Question

Why melting of butter is reversible whereas curdling of milk is irreversible?

Answer 1

Formation of curd from milk is an irreversible change, as milk cannot be obtained back from curd.So it is a irreversible change.
But when butter melts, it goes from a solid state to a liquid state. That’s a physical change. It’s easily reversible if you return the butter to a cold place where it solidifies again.