In a problem in Callen's book I was asked to say what was the latent heat of fusion at the triple point for ammonia. I answered "without performing any algebra, 0J". Because I remember a video I saw on youtube about the triple point () and now I read on wikipedia which seem to confirm that indeed, the system can undergo a phase transition without any external heat transfer. So would the triple point be equivalent to a critical point in some way? In the sense that if I understood well, in a critical point the system can change its phase without any external heat, just like in the triple point?