Se + 2 Cl₂ + 3 H₂O → H₂SeO₃ + 4 HCl

This is an oxidation-reduction (redox) reaction:

Se is a reducing agent, Cl₂ is an oxidizing agent.

Reactants:

• Se
  • Names: Selenium , Se , Selen ... show moreElement 34 , 34Se , Elemental selenium , Selenium alloy show less
  • Appearance: Grey solid in various forms ; Amorphous or crystalline, red to gray solid. [Note: Occurs as an impurity in most sulfide ores.]
• Cl₂
  • Names: Chlorine , Molecular chlorine
  • Appearance: Greenish-yellow compressed liquefied gas with pungent odour ; Greenish-yellow gas with a pungent, irritating odor. [Note: Shipped as a liquefied compressed gas.]
• H₂O – Water, oxidane
  • Other names: Water (H2O) , Hydrogen hydroxide (HH or HOH) , Hydrogen oxide ... show moreDihydrogen monoxide (DHMO) (systematic name) , Hydrogen monoxide , Dihydrogen oxide , Hydric acid , Hydrohydroxic acid , Hydroxic acid , Hydrol , Μ-oxido dihydrogen , Κ1-hydroxyl hydrogen(0) show less
  • Appearance: White crystalline solid, almost colorless liquid with a hint of blue, colorless gas

Products:

• H₂SeO₃ – Selenous acid , Selenic(IV) acid
  • Other names: Selenious acid , [SeO(OH)2] , Selenige Säure , Monohydrated selenium dioxide
  • Appearance: White hygroscopic crystals ; Colourless hygroscopic crystals
• HCl – Chlorane , Hydrogen chloride
  • Other names: Hydrochloric acid , M , Uriatic acid ... show moreHydronium chloride , Chlorhydric Acid , Hydrochloric acid gas , HCl , Anhydrous hydrogen chloride , Hydrochloric acid, anhydrous , Aqueous hydrogen chloride , [Note: Often used in an aqueous solution.] show less
  • Appearance: Colorless, transparent liquid, fumes in air if concentrated ; Colorless gas ; Colourless compressed liquefied gas with pungent odour ; Colorless to slightly yellow gas with a pungent, irritating odor. [Note: Shipped as a liquefied compressed gas.]