[cliverlong]

If aluminium is a fairly reactive metal, I'm guessing it forms an oxide layer on contact with the atmosphere. This, I'm guessing, would make the metal dull in comparison to "new" aluminium.

If true, then why is aluminium foil shiny?

Thanks

Clive

[skyjumper]

Aluminum actually takes quite a while to oxidize in our atmosphere (relatively speaking) There is also a good chance there is some kind of barrier coating on the metal, stooping or at least slowing the rate of oxidation

[NaOH]

Probably it oxidices, I've noticed it get dull and unshiny when you leave it on a humid place like a closet for a long time.

[Borek]

Aluminum is always (assuming it is in contact with air) covered by thin layer of oxide, layer that prevents further oxidation. If this layer is thin enough, aluminum looks shiny.