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OT - Mythbuster
 

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April
Joined: 13 Dec 2003
Posts: 6592
Location: Westchester/NYC

1/14/13 10:40 PM

OT - Mythbuster

A while back, an e-mail made its way into my inbox. In it, a 'warning' that one shouldn't microwave water as a way to heat it up, because it might explode.

Reading through the 'reasoning', it said the water might superheat to beyong its boiling point, and then upon contact with anything else, will explode.

Being a physics major, I thought that's not very likely. The only way water can be superheated is if there's NO impurity in it. Otherwise, the water next to any impurity will boil first so the water never got to be superheated...

Funny enough, the TV show Mythbuster did a segment on that and that was their conclusion too. They did an experiment with distill water (pure) and prooved it could be superheated in a microwave and went boom! But for some reason, they didn't bother to experiment with tap water but only said it can't be superheated. And that's their way to say the myth has been busted.

But I didn't think I needed that proof. I reached the same conclusion independently.

Lo and behold, today I put some water in the microwave and set the cook time to 5 minutes (it's a big cup). Since that's a long time, I went out of the kitchen and got busy with other things... "Boooom!" I rushed into the now slightly flooded kitchen. The microwave door was blown open and water was everywhere except in the cup!!!

Not believeing my eyes, I dutifully fill up another cup of tap water and set the microwave timer to 5 minutes again... and stay well away... "Booooom!" AGAIN!!!

Still haven't quite figure out why it does (superheated). But it's reproducible, at least from my tap!

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ErikS
Joined: 19 May 2005
Posts: 8320
Location: Slowing boiling over in the steamy south, Global Warming is real

1/15/13 4:17 AM

Pure water freezes strangely too.

There was an airbus flight from south America to France that crashed because of lash freezing of water.

Out over the Atlantic the water in the storms that brew there is technically distiller because of no dust etc being in the atmosphere. Water won't freeze until it has impurities in it. The aircraft hit an storm and even with probe heat on the water flash froze on the air data froze and the flight computers failed.

The testing proved the event and it is wild to see the water freeze instantly when they put a stirrer in the beaker.

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daddy-o
Joined: 12 Apr 2004
Posts: 3304
Location: Springfield

1/15/13 7:07 AM

It's the cup, not the water

The "impurity" that makes water boil is a flaw in the boiling vessel.

If you just heat it up for a little less time than it takes to 'boom' and then put in a spoonfull of something in it like coffee, sugar, salt, rice and so on; it will boil furiously. If you just put in one grain of rice, or a tiny pinch of the granular items, it will boil but not so crazy. Even sand would work, but since we're talking food.

A newly glazed cup or glass measuring cup (bowl, etc) doesn't have the flaw (scratches) to make the water boil predictably.

I've seen this happen a couple of times too many for my liking. These days I'll put one piece of rice or a couple of coffee grounds, or even a little piece of paper in with the water to act as a boiling indicator designed to avoid an accident like you described or an accident like I described that happens when I put in a thousand grains of salt, sugar, coffee, etc.

Try it! It isn't dangerous considering what you've witnessed so far. One experiment is just messy, the safe one will become a standby.

FWIW the water I use is from a Brita filter pitcher, but I've seen it happen with a couple of localities' tap water too.

Freezing is similar too, but I haven't studied it like Erik. Notice how ice cubes from your freezer will freeze together the first time you pour something over them? Their surface is flawed and crystals can form rapidly and form bridges between the cubes. After first drink is done, the flaws have melted away and the surface is polished smooth. The drink machine at 7-11 delivers partially melted ice, the flaws are melted away and the ice doesn't freeze into a solid mass that is less appetizing (convient.) That's a crazy story Erik. I remember the crash, way out in the middle of the Atlantic, Air France IIRC.

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April
Joined: 13 Dec 2003
Posts: 6592
Location: Westchester/NYC

1/15/13 9:19 AM


quote:
A newly glazed cup or glass measuring cup (bowl, etc) doesn't have the flaw (scratches) to make the water boil predictably.

The cup is about 10 year old (for me, how many years before I bought it in the yard sale is unclear)!

Yes, it occurs to me I would in the future put a bit of whatever I was going to add to the water as a boiling seed.

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Sparky
Joined: 08 Dec 2003
Posts: 17552
Location: Portland, OR

1/15/13 9:21 AM

IIRC, putting a coffee stir stick or chop stick which breaks the waters top surface tension keeps it from reacting violently at that temperature event horizon.

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KerryIrons
Joined: 12 Jan 2004
Posts: 3018
Location: Midland, MI

1/15/13 7:52 PM

Thermodynamics

The explosion comes because you are driving past the boiling point rapidly by pouring large amounts of energy via the microwave oven. You could not produce this same effect if you heated the water via conduction (on the stove top. It is not a matter of whether the water is pure or not for superheating but due to the very rapid heatup. Superheating is about nucleation (physical solids) not about chemical purity. And generally tap water is relatively free of particulates.

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April
Joined: 13 Dec 2003
Posts: 6592
Location: Westchester/NYC

1/15/13 9:05 PM

You're right Kerry. I used the incorrect term. It's the physical particle we're talking about.

Still, I think tap water has plenty of those physical particles. It's not a prestine environment the water travels through.

I can see your point about rapid energy absorption too. But I'm not sure it's rapid enough to create superheated liquids

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bboston75
Joined: 16 Jan 2004
Posts: 365
Location: philadelphia

1/15/13 9:22 PM

Doesn't the microwave heat all the water pretty uniformly at once, rather than depending on convection the way stove heating would? If the whole mass boils at one time....

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Sparky
Joined: 08 Dec 2003
Posts: 17552
Location: Portland, OR

1/15/13 10:02 PM

BTW, IIRC boiling water in a micro wave negates some of the minerals which are actually good for you in the water.

I have a Reverse Osmosis filter for drinking water, both at the fridge and the sink, and the ice maker also is fed from the RO. I am adding this because RO filtered water removes even more, but also remove not so good stuff too. ;)

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daddy-o
Joined: 12 Apr 2004
Posts: 3304
Location: Springfield

1/15/13 11:59 PM

Just try the trick outlined above. If it boils regular-like, use a different cup but take note of the time to boil and use that as a gauge for superheating. You really don't want it super-duper heated.

Microwaving does not heat evenly. Reheating leftovers from the microwave proves that. They always need to be stirred, even on a turntable microwave. It probably heats more evenly than a stove though. Imagine not stirring something on a stove.

Yes, the water is superheated and microwaving makes it easy. April used the best term, boiling seed. The same scratch in a pot will be where the water always boils.

Try microwaving some oil, I've never been able to make it warm up much at all.

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