When iron and oxygen combine, rust forms. Iron can rust in contact with water, oxygen, or acids. Iron rusting is a redox reaction. Iron plus water plus air forms iron oxide or rust. Iron will lose electrons in this reaction and become a positively charged ion in water. The electrons that are lost are used to reduce the oxygen dissolved in water. The iron ions and the hydroxide ions that are negatively charged in water will react, to produce iron hydroxide. This eventually will form Fe2O3 or rust. When iron rusts, it is an exothermic reaction. This means that heat is released, however, in this particular reaction, it is occurring at a slow pace and a small amount of heat is released.