[Tayl]

I'm not sure what forum this should go in

I recently put a few pennies in a water bottle with a vinegar and salt solution in it.

Now as far as I know, pennies are made of copper plated zinc, but I'm not a chemist so I don't know what's happening.

The pennies(I'll call them Cu, even though they're nowhere near completely copper) are dissolving in the vinegar(CH3COOH?)+salt(NaCl) solution

The sealed bottle is expanding with some sort of gas, can someone tell me what it is?

I'm sorry if this is completely obvious. I know nothing about chemistry

[Benzenelover]

It sounds to me like it's hydrogen gas, from the acetic acid.

[Tayl]

Edit: what gas would be produced when steel is dissolved in a vinegar + salt solution?

[Arkcon]

Well, mild steel won't react with a weak acid like vinegar to produce hydrogen, though it may combine and corrode, the same for copper..  If you had a stronger acid, like moderately concentrated sulfuric, or moderately concentrated nitric, you can expect hydrogen gas too.  Warning: stronger acids give different reactions.  Most likely, your penny experiment is the acetic acid in vinegar attacking the zinc core of a modern penny, seeping past the copper plating at a thin point, or crack, chip, micro-scratch, pore whatever.  So you should consider that experiment a reaction between zinc and acetic acid.

[Fluoroantimonicacid]

Well, mild steel won't react with a weak acid like vinegar to produce hydrogen, though it may combine and corrode, the same for copper..  If you had a stronger acid, like moderately concentrated sulfuric, or moderately concentrated nitric, you can expect hydrogen gas too.  Warning: stronger acids give different reactions.  Most likely, your penny experiment is the acetic acid in vinegar attacking the zinc core of a modern penny, seeping past the copper plating at a thin point, or crack, chip, micro-scratch, pore whatever.  So you should consider that experiment a reaction between zinc and acetic acid.

He is right, Cu is a very noble metal. Cu + 2H⁺ Cu²⁺ + H₂ reaction won't work.
But same with Zn works.
Also if Cu was reacted, Zn would reduce it to Cu again.