Why is a block of gold more dense than a block of iron of equal size ?

-- Gold is a more dense substance than iron.  At 19.3 times the density
of water, it's in the top few most dense substances in all of nature.
The density of iron is only 7.87 ... only 40% the density of gold !

-- Density doesn't depend on how much of the substance you have.
A flake of gold foil is still more dense than an iron battleship.

490 n  f=125n a=2.5m/s2