• Brownian motion of particles is the phenomenon when:

Small particles (such as pollen or smoke particles) suspended in a liquid or gas are observed to move around in a random, erratic fashion

Brownian motion is the erratic motion of small particles when observed through a microscope

• Brownian motion:
  • Can be observed under a microscope
  • Provides evidence for the existence of molecules in a gas or liquids

   

• The particles are said to be in random motion, this means that they have:
  • A range of speeds
  • No preferred direction of movement

   

• The observable particles in Brownian motion are significantly bigger than the molecules that cause the motion
  • In most cases, these were observed as smoke particles in air
  • The air particles cause the observable motion of the smoke particles that we see
  • This means that the air particles were small and light and the smoke particles were large and heavy

   

• The collisions cause larger particles to change their speed and directions randomly
  • This effect provides important evidence concerning the behaviour of molecules in a gas, especially the concept of pressure

   

• The small molecules are able to affect the larger particles in this way because:
  • They are travelling at a speed much higher than the larger particles
  • They have a lot of momentum, which they transfer to the larger particles when they collide