Under normal conditions, you can only heat water to 212° F., then
it begins boiling. Once boiling, any more energy delivered to the water
only makes it boil more vigorously.
Superheating is an unusual event that can happen to anyone at anytime,
although it never seems to happen to bad people, only to nice people like
you and I.
Normally a pot of water on the surface of a stove begins to boil as the
temperature of the water at the bottom reaches 212° F. Then as steam
bubbles begin to form, they rise to the top and are released as steam. A
microwave oven heats all areas of the cup of water somewhat evenly.
It is possible to heat a liquid hotter than 212° F at normal
atmospheric pressure, and the liquid NOT boil! Of course, if
you continue heating it, it usually begins to eventually boil.
But when it's above the boiling temperature, the water is called
'superheated'.
And just the slightest disturbance of the surface causes
the water to suddenly and violently turn
to steam. When an amount of water is turned to steam, the vapor expands
roughly 1000 times the volume of the water!
It's not unlike the effect that drives
geysers. Part of the magic allowing the superheated condition is a
container with a very smooth surface (like glass) that doesn't hold small
or microscopic air bubbles when filled. If any bubbles are formed, and
released, when they break the surface, the boiling begins and prevents
superheating.
If a person reaches into the microwave oven and picks up the cup with
the superheated liquid, they may or may not be able to clear the microwave
oven before
they are severely burned with second and third-degree burns from the
water. This is why most recent microwave instructions include
'...then allow it to sit for 2 minutes before removing'
Remember the effect of the line voltage on the microwave's power?
If someone normally needs 1 minute to heat their coffee just below
boiling, then a 10% higher line voltage from the wall socket
can push the coffee into the
superheated region, making it a trap just waiting for you to touch it.
Another way you can accidentally make superheated liquids is to start
heating a cup of coffee, get interrupted with perhaps a phone call,
eventually return and reheat the coffee. If you waited longer than
30-40 minutes, the almost boiling coffee you nuked in the first session
may have cooled sufficiently. Any less time, and you will be guilty of
'stacking' - you reheat for your 'normal' time, but with coffee already
semi-hot - you really only needed a few seconds to heat it back to near
boiling, yet using your full burn time superheats it. Surprise!
How can you protect yourself? After heating a liquid, either allow it to
cool for at least 30 seconds to see if automatically progresses from a
superheated condition to boiling, or whether the temperature drops below
the potential danger zone. OR, if you are in a
hurry and can't wait - disturb the surface, either by touching it
with a spoon or bumping the cup with a spoon or pencil to see if
it suddenly explodes. Maybe you can get your arm back out of the oven in
time to prevent getting scalded. Maybe not. Your call.
Most discussions of superheating all assume it only occurs
in microwave ovens and labs, never with a standard kitchen stove. Wrong!
- Been there, done that!
I once assumed water, at one atmosphere pressure, would always boil at
212° Fahrenheit. So, to verify the temperature settings on a standard
kitchen
oven, I put a small Pyrex bowl filled with water in the oven and set
the temperature for a very low value - just below 200°. I let
the oven come up to temperature, checking the water status, while
peering through the glass oven window using the
oven's internal light. Then I began raising the oven temperature
in small increments - when I got above 212, I assumed I would see the
water boil - just like casseroles bubble and boil!
Time passed. MUCH time passed as I slowly raised the dial.
I felt some satisfaction knowing my original suspicion was correct,
the damn oven dial was incorrect. I started using larger increments,
deciding I would 'bracket it' and zero in on a second
attempt. But first I needed to get some boiling started.
As I reached the 500° mark, I knew something was amiss. The
water was not boiling, yet the oven was bristling with heat,
hot enough to feel several feet away from the oven. At this point
I actually opened the oven door to look directly at the water,
just in the event the image of the cool, unboiling water had somehow
etched itself into the glass of the oven door, preventing me from seeing
the real water. No such luck, the water was still there, and
not boiling. I turned the oven off and left the bowl in
place (thank goodness!!!!!) because it was obviously very hot.
This was just another failed experiment of mine - another 'learning
left in the lurch'
example. But the search for the reason for that failure haunted me
for a while afterwards.
Finally I latched onto the correct reason, almost. I theorized, as many
microwave oven superheat enthusiasts do, the water didn't boil because
it was evenly heated from top to bottom. On the stove top, the heat is
transferred to the bottom of the pot (and some to the sides), and there
is always some gradient of temperature from bottom to top. As the
bottom is heated to 212F, the water rises through the colder water and
is slightly cooled while the colder upper water replaces it. I
also assumed the temperature of the rising water would stay at 212F
until it reached the surface, where it would vaporize into steam, starting
the actual boiling process. And other small portions of water would
be heated sufficiently to turn to little steam bubbles that would
also rise to the top and each little bubble would 'violently' turn
to steam as it was released.
Yes, just like this oven, the microwave oven also heats evenly, and the
lack of swirling water of a pan on the stove top is definitely a
factor in superheating. But the real magic is the bubble formation
AND the release of a bubble or two to the disturb the surface,
then you have boiling! You might wonder about those casseroles
you pull bubbling from the oven, why don't they get superheated? It's
because they contain chunks of 'foreign' objects, like meat and
vegetables that cause uneven heating. If you have water or
'coffee-water' or even very well-stirred cocoa, you can get
superheating under the right conditions. Be careful!
And don't start with me about that crap saying injuries from
superheating are just urban myths! Using only the intelligence found
in ant feces, you will realize why more superheat incidents aren't well
reported. A person reaches in the microwave for their coffee, it
explodes burning their arm, and if it's severe enough, they go to the
hospital emergency room for treatment. Or perhaps someone is boiling
water on the stove making spaghetti, and as they pour it into the
colandar in the sink, they slip, pouring the water much too fast, it
splashes back on them, severely burning their arm. The doctor reports
both incidents the same - 'cooking accident - burned with
boiling water'. Even if the patient starts with the microwave story
or adds a routine about aliens bursting in while they were cooking and
burned them by peeing on them with boiling water, the doctor's report
will still be the same, 'cooking accident - burned with boiling water'.
Got that?
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