Which one is a chemical change: Rusting of iron or melting of iron?

Silver articles become black on prolonged exposure to air. This is due to the formation of

State whether the following statement is true or false:
Keeping food in airtight containers helps to slow down oxidation.

Silver articles become black on prolonged exposure to air. This is due to the formation of:

$A$ silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.
$(a)$ Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.
$(b)$ Name the black substance formed and give its chemical formula.

Explain the term 'Rancidity' with one example.