Analysis and Improvement of an Experiment on Diffusion Rate of Gases Introduced in High School Chemistry Textbooks

Abstract

This study is to analyze the problems observed in the experiment on ‘diffusion rate of gases’ in high school Chemistry II text books and investigate the reason followed by suggestion of an improved experimental method. As typically introduced in textbooks, a few drops of HCl(aq) and NH3(aq) were dropped at both ends of a glass tube, and then the position of ammonium chloride white ring created by the reaction with HCl(g) and NH3(g) was measured. The position of the white ring changed with time. In order to investigate the reason of such change, the experiment was repeated using HCl and NH3 gases, and the position of white ring did not changed with time. Meanwhile, the relative diffusion distance did not match with the theoretical expectation of the Graham's diffusion experiment but did with that of the Loschmidt's diffusion experiment. Based on this result experimental condition allowing equal partial pressure of HCl and NH3 gases in hydrochloric acid and aqueous ammonia solution, respectively was found, and under the obtained condition the position of white ring did not change and was consistent with the theoretical expectation. This study is anticipated to help learners to understand the scientific concept of diffusion through the improved experiment.