Water and coal are made to react by passing steam over red hot coke at very high temperature, producing a mixture of carbon monoxide and hydrogen which is called water gas.Carbon monoxide is converted into carbon dioxide by passing the water gas and excess steam over the catalyst iron (III) oxide and chromium (III) oxide at very high temperature.Carbon monoxide is separated from hydrogen gas by passing it through water at a pressure of 30 atmospheres due to which carbon monoxide is absorbed in water unlike hydrogen gas.