Determination of End Point in Benzoic Acid Crystallization via Laser Irradiation

The recrystallization experiment is a significant component of chemistry laboratory instruction in both middle schools and universities. The method for identifying the crystallization end point is absent from the textbooks. In this paper, the characteristic of light propagation in nonuniform media was employed to identify the crystallization end point. Filter the crude benzoic acid solution while it is hot, allowing the laser beam to pass through the center of the filtrate. Observe the alteration in the light spot projected onto the white screen through the beam. As the concentration gradient of the solution changes, the gradual recovery of the light spot from the reticulated stripes of light and dark indicates completion of the crystallization process, signaling readiness for the subsequent filtration experiment. This method offers a simple means to determine the end point of crystallization, and the phenomenon is intuitive, making it suitable for chemistry teaching experiments.