Purification of gas is a prerequisite for many processes.

(1) If heating is used to remove impurities, it is generally dry before. For example, if O2, CO2 and H2O (g) are mixed in N2, CO2 and H2O (g) can be removed by passing the mixed gas through alkali lime, and then the gas derived from the drying pipe can be removed by passing through the hard glass pipe equipped with hot copper mesh to get dry and pure N2. If the mixed gas is first passed through the hot copper mesh, the hard glass tube is easy to crack due to the water vapor mixed in the gas.

(2) If solution is used for impurity removal, it should be before impurity removal and after drying. The reason is: water vapor will be brought into the solution during the gas impurity removal process, and the desiccant can remove the water vapor. For example, in the laboratory, marble and dilute hydrochloric acid are used to prepare CO2. To prepare dry and pure CO2, the generated gas can be first passed through saturated NaHCO3 solution to remove the mixed HCl gas, and then the water vapor can be removed through concentrated sulfuric acid.