Why the flask must be sealed with the stopper while the reaction takes place, please answer ​

Identify a specific example for each type of technology

Silver tarnishes as silver metal reacts with hydrogen sulfide, h2s, in the air. in this reaction, dark silver sulfide, au2s, covers the surface of silver. when silver is polished, this coating of silver sulfide can be removed from the surface. this makes the silver shiny again. enter the coefficients that balance the tarnishing reaction equation. (type 1 for no coefficient.)

What forms when chemical reactions combine pollution with sunlight?