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

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

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

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

An iron nail dipped in copper sulphate solution turns brown. This is due to which one of the following types of reactions?

Copper sulphate crystals available in the market are blue coloured crystals. By careful heating, they turn to white colour. Which one of the following is responsible for the blue colour ?