Why do the resurrected bodies disappear in the impure world resurrection jutsu?
The souls can be freed from the Impure World Reincarnation if they are given closure. Experiencing a emotional change allows them to break out of the jutsu and the soul can ascend.  

Orochimaru stated that Hashirama Senju could break the summoning jutsu if he wanted. I'm not sure why he chose not to during the fight with Hiruzen Sarutobi.