When sodium chloride is heated with conc. H2SO4 in presence of K2Cr2O7 , deep red vapours of chromyl chloride are evolved.
When these vapour are passed through NaOH solution, the solution becomes yellow due to the formation of sodium chromate.
The yellow solution is neutralised with acetic acid and on addition of lead acetate gives a yellow precipitate of lead chromate.
Note:
(i) This test is not given by the chlorides of mercury, tin, silver, lead and antimony. In such cases this test may be performed by taking the residue obtained after evaporation of sodium carbonate extract.
(ii) The chromyl chloride test is always to be performed in a dry test tube; otherwise the chromyl chloride vapours will be hydrolysed in the test tube.
(iii) The test is said to be positive when all the three observations viz., orange-yellow (red) vapours of chromyl chloride, yellow solution of sodium chromate, yellow precipitate of lead chromate, are correct.
(iv) Sometimes, a white precipitate is obtained after the addition of lead acetate solution even in absence of chloride. This may be due to strong heating of the mixture with conc. H2SO4 when H2SO4 vapours are absorbed in NaOH solution or due to incomplete neutralisation of NaOH solution which reacts with lead acetate to form lead hydroxide.
(v) Bromides and iodides do not give this test.
