Why metal(eg. iron)placed at aqueous solution will be more easily occur rusting?Is it due to the mobile ions in the aqueous solution ?Please give me an detail explanation ,thank you.

Answer 1

See "oxidation of iron metal" at
http://en.wikipedia.org/wiki/Rust

Rust results in the formation of iron hydroxide at the surface, under the combined action of water and oxygen. The iron oxide does not form a protective layer, but instead flakes off.