How does the addition of alum to muddy water help in its purification? Name and define the process involved.

Farhad heated some water until it was boiling. He then put in an egg, and let it cook in the boiling water. He told his father that as the boiling of water is a reversible physical change, the eggs that were placed in the boiling water too underwent a reversible physical change. Was Farhad correct about the change in the eggs? Why, or why not?