SCIE 4001
Unit 1
About Matter
Nature of Matter
Properties of Matter
States of Matter
Kinetic Theory of Gases
Brownian Motion

Unit 1-1.d Brownian Motion.


374Fig 1.1.42

When smoke particeles are observed under the microscope (fig 1.1.42), the  bright smoke specks seem to dance about in a jerky, erratic or random way like in Fig 1.1.43.

How does this evidence support the hypothesis that molecules in gases can vibrate, rotate and translate freely?

Smoke particles are very small and when the collisions with smaller air molecules on all sides are unequal it move in the direction of the unbalanced force as in fig 1.1.44.

Why don't we see the air molecules?

The air molecules are too small to be seen with the microscope.

This effect was first observed by biologist Robert Brown, the first to study and report on the phenomenon (1827). He was studying pollen grains in water under the microscope.

How did this help in developing the Kinetic theory of gases?

If the cause was in the molecular motion in the environment then a smaller particle, a less viscous fluid, and a higher temperatures woul all lead to more-rapid Brownian motion. Experimental evidence bears out these relationships.

Why don't regular size objects in fluids show Brownian Motion?

The unballanced force on large objects in fluids is too small to move them.


Brownian motion is the continual and random motion of the minute particles (e.g. smoke particles or pollen in water) suspended in a fluid (liquid or gas).



How do liquid or gas molecules move?

They rotate, vibrate and translate in a continuous and random motion, with a wide range of velocities.

Why can't we see the molecules in motion?

They are too small to be seen by the naked eye or even the most powerful microscope.

What is the evidence?

The effects called Brownian motion and diffusion.



Fig 1.1.43


Fig 1.1.44


Fig 1.1.45





Concept by Kishore Lal. Programmed by Kishore Lal... Copyright © 2015 Kishore Lal. All rights reserved.