An experimental approach to teaching phase transitions: Linking cooling curves, phase diagrams, and microstructures in the Pb–Sn system

This work presents an experimental approach to explore thermodynamic phase transitions through the construction of cooling curves, phase diagrams, and microstructures in the Pb–Sn alloy system. The experiment, implemented in an undergraduate laboratory, allows students to directly relate temperature–time measurements to equilibrium phase behavior. The cooling curves reveal characteristic features such as isothermal plateaus in pure components and temperature intervals associated with off-eutectic solidification. The eutectic composition exhibits a near-isothermal transformation corresponding to the invariant reaction <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi>L</mml:mi> <mml:mo stretchy="false">→</mml:mo> <mml:mi>α</mml:mi> <mml:mo>+</mml:mo> <mml:mi>β</mml:mi> <mml:mo>.</mml:mo> </mml:math> These observations are used to construct an experimental phase diagram, which is further complemented by metallographic analysis. The combination of thermal data and microstructural observations provides a direct link between latent heat, phase coexistence, and the resulting spatial distribution of phases. This integrated approach enables a deeper understanding of thermodynamic equilibrium and phase transformations, bridging the gap between abstract theory and experimentally observable phenomena. The experiment also highlights the role of measurement uncertainty and non-ideal conditions in real systems.