In this paper we present a didactic activity on the phase diagrams which are at the basis of different technological processes involving materials with peculiar properties, but which are also relevant to everyday life substances. Our proposal is a learning tool on phase transitions, based on computer on line experiments and problem solving activities. In particular it is oriented to the analysis of: a) unary pressure-temperature diagram, where equilibrium lines between phases are present which converge into the triple point; b) binary diagram, where the lines delimit the region of coexistence of solid and liquid phases of two completely miscible substances; c) eutectic diagram, where lines incident in the eutectic point mark the boundaries of adjacent regions of coexistence of solid and liquid phases of two substances with limited solubility.