Selenic Acid, H₂SeO₄, is also a colourless syrupy liquid; it can be produced by direct oxidation of selenium by chlorine water, but on concentration the resulting hydrochloric acid reduces the selenic acid to selenious acid, as hydriodic acid reduces sulphuric acid. It is best prepared by addition of copper carbonate to the mixture of selenic and hydrochloric acids obtained in that way; selenate and chloride of copper are formed; the mixture is evaporated to dryness, and the copper chloride is dissolved out with alcohol, leaving the insoluble selenate behind. The selenate is dissolved in water, and on treatment with sulphuretted hydrogen, copper sulphide is precipitated, and removed by filtration ; the selenic acid is then concentrated; if it contains a trace of water, it is a heavy liquid; but if quite anhydrous, it forms a solid, melting at 5 8°.