Speaker: There is this water-bottle over there. Can you estimate the number of water molecules in this?
Listeners (everyone): No, Sir.
Speaker: One mole of water has how much grams of water?
Listener 1: Eighteen grams of water.
Speaker: This water-bottle has not less than four grams of water. So, you can estimate the enormous number of molecules present in the bottle. What is happening inside? What is the typical r.m.s. velocity at the room temperature and normal atmospheric pressure?
It is of the order of hundreds of meters per second. Right? So much of the movement is taking place there. Look at any individual molecule. What is it doing? A three-dimensional random movement. Correct. Why cannot that molecule move in one particular direction and keep moving in that direction? Why?
Listener 2: There is a disturbance, an obstruction.
Speaker: Obstructed by whom?
Listener 3: By others.
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