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Stainless steel U tubes can be used to dry gas and collect low boiling point liquids. In the U tube into the desiccant, by gas drying, in addition, the tube inserted in the anhydrous CuSO4 white powder in U, can through the gas check operation of the water, anhydrous copper sulfate is not stable, easy to absorb water, and the formation of a stable five hydrated copper sulfate CuSO4 - 5H2O. Blue, the color change can prove that the gas containing water vapor, the reaction is quick and sensitive.