Have you ever seen hordes of small annoying midges, randomly swarming above your head? Sometimes it seems that they seem to hang motionless in the air. On the one hand, this swarm is motionless, on the other, the insects inside it move non-stop right or left, then up, then down, constantly colliding with each other and scattering again within this cloud, as if an invisible force holds them together.
The movements of the molecules are similarly chaotic, while the body remains in a stable shape. This motion is called the thermal motion of the molecules.
Brownian motion
Back in 1827, the famous British botanist Robert Brown, using a microscope, studied the behavior of microscopic particles of flower pollen in water. He drew attention to the fact that the particles constantly moved in a chaotic order that could not be explained logically, and this random movement did not depend on the movement of the liquid in which they were located, or on its evaporation. The smallest particles of pollen described complex, mysterious trajectories. It is interesting that the intensity of such motion does not decrease with time and is not related to the chemical properties of the medium, but only increases if the viscosity of this medium or the size of moving particles decreases. In addition, temperature has a great influence on the speed of movement of molecules: the higher it is, the particles move faster.
Diffusion
Once upon a time, people realized that all substances in the world consist of the smallest particles: ions, atoms, molecules, and there are gaps between them, and these particles constantly and randomly move.
A consequence of the thermal motion of molecules is diffusion. Examples we can observe almost everywhere in everyday life: both in everyday life and in nature. This is the spread of odors, bonding various solid objects, mixing liquids.
In scientific terms, diffusion is the phenomenon of the penetration of molecules of one substance into the gaps between the molecules of another substance.
Gases and diffusion
The simplest example of diffusion in gases is the rather rapid spread of odors in the air (both pleasant and not so good).
Diffusion in gases can be extremely dangerous, because of this phenomenon poisoning by carbon monoxide and other poisonous gases proceeds instantly.
Fluid diffusion
If diffusion in gases occurs quickly, most often in a matter of seconds, then diffusion in liquids takes whole minutes and sometimes even hours. It depends on density and temperature.
One example of diffusion in liquids is the very rapid dissolution of salts, alcohols and acids, which form homogeneous solutions in a short time.
Solid Diffusion
In solids, diffusion is most difficult, at ordinary room or street temperature it is invisible. In all modern and old school textbooks, as an example of diffusion in solids, the experience with lead and gold plates is described. This experiment showed that only after more than four years, an insignificant amount of gold penetrated into lead, and lead penetrated into gold to a depth of not more than five millimeters. This difference is due to the fact that the density of lead is much higher than the density of gold.
Consequently, the speed and intensity of diffusion, not least, depends on the density of the substance and the speed of the random movement of molecules, and the speed, in turn, depends on temperature. Diffusion is more intense and faster at higher temperatures.
Examples of diffusion in everyday life
We don’t even think about the fact that every day at almost every step we encounter the phenomenon of diffusion. That is why this phenomenon is considered one of the most significant and interesting in physics.
One of the simplest examples of diffusion in everyday life is the dissolution of sugar in tea or coffee. If you place a piece of sugar in a glass of boiling water, after a while it will disappear without a trace, even the volume of liquid will not practically change.
If you carefully look around, you can find many examples of diffusion that make our life easier:
- dissolution of washing powder, potassium permanganate, salt;
- spray air fresheners;
- throat sprays;
- leaching dirt from the surface of the laundry;
- mixing paints by an artist;
- kneading dough;
- cooking rich broths, soups, and gravy, sweet compotes and fruit drinks.
In 1638, returning from Mongolia, Ambassador Vasily Starkov presented the Russian Tsar Mikhail Fedorovich with a gift of almost 66 kg of dried leaves with a strange tart aroma. This dried plant was really liked by the Muscovites who have never tried it, and they still enjoy it with pleasure. Did you recognize him? Of course, this is tea that is brewed due to the phenomenon of diffusion.
Examples of diffusion in the outside world
The role of diffusion in the world around us is very great. One of the most important examples of diffusion is blood circulation in living organisms. Oxygen from the air penetrates into the blood capillaries located in the lungs, after which it dissolves in them and spreads throughout the body. In turn, carbon dioxide diffuses from the capillaries into the alveoli of the lungs. Nutrients released from food by diffusion penetrate the cells.
In herbaceous plant species, diffusion occurs through their entire green surface, in larger flowering plants through leaves and stems, in shrubs and trees through cracks in the bark of trunks and branches and lentils.
In addition, an example of diffusion in the surrounding world is the absorption of water and minerals dissolved in it by the root system of plants from the soil.
It is diffusion that is the reason that the composition of the lower atmosphere is inhomogeneous and consists of several gases.
Unfortunately, in our imperfect world there are very few people who do not know what an injection is, also known as an “injection”. This type of painful but effective treatment is also based on the phenomenon of diffusion.
Environmental pollution: soil, air, water bodies - these are also examples of diffusion in nature.
White clouds melting in the blue sky, so beloved by poets of all time - it is also a diffusion known to every student of the middle and senior classes!
So, diffusion is something without which our life would not only be more difficult, but practically impossible.