Demi Lovato contributed the song 'Heart by Heart' to the soundtrack of the movie 'The Mortal Instruments: City of Bone,' which is out Aug. 20. Consequently, that's the singer's 21st birthday.

A minute-long snippet of the track is posted below for your listening pleasure. It's a big ballad and a vehicle for Lovato's soulful voice. It's pretty much Lovato backed by a piano, and it sounds like it's ripe for the end credits of a movie.

Listen to Demi Lovato 'Heart by Heart'