This question already has an answer here:
While reading about cis-trans isomers I found the concept of free rotation of atoms.it says 2 atoms connected by sigma bond do not have trans sis isomer because of free rotation.while in case of pi bond free rotation is not possible. My question is : do we mean actual rotation of atoms in molecule when we say free rotation? Does the rotation happen spontaneously? What things are responsible for the rotation? Or is it just a concept to understand the concept of cis-trans isomers?