At work (pizza delivery place), we cook our chicken wings by placing a sheet of aluminum foil on one of our pizza screens, a piece of “parchment” paper on top of the aluminum, then the wings on the paper.
When it comes out of our 560[sup]o[/sup] oven, we casually grab the foil, place it in a wing box, remove the paper, and place sauce on them.
My question: why aren’t we burned grabbing a piece of metal that’s been in a 560[sup]o[/sup] oven for 10 minutes?
IANAChemist but I believe that it has to do with a combination of Aluminum’s heat capacity (the amount of heat which is stored in a certain amount of a substance), it’s low density (less mass = less heat stored), and it’s thermal-conductive properties. I think that the biggest overall factor in this case would be the tiny mass of a piece of foil not storing enough heat from the oven to damage your fingers. Water (what your fingers are mostly made of) has a heat capacity that’s almoat 3 times greater than aluminum, meaning that it takes 3 times as much foil to store the heat required to heat up your finger by a comperable amount. The light weight foil, despite it’s high thermo-conductivity, just doesn’t have it in it to boil your water.
Mirage has it - just not enough heat capacity in the foil is the main reason.
Mirage has it. Aluminum foil at 560F doesn’t transfer enough heat (which is a function of temperature, mass, thermal conductivity, duration of contact, and heat capacity) to raise your skin temp very much.
Similarly, you can pour liquid nitrogen over your hand without it feeling very cold.
Remember, boys annd girls:
Q = MC[sym]D[/sym]T
Where Q is the amount of heat energy transferred, M is the mass of the object, C is the heat capacity of the material, and [sym]D[/sym]T is the change in temperature. Since aluminum has a low C, it doesn’t release much energy in cooling down.
Oh, admit it - you just wanted to show off your [sym]D[/sym]. 
Actually, I think the main reason why the aluminum doesn’t burn you is that it is not 560 F. The oven you’re using heats primarily by radiation, and since the aluminum foil is highly reflective (low emissivity) and doesn’t heat up quickly.
Pure aluminum has a specific heat roughly twice that of carbon steel, but if you put a thin sheet of steel in your oven I suspect it would burn you, even though it has a much lower thermal conductivity.
I disagree cynic. I think the foil would almost instantly reach the oven air temperature.
This cite has a good explanation of why there is so little heat transfer to the foil:
http://www.u-b-kool.com/PoF3.htm
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The heating rate is small, but the equilibrium temperature is still the air temperature of the oven. It shouldn’t take long for everything in the oven - including air, walls, food and the aluminum foil - to reach the same temperature. (At least, the surface of the food - I know the center of the turkey may not reach the oven temperature for hours).
Cynic, I think you’ve missed the point. In your purely radiative oven, sure, the foil would take some time to warm up, but in a real oven, the foil is not only in contact with the air, but also (unless it’s suspended) in contact with other surfaces (at some points along its surface). Conduction from the air and from the surfaces in contact with other hot surfaces will be extremely fast.
The large surface area/volume ratio of foil will allow it to cool down very rapidly once it leaves the oven.
Well, I AM a chemist, and I must state with the utmost emphasis, DO NOT TRY THIS AT HOME!!!
Having poured/dispensed gobs of LN[sub]2[/sub] in my day, I can attest to skin damage if even a small drop hits soft tissue (inside of arms, etc.)
I use the stuff in lab all the time, too. I’ve tried the experiment I mention and have suffered no ill effects. The secret is to pour a small, steady stream over the palm. It feels cool, but not cold. Skin damage probably occurs if a droplet becomes lodged between skin surfaces. This instantly vaporizes the drop with resultant rapid cooling of the skin.
My experiences notwithstanding, I’m sure your friendly safety officer would frown on this type of recreation. Please don’t try it.
How 'bout liquid aluminum? :D:D
How 'bout liquid aluminum? :D:D
Actually, the C in your equation is the specific heat of aluminum (an intensive property), which is equal to the heat capacity of the object in question divided by its mass.
Since heat capacity equals the mass of the object times its specific heat, Anthracite is absolutely correct, as usual. 
However, if we look at waterj2’s equation and consider the effect of specific heat vs. the mass of the object, I would argue that the main reason is the small mass of the aluminum foil. If you picked up a 1-kg ingot of aluminum that had just been removed from the oven, I think you would agree.
Also, as ticker mentioned, the foil’s large surface area to volume ratio allows the aluminum to cool off very rapidly once removed from the oven.
Yes, this experiment works, but not because it has much to do with the specific heat of liquid nitrogen. It is a demonstration of the Leidenfrost Effect, the same thing that causes a bead of water to skitter across a hot skillet. A thin film of water turns to steam and allows the rest of the drop to “float” without touching the skillet. Likewise, if you hold your hand taught and flat, a small amount of liquid nitrogen (too small to absorb much heat from your hand, or to make it feel more than “cool”) vaporizes and allows the rest of the liquid nitrogen to pass over your hand without touching it. If you held the liquid nitrogen in a cupped part of your hand, or allowed it any lingering contact with your skin, it is plenty cold to do some major damage.
However, I’m willing to bet that thirty seconds after you take the foil out of the oven, you could ball it up and squeeze it between your palms with no ill effects. Not the Leidenfrost Effect, but the fact that it’s low specific heat has already gotten rid of the energy.
-b
Yep, if kids learn only ONE physics concept, they should be taught the difference between heat and temperature. Most people think these are the same thing, but if we understand the difference we can understand all sorts of things more easily.
Well, maybe they should learn that heat=kinetic energy first. But definately heat!= temperature second.
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True, true. While the mechanism is somewhat different from why hot foil doesn’t burn the skin, my point was that asin the case of the foil, less heat is transferred than you’d imagine.