious safety issue. The worst case is to heat the water to boiling and then let it stand. The first boiling will drive off most of the disolved gasses that play a role in nucleation. Then re-heat the water. The water will be clam and still. And when you add a powder to it, place a spoon in it, or pour it, it will boil explosivly. This can cause serious burns.

Often when you boil water in a microwave the 'rolling boil' is mostly outgassing, not actually steam bubbles.

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[rmw]

GeeeWhiz. And just when I thought it was safe to go near the microwave again. Great thread. Who says you guys can't teach an old dog new tricks.

rmw

PS BronYrAur, I thought it a very appropriate topic for this forum.

 

[BronYrAur]

rmw, thanks for the reassurance about the validity of my question. I was "scolded" once in the electrical forum for asking what may be causing a high-voltage problem at my house.

 

[AbbyNormal]

Is boiled bottle then mainly water vapour over a liquid? Just wondering if the 40F coils the vapour, drops the pressure maybe?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[BronYrAur]

AbbyNormal, The bottle never boils. I just put it into a boiling water bath. The bottle is sealed and cold.

 

[AbbyNormal]

Well maybe the vapour cools fast and the pressure drops. In this video they had the water up to boiling first.

http://demo.physics.uiuc.edu/lectdemo/descript/1092/More info.html

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

I misread you, the water in the bowl starts boiling when you put the cold mik bottle in, my bad

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[imok2]

Fill a clean mug about 1/3 full of clean water (DON'T FILL IT TO THE TOP!), then heat it for about five minutes in the microwave oven. Now carefully take it out and immediately plunk it firmly onto the tabletop (whack it hard, but not so hard that it breaks.) The boiling water will burst into froth. DON'T BURN YOURSELF! The superheated water acts almost like warm carbonated cola: if you strike the container, it will foam up instantly.


Another trick: heat up the water to boiling again, remove it from the oven, then immediately insert a dry wooden coffee-stirrer, or a wooden popcicle stick into the water. Foosh! The water boils violently. The dry wood contributes a layer of air to the water, and the air fills with steam and expands into a mass of hot foam.