To make learning fun a teacher must first know how to have fun. They have to be passionate for the subject and have a good personality. The teacher does not have to be hyper but enthusiastic. If a teacher does not seem like they care then what makes them think that the students are going to enjoy what they are learning.
With chemistry I think it is usually easy to keep students interested because the teacher is able to do experiments in front of the class to show them what they are learning. Also with chemistry the teacher has to enjoy what they are doing and talking about. To engage the students it is important to use different media sources other than just an over head with notes. The teacher can show movies and even get the students up to do labs and have hands on experiences. But as states before the key to making learning fun is for the teacher to be passionate for the subject and be enthusiastic when in the classroom.