Hi, I don't think they are too young to understand the topic, but they would certainly need some visual and hands on aids, i.e. good models that can be used to help them get a clear picture of what happens. Personally, I would find it quite difficult to teach changes of stated without mentioning particles, but I come from a Secondary background! I think focussing on different models and outlining their strengths and weaknesses would make a nice dissertation. You could also evidence it with examples of pupils' work, quotes, etc... And if you have enough time, you could try to move to models for electric circuits, maybe! Hope this helps, Alessio.