Why you use only sulphur powder in hays test?

In the Hays test for the detection of arsenic, you do not use only sulphur powder. The test involves a three-step procedure that utilizes three different reagents:

1. Distillation: In this step, you use concentrated sulfuric acid (H2SO4) to decompose the organic matter in the sample and liberate arsenic, if present, as arsine gas (AsH3).

2. Generation of arsine gas: The liberated arsine gas from the previous step is passed through a glass tube containing lead acetate paper, which turns black if arsine is present.

3. Confirmation of arsenic: To confirm the presence of arsenic, you then introduce hydrogen sulfide gas (H2S) into the arsine-containing tube. This results in the formation of a yellow precipitate of arsenic sulfide (As2S3) on the paper.

So, while sulphur powder (in the form of hydrogen sulfide gas) plays a crucial role in the confirmation step of the Hays test, it's not the only reagent used in the entire procedure.