Aluminum Foil - Why does it burn

[GoodSmeagol]

Hello.
Sciencey question for all of you.

Aluminum!

Melting point Al: 933.57 K (660.32 oC)

The reported temperature for a bic lighter: 600 - 800 C.

Aluminum burns at over 4,100 K (6920°F),

How come when I hold a bic lighter to the edges of aluminum foil, it seems to burn??

Thanks a bunch

[Pappa]

Is it just because it's such a thin section of metal, so it's able to burn at a lower temperature?

[Ironclad]

What's aluminum?

[GoodSmeagol]

Is it just because it's such a thin section of metal, so it's able to burn at a lower temperature?

I think that has something to do with it, however I think what happens is the metal oxidizes and the bonds between the particles break down. I also believe that it is not the actual metal that burns but the AlO (Aluminum Oxide) that does.

However I know this only from reading a few web pages, I am hoping someone can explain it to me in a better way.

[Ulven]

This is an educated guess.

When you heat Al up to, let's say 800 degrees, some oxidation will take place. Not enough to cause an impression of burning, but nevertheless an oxidation. The oxidation process in itself gives off heat which will cause the temperature to rise further. Because a thin sheet of aluminium conducts less heat than a thick slab or an ingot, the local temperature will eventually rise to the point where the foil actually burns. In a thick slab of aluminium, the heat will be conducted in three dimensions and therefore removed more efficiently from the heated area. The temperature never rises to the point where the aluminium burns.

[Warren Dew]

If you move the lighter away from the foil, does the foil continue to burn on its own?

If not, I think it's not actually burning, just oxidizing - rusting - more rapidly than usual.

[GoodSmeagol]

If you move the lighter away from the foil, does the foil continue to burn on its own?

If not, I think it's not actually burning, just oxidizing - rusting - more rapidly than usual.

This is close to what I have read, and what I had presumed was going on.

This would however force me to think about all those hot dogs I have thrown in a fire wrapped in aluminum foil....

[Bella Fortuna]

What's aluminum?

It's aluminium with the stick removed from its arse.

[FBM]

What's aluminum?

It's aluminium with the stick removed from its arse.

[JimC]

Aluninium metal is actually highly reactive. However, it is always found coated with a thin, tough, transparent layer of aluminium oxide. A freshly cut surface forms this layer instantly.

Unlike iron oxide, aluminium oxide bonds firmly to the pure metal underneath, meaning that aluminium needs less corrosion protection than many other, technically less reactive metals.

However, when aluminium (and other reactive metals) are powdered, all bets are off. The small grain size and the huge surface area mean that, on reaching ignition point in an atmosphere containing oxygen, the protective layer is not stable enough to prevent a highly energetic combustion reaction. Thermite is actually a mixture of aluminium powder and an oxidising agent, bonded with resin.

I suspect that the conditions at the edge of aluminium foil destabilize the layer of oxide, allowing oxygen molecules to reach the metal atoms themselves.

[GoodSmeagol]

Thanks JimC!

[mistermack]

It's a confusion of terms here. Something burns if it gets so hot, it produces a gas which combines with oxygen. The burning gas is called a flame. Melting doesn't necessarily come into it, although a metal would normallly melt before it vapourised.
The aluminium doesn't need to melt or vapourise to oxidise, it can oxidise at much lower temperature, but it usually stops once the oxide forms a skin and stops the oxygen getting to the unoxidised metal.

I think foil is so thin, the heat can make it move around, causing splits in the oxide coating, so it can carry on oxidising, till it's completely oxidised.
It didn't burn, because it never vapourised, and it didn't need to melt to oxidise.

[Tigger]

What's aluminum?

It's aluminium with the stick removed from its arse.

There.

[Faithfree]

Same thing with iron, which has a much higher melting point than aluminium. Get some "steel wool" and touch it for just a second with a flame of any sort and take away the source of heat. The iron continues to burn rapidly. No melting involved, it's just the thinness of the filaments that allow it to burn.

[mistermack]

Same thing with iron, which has a much higher melting point than aluminium. Get some "steel wool" and touch it for just a second with a flame of any sort and take away the source of heat. The iron continues to burn rapidly. No melting involved, it's just the thinness of the filaments that allow it to burn.

Yes, I don't know if that is defined as burning or smouldering. I thought burning was with a flame, and smouldering was a flameless process, I'm not so sure now.
But it's all oxidation, or combustion.