phenol loses its H+ when kept in an acidic medium. due to which phenate ion is formed. this ion formed, is more stable than phenol, due to it's resonating character. hence phenate formation is favored. and the release of H+ is there. this release of H+ makes phenol acidic

phenoxide ion is more stable than phenol. Hence the hydrogen leaves easily!

Test like ... refer Ncert text book in chapter alcohols and phenols!there are some of the test which prove that phenol is acidic!:))

because of presence of l.p. on oxygen and as it is attached to conjugated benzene ring a conjugated sys. ensures that l.p. of oxygen due to resonance rotates in the ring and as there is deficiency of charge i.e. ve charge is there at c-o bond leading to acidity