(a) No reaction takes place when dilute sulphuric acid is poured on a copper plate.
However, when concentrated sulphuric acid is poured on a copper plate, hydrogen gas evolves along with the formation of copper sulphate. When the product is allowed to cool down, there is formation of blue coloured crystals of copper sulphate.
The chemical reaction of concentrated sulphuric acid and copper is:
Cu(s) + H2SO4 (conc.) ---> CuSO4(l) + H2(g)
(b) When iron nails are placed in a copper sulphate solution then iron being more reactive than copper, it displaces copper from its solution.
With the formation of ferrous sulphate, the blue colour of copper sulphate fades and the solution appears white in colour (colour of ferrous sulphate in water).Copper gets deposited on the iron nail.
Fe(s) + CuSO4(l) ---> FeSO4(l) + Cu(s)