Rust !
Yes, iron can rust in water. When iron is exposed to both water and oxygen, a chemical reaction occurs where the iron oxidizes, forming iron oxide or rust. This process is accelerated in the presence of impurities in the water, such as salts and minerals.
Two different molecules are water (H2O) and carbon dioxide (CO2). Water is a compound composed of two hydrogen atoms and one oxygen atom, while carbon dioxide is a compound made of one carbon atom and two oxygen atoms.