Understanding Diffusion: The Role of Statistical Properties in Particle Movement

In summary, particles move from a region of high concentration to a region of lower concentration due to random motion and to maximize entropy. This phenomenon is not specific to air molecules and can occur with any type of particle. The random motion is a statistical property of the system, making it difficult to accurately predict the motion of individual particles. Therefore, temperature, pressure, and other statistical properties are used to describe the state of the system.
  • #1
hexrd
6
0
what makes the particles move from a high concentration to a lower concentration?
 
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  • #2
Consider two adjacent regions A,B,(of water, for example) which has different concentrations of a solvent (for example, salt).

Let the boundary between these two regions be permeable for the solvent.
(Think of A and B as the two halves of a tank)

Now, every molecule of the solvent moves in a random fashion.
Hence, the fraction of molecules in A that passes over into B (that is, the percentege of the molecules contained in A) should equal the percentage of molecules originally in B that passes into A.

But then it follows that the actual number of molecules passing from the region of higher concentration must be higher than the number of molecules passing over from the region of lower concentration.

Hence, you have a net drift of molecules from the region of higher concentration into the region of lower concentration.

It is important to realize that the molecules themselves couldn't care less about which region they belong to; diffusion is the net effect of random motion.
 
  • #3
"Perfectly" random motion.
 
  • #4
hexrd said:
what makes the particles move from a high concentration to a lower concentration?

To maximize entropy. It's not that they specifically seek out regions of low concentration, though it could be viewed that way. They just get randomly shoved around (as arildno pointed out) till they are pretty much spread out uniformly. Even though I use the word "uniformly", the system is now more disordered, and it's harder for me to guess where a certain type of particle can be found. In the initial state of the system, I would have better odds of correctly guessing the location of this type of particle - since I know they are all concentrated in a certain region.
 
  • #5
Gokul43201 said:
To maximize entropy.

Isn't this more of an answer to the question:
"Why do the particles exhibit (practically perfect) random motion?"

It seems to me that you chose to explain this fundamental phenomenon in your reply (which I merely stated as a fact)
 
  • #6
Perhaps, but I was also setting up a framework for analysing other such problems - the concept of maximizing entropy is a useful tool and allows you to not have to go down to first priciples each time.
 
  • #7
Is this random motion caused by the bombardment of air molecules? Like Brownian Motion?
 
  • #8
Not necessarily. You can have random motion with a pure species that is not one of the species found in air. The random nature is due to our inability to monitor the particles individually (since there can be billions of billions of billions of them). The temperature and pressure of a gas are statistical properties. It would be another thing entirely to specify the state of a system by the positions and momenta of every particle in the system. Since we cannot do that (not practically nor even theoretically AFAIK), we use temperature, pressure, volume, entropy, and such to specify the state of the system. Since these properties are statistical wrt what each particle in the system is doing, then the system can be in a well defined statistical state (i.e. T=300K, P=1atm) while any given particle undergoes completely unspecified motion.
 

1. What is diffusion?

Diffusion is the movement of particles from an area of high concentration to an area of low concentration. This process occurs naturally and is driven by the random movement of molecules.

2. How does diffusion work?

Diffusion works by the random movement of particles, also known as Brownian motion. Particles in a high concentration area are more likely to collide with each other and move to a lower concentration area, resulting in an overall net movement of particles from high to low concentration.

3. What factors affect the rate of diffusion?

The rate of diffusion is affected by several factors, including temperature, molecular weight, concentration gradient, and surface area. Higher temperatures, smaller molecular weight, larger concentration gradient, and larger surface area all contribute to a faster diffusion rate.

4. How is diffusion important in biological systems?

Diffusion is vital in biological systems as it allows for the exchange of molecules and nutrients between cells and their surrounding environment. It is also responsible for the transport of oxygen and carbon dioxide in the respiratory system.

5. Can diffusion be observed in everyday life?

Yes, diffusion can be observed in everyday life. For example, when you add a drop of food coloring to a glass of water, you can see the color gradually spreading throughout the water due to diffusion. It also occurs when you open a bottle of perfume and can smell it from a distance, or when you smell food cooking in the kitchen from a different room.

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