Investigation of Ion-Water Molecule Collision Cross-Section in Dilute KCL Solution Based on Conductivity Experiment

Water, an essential substance for life, commonly exists in the form of aqueous solutions in nature. When strong electrolytes dissolve in water, they fully dissociate into free ions, resulting in conductive solutions whose conductivity is closely related to ion concentration. Based on L. Onsager’s limiting law, ionic mobility in infinitely diluted solutions stabilizes, affected mainly by water molecular thermal motion. This study focuses on KCl dilute solutions. At room temperature, it uses low-voltage, high-precision conductivity measurements to get conductivity data at different concentrations. The data is analyzed and fitted to find limiting ionic mobility and solvent viscosity. With the kinetic theory of molecules, average collision time is calculated. An equivalent hard-sphere model estimates the effective collision cross-section between ions and water molecules in thermal motion. Results show ionic mobility saturates at lower concentrations, and the effective collision cross-section exceeds the geometric one, indicating significant Coulombic interactions during collisions. The experimental methods and analytical models here, based on college level physics and statistical physics, are highly valuable for teaching. They can be used in relevant courses to expand on topics like thermal motion and electrolyte transport, helping students link abstract physical models to real-world processes.