the reason is that in a solid state the molecules have little kinetic energy to move and so they have restricted movement and so it remains solid once heat is applied the molecules gain enough energy to move and so they turn into a liquid and move according to the laws of physics
The product of replacement reaction of 2Fe + 3 CUSO4 yield 2Fe(SO4)3 + 3 Cu
during replacement reaction involve replacement of less reactive element in the reactivity series from its compound. Fe is more reactive in the reactivity series than cu therefore it Fe replaces cu from CUSO4
