14.4 Brownian Motion In Liquids

Chemical Concept Demonstrated: Brownian motion


The flask contains a saturated solution of excess sulfur in CS2. The crystallizing dish is half-filled with the solution, covered with a glass plate, and placed on the overhead projector.


    Rhombic sulfur crystals form in the solution and can be seen moving randomly in the dish.  Eventually, the proliferation of crystals will  interfere with this movement.


    Molecules in fluids are in a constant state of random motion.   A particle suspended in a fluid is constantly and randomly bombarded from all sides by molecules of the fluid, and this is noticable, provided the particle is small and light enough (we do not, for example, notice the fluid of the atmoshere pushing around billiard balls).  The random motion of the crystals, not the molecules, is referred to as Brownian motion.