In this paper an educational laboratory work on phase diagrams is presented. The phase diagrams are at the basis of many technological processes involving materials with peculiar properties to produce artifacts of our everyday life. 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, b) binary lens-like diagram, and c) binary eutectic diagram. The proposal is thought to gradually proceed towards increasing difficulties so that the activities can be selected and used at various levels.