Demonstrating Brownian motion and estimating Avogadro’s number using simple tools: a practical guide for students

Abstract Brownian motion provides crucial observational evidence for the kinetic molecular theory by demonstrating the random perpetual movement of small particles. This phenomenon underscores the existence and behaviour of atoms and molecules, fundamentally supporting our current understanding of molecular kinetic theory. Remarkably, in 1905 Albert Einstein was able to predict this motion exactly purely from a theoretical point of view and in 1908 Jean Perrin provided experimental proof. This provided experimental proof of the existence of atoms and molecules. This paper outlines a method to demonstrate and record Brownian motion using a drop of homogenised low fat milk, a microscope and a mobile phone. The captured video files can be analysed with Physics Tracker software, allowing for the estimation of Avogadro’s number.