Intermediate Topic 316 of 1102 States of Matter & Solutions
Brownian Motion
Random zigzag movement of colloidal particles caused by molecular collisions.
Practise States of Matter & Solutions All States of Matter & Solutions topics
In-depth explanation of Brownian Motion
Brownian motion is the random, zigzag movement of small particles suspended in a fluid, caused by uneven bombardment by molecules of the fluid. Robert Brown first observed it in 1827 using pollen grains in water. The motion arises because fluid molecules, moving randomly with thermal energy, strike the colloidal particle unevenly on different sides at any moment. The intensity of the motion increases with temperature and decreases with particle size. Quantitative analysis of the motion by Albert Einstein and Jean Perrin later confirmed the atomic nature of matter and helped estimate Avogadro's number.
Going deeper: Brownian Motion
Key ideas: Mean square displacement grows linearly with time, x^2 = 2Dt, with D from Stokes-Einstein: D = kT / (6 pi eta r). Smaller particles and lower viscosity give faster motion. It prevents colloidal particles from settling because the motion counters gravity for sizes below about a micron.
Common mistakes and connections: Students think the particles move because they are alive or are driven by convection; the cause is molecular collisions. Another error is expecting the motion to stop at constant temperature; it persists. Also, the motion of individual molecules is not directly seen, only the effect on larger particles. Links to kinetic theory, colloid stability, diffusion and the determination of Avogadro's number.
Visualization of Brownian Motion
Brownian motion is the random zigzag path of colloid particles driven by uneven molecular collisions, which helps keep them dispersed. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Small smoke particles in still air show a random zigzag motion when viewed under a microscope.
Frequently asked questions about Brownian Motion
What is Brownian Motion?
Random zigzag movement of colloidal particles caused by molecular collisions.
Is Brownian Motion a beginner, intermediate or advanced topic?
Brownian Motion is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
What should I learn before Brownian Motion?
Study these earlier topics in States of Matter & Solutions first: Abnormal Molar Mass, Colloids, Tyndall Effect.
What comes after Brownian Motion?
The next topics in States of Matter & Solutions are Emulsions and Micelles.
Can I practise Brownian Motion with quizzes?
Yes. The interactive lesson includes a self-check question on Brownian Motion, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Brownian Motion fits in States of Matter & Solutions
Brownian Motion is topic 97 of 109 in States of Matter & Solutions.