In a heating curve, when it is plateau, where does all the energy go since the temperature does not get higher?
You may have learnt at some point that a change of phase is endothermic (i.e. needs heat to happen). So, when you transition from a phase to another and keep adding heat, the temperature will remain the same, as all the heat you'll add will be used for the change of phase. Hence, the temperature will remain the same until the transition is over (i.e. you gave your system enough heat for the transition to be complete), then will vary again.