The deep red vapours obtained when a chloride is heated with $\mathrm{K}{}_2\mathrm{Cr}{}_2\mathrm{O}{}_7$ and concentrated $\mathrm{H}{}_2\mathrm{SO}{}_4$ consist of

Chloride, dichromate and concentrated sulphuric acid give volatile chromyl chloride, $\mathrm{CrO}{}_2\mathrm{Cl}{}_2$, as orange-red to deep red vapours. It is the basis of the chromyl chloride test for chloride.