Answer: hyperbole
Step-by-step explanation:
"Evaporated into thin air" is an hyperbole. It simply means that something has completely vanished or in a situation whereby nobody knows the whereabout of someone or a particular thing.
For example, "The book on my table has evaporated into thin air" simply means that the book was on my table but now, I don't know where it is as it can't be found.