John Dudek (Hartwick College, USA), Jason Hofstein (Siena College, USA)
We are using Vernier datalogging hardware and software as a low cost solution to study critical opalescence of binary liquid solutions in physical chemistry laboratory. These solutions are prepared at a constant total volume by varying the mole fractions of the components and then heating these solutions until one phase is observed. While the solutions cool, an attenuated helium-neon laser passes through these one-phase solutions to a Vernier light sensor. When the solution becomes opaque and the absorption drops, critical opalescence is observed and the critical demixing temperature for each solution is measured. From the polynomial fit of the critical demixing temperature versus mole fraction graph, the activity and activity coefficient of both binary species can be determined. This approach allows physical chemistry students to get a better understand of binary solutions, activity and activity coefficients.