Random systems are those which contain an element of unpredictability. For example, before a coin is tossed, it is impossible to know whether it will land with heads or tails facing up. Before students take a test, it is impossible to know with certainty what their grades will be. If a leaf is selected from a tree and measured, its length cannot be accurately known before the leaf is chosen.
In chemistry, many processes contain a great deal of randomness. When two chemicals A and B are mixed to form new compound AB, it is not known in advance which molecules of A will react soonest and which will remain unreacted until the reaction is almost complete. In an ideal gas of colliding atoms, it is impossible to know in advance the speed of a particular atom at any given time. Each atom's speed is the by-product of all its collisions with other atoms and with the walls of the container.
Exercises to Illustrate Randomness
The Big Picture
© Andrew M. Rappe