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Molecular speeds.

posted Feb 2, 2015, 1:24 AM by Ranmini Perera   [ updated Jul 17, 2016, 7:43 PM by Upali Salpadoru ]

Speeding Molecules.

Fig.1  Molecules of a perfume spreading
 in all directions. 

Ali and Nelly have been told that there are
 molecules in the air and they scatter in all 
directions like a swarm of bees. 

It never sinked into their brains. 
One day as Ali was coming to Nellie’s house he got a very strong smell 

He looked around expecting to see  strong smelling flowers. But there were none. Nearer the house, stronger was the smell. Then he knew it was from the house. It all happened this way. Nellie had received an expensive bottle of perfume on her birthday.At the time she took just one drop, just to test the smell; what a surprise, it has spread all over the house and had even spilled out and reached the gate to greet a visitor . Two of them discussed the problem how a tiny drop could have sent a signal over such a vast area .  

Ali said, “For me to have got the smell, at least one particle of the scent must have
entered my nose” Nellie agreed and decided to investigate further.  

If the molecules are moving, though we cannot see them , they must be having a speed.Ali got
a brain wave. "I'll stand a known distance away from you. You open the bottle giving me a
signal. Ill keep a  watch and find the time for it to come to my nose"

Fig.2  Path of a random particle.

Prior to the experiment they consulted the teacher too. She called this phenomenon,  
diffusion. "The perfumes are oil like substances, which throw their molecules into the air
rapidly. These 'essential oils', that’s how they are called, are dissolved in a liquid
called alcohol.  The molecules that escape cannot be seen,  yet experiments may be conducted
to find out their speed.” So they decided to  find out how fast they are moving
Unlike a bee, which would take a crooked line, gas particles shoot straight unless or until
they bang into another molecule. There are plenty of molecules in the air; nitrogen, oxygen,
water and dust particles. Dust particles are huge compared to molecules.

Next day they carried out the experiment. They measured the distance a particle can take
from the bottle of scent to where Ali was seated. It was 28 meters. Ali had a watch
indicating seconds.  As Nellie opened the bottle, her sister Mali rang the door bell. When

Ali heard the bell, he noted the time. It was  4:30 and 5 seconds. Ali got the smell at 4:30
and 25 seconds.

 Activity 1

  • Find out the speed of the scent molecule according to the  results.


The distance of the path a molecule may take .. .   =   ………. meters.
Time taken  by the  molecule                         ..     .     = ………..seconds

The speed of the  molecule  =   Distance / Time .. =   --------------------------

                                                         Answer ………………………meters per second.

Activity  2
2.    Do the experiment yourself.

           (An option would be to use lighting of a cracker. Start timing from the sound and stop when you get the smell at a distance. )
3.    If you need an accurate result you must do the experiment several times in different places.
4.    Discuss the  results with your brighter class mates and present them to your teacher. You may even E-mail the results to us.
Click to get the meanings of these words.
1.    Speed      .
2.    Molecule .
3.    Diffusion