Why copper coin when put in a beaker of water doesnot get oxidised for many days but when another copper coin made just wet and left gets tarnished in few days and then oxidised to a black layer after few days

Question

Why copper coin when put in a beaker of water doesnot get oxidised for many days but when another copper coin made just wet and left gets tarnished in few days and then oxidised to a black  layer after few days

anonymous · Answer

What do you need to be present for the copper to oxidise ?

anonymous · Answer

Oxygen

anonymous · Answer

But doesnot the water contain oxygen too. What is the difference in this case... Please help me understand

anonymous · Answer

Well I'm not an expert in this area, but surely the concentration of oxygen would make a difference, don't you think ?

anonymous · Answer

Okay. Let me think in that direction. Thank you.

anonymous · Answer

I think the concentration of oxygen in air is about 25 times greater than the concentration of oxygen in water at normal pressure and room temperature.

anonymous · Answer

I think that answers it.