Silver articles become black after some time when exposed to air because

Silver articles turn black when kept in the open for longer time due to the formation of

Which one of the following substances do silver articles react with, that makes the silver articles black ?

Food cans are coated with tin and not with zinc because

When air is blown from mouth into a test tube containing limewater, the limewater turns milky. This is due to the presence of

Which one of the following compounds is used in 'black and white' photography?