what common compund would react with blackened paint by h2s to give original colour

The common compound that would react with blackened paint, typically black due to the presence of iron oxide or a similar pigment, with hydrogen sulfide (H₂S) to restore the original color is zinc oxide (ZnO).

Explanation:

1. H₂S Reaction: Hydrogen sulfide reacts with metal oxides to form metal sulfides, which can change the color of the paint.
2. Zinc Oxide (ZnO): ZnO can react with H₂S to produce zinc sulfide (ZnS), often a whitish or lighter compound, which can restore the original appearance of a black paint by reacting with other compounds present.

In summary, applying a compound like zinc oxide in the presence of H₂S can help in reverting blackened paint back to its original color.