Which of the following metals is the least reactive?

Four groups are given linking a metal, its ore, and the chemical formula of the main component of the ore. How many of these are correct?
$(1)$ Iron - Magnetite - $Fe_{3}O_{4}$
$(2)$ Iron - Siderite - $FeS_{2}$
$(3)$ Copper - Copper glance - $Cu_{2}S$
$(4)$ Copper - Dolomite - $Al_{2}O_{3}$

Which of the following is a mineral of sulfur?

Write chemical equations for the reactions taking place when:
$(i)$ iron is strongly heated in air.
$(ii)$ lead carbonate is calcined.
$(iii)$ chromium oxide $(Cr_{2}O_{3})$ is heated with aluminium powder.

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:
Those minerals which contain a very high percentage of a particular metal and from which the metal can be profitably extracted are called ores.