Liquid–Liquid Demonstrations: Critical Opalescence

• J. Charles Williamson ^[1]
  1. [1] Willamette University

     Willamette University

     Estados Unidos

     
• Localización: Journal of chemical education, ISSN 0021-9584, Vol. 98, Nº 7, 2021, págs. 2364-2369
• Idioma: inglés
• Texto completo no disponible (Saber más ...)
• Resumen

  • In the vicinity of a critical point, the impact of a substance’s chemical identity is superseded by universal critical behavior for a number of physical properties, such as visible light scattering. In this classroom demonstration, students observe a single-phase critical mixture of partially miscible isobutyric acid + water (IAW) as the liquid–liquid sample cools down to its critical temperature. Students see the sample change from colorless and transparent, to pale blue, to white and turbid. This optical behavior is called critical opalescence. No pressure control is necessary for the demonstration since the system is a condensed phase, so the sample can be large (100 mL here) and suitable for classroom visualization. The experiment is easy to set up and highly repeatable because the critical temperature of the IAW system is just above room temperature. The demonstration can be used to show the difference in angular dependence for light scattering from a single-phase sample versus a two-phase sample containing a suspension of droplets, and also to segue to a conversation on why the sky is blue. Furthermore, the critical IAW sample may be incorporated into a quantitative classroom demonstration or laboratory project in which laser light scattering measured at 90° of incidence is used to calculate the correlation length amplitude ξ0 of the IAW system and the universal critical exponent ν.