Let's start with the fact that the liquid is an intermediate state of aggregation. At the critical boiling point, it is similar to gases, and at low temperatures, characteristics similar to a solid appear. A liquid does not have an ideal model, which significantly complicates the description of its equilibrium thermodynamic properties, freezing point, viscosity, diffusion, thermal conductivity, surface tension, entropy, enthalpy.
Definition
What is diffusion? This is the spreading, distribution, movement of particles of the medium, which leads to the transfer of matter, the establishment of equilibrium concentrations. In the absence of external influences, this process is determined by the thermal motion of the particles. In this case, the diffusion process is directly proportional to the concentration. The diffusion flux will change similarly to the concentration gradient.
Varieties
If diffusion in a liquid proceeds with a change in temperature, it is called thermal diffusion, in an electric field - electrodiffusion.
The process of movement of large particles in a liquid or gas occurs underlaws of Brownian motion.
Features of flow
Diffusion in gases, liquids and solids proceeds at different speeds. Due to differences in the nature of the thermal motion of particles in different media, the process has the maximum speed in gases, and the minimum rate - in solids.
The trajectory of the particle is a broken line, since the direction and speed periodically change. Due to the disordered motion, a gradual removal of the particle from its original position is observed. Its displacement along a straight line is much shorter than the path that takes place along a broken path.
Fick's Law
Diffusion in a liquid obeys two Fick's laws:
- diffusion flux density is directly proportional to concentration with diffusion coefficient;
- The rate of change in the diffusion flux density is directly proportional to the rate of change in concentration and has the opposite direction.
Diffusion in a liquid is characterized by jumps of molecules from one equilibrium position to another. Each such jump is observed when energy is imparted to the molecule in a volume sufficient to break the bond with other particles. The average jump does not exceed the distance between molecules.
When discussing what diffusion in a liquid is, we note that the process depends on temperature. With its increase, “loosening” of the liquid structure occurs, as a result of which there is a sharp increase inthe number of jumps per unit of time.
Diffusion in gases, liquids and solids has some distinctive characteristics. For example, in solids, the mechanism is associated with the movement of atoms within the crystal lattice.
Features of the phenomenon
Diffusion in a liquid is of practical interest due to the fact that it is accompanied by the equalization of the concentration of a substance in an initially inhomogeneous medium. Significantly more particles escape from areas with a high concentration.
Experiments
Experiments with liquids have shown that diffusion is of particular importance in chemical kinetics. During the course of a chemical process on the surface of reactants or a catalyst, this process contributes to the determination of the rate of removal of reaction products and the addition of initial reagents.
What explains diffusion in liquids? Solvent molecules are able to penetrate through translucent membranes, resulting in an osmotic pressure. This phenomenon has found application in chemical and physical methods for the separation of substances.
Biological systems
In this case, diffusion models can be considered on the example of air oxygen entering the lungs, absorption of digestive products from the intestine into the blood, absorption of mineral elements by root hairs. Diffusion of ions occurs during the generation of bioelectric impulses by muscle and nerve cells.
Physical factor that affectsthe selectivity of the accumulation of certain elements in the cells of the body is the different rate of penetration of ions through the cell membranes. This process can be expressed by Fick's law, replacing the value of the diffusion coefficient with the membrane permeability, and instead of the concentration gradient, use the difference in values on both sides of the membrane. With the diffusion penetration of water and gases into the cell, the osmotic pressure indicators outside and inside the cell change.
Analyzing what diffusion depends on, we note that there are several types of this process. The simple form is associated with the free transfer of ions and molecules towards the gradient of their electrochemical potential. For example, a similar option is suitable for those substances whose molecules are of small size, for example, methyl alcohol, water.
Limited variant assumes a weak transfer of matter. For example, even small particles cannot penetrate into a cell.
History Pages
Diffusion was discovered during the heyday of ancient Greek culture. Democritus and Anaxogoras were convinced that any substance consists of atoms. They explained the variety of substances common in nature by the connections between individual atoms. They assumed that these particles could mix to form new substances. Among the founders of the molecular-kinetic theory, which explained the mechanism of diffusion, Mikhail Lomonosov played a special role. They gave a definition to a molecule, an atom, and explained the mechanism of dissolution.
Experiments
Experience with sugar allows you to understand all the features of diffusion. If you put a piece of sugar in cold tea, a thick syrup will gradually form at the bottom of the cup. It is visible to the naked eye. After some time, the syrup will be evenly distributed throughout the volume of the liquid and will no longer be visible. This process proceeds spontaneously and does not involve mixing the components of the solution. Similarly, the fragrance of perfume spreads throughout the room.
The above experiments show that diffusion is a spontaneous process of penetration of molecules of one substance into another. The spread of matter occurs in all directions, despite the presence of gravity. Such a process is a direct confirmation of the constant movement of the molecules of matter.
So, in the above example, the diffusion of sugar and water molecules is carried out, which is accompanied by a uniform distribution of organic matter molecules throughout the volume of the liquid.
Experiments make it possible to detect diffusion not only in liquids, but also in gaseous substances. For example, you can install a container with ether vapor on the scales. Gradually, the cups will come into balance, then the glass of ether will be heavier. What is the reason for this phenomenon?
Over time, ether molecules mix with air particles, and a specific smell begins to be felt in the room. In a high school physics course, an experiment is considered in which a teacher dissolves a grain of potassium permanganate (potassium permanganate) in water. At first, a clear trajectory of grain movement is visible,but gradually the whole solution acquires a uniform shade. Based on the experiment, the teacher explains the features of diffusion.
To identify the factors that affect the speed of the process in liquids, you can use water of different temperatures. In a hot liquid, the process of mutual mixing of molecules is observed much faster, therefore, there is a direct relationship between the temperature value and the diffusion rate.
Conclusion
Experiments conducted with gases, liquids and solids make it possible to formulate the laws of physics, to establish the relationship between individual quantities.
It was as a result of the experiments that the mechanism of mutual penetration of particles of one substance into another was established, the chaotic nature of their movement was proved. Empirically, it was found that diffusion occurs fastest in gaseous substances. This process is of great importance for wildlife, is used in science and technology.
Thanks to this phenomenon, the homogeneous composition of the earth's atmosphere is maintained. Otherwise, the stratification of the troposphere into separate gaseous substances would be observed, and heavy carbon dioxide, unsuitable for breathing, would be closest to the surface of our planet. What would it lead to? Wildlife would simply cease to exist.
The role of diffusion in the plant world is also great. The lush crown of trees can be explained by diffusion exchange through the surface of the leaves. As a result, not only breathing is carried out, but also the nutrition of the tree. Currently in agriculturefoliar feeding of shrubs and trees is used, which involves spraying the crown with special chemical compounds.
It is during diffusion that the plant receives nutrients from the soil. Physiological processes occurring in living organisms are also associated with this phenomenon. For example, s alt balance is impossible without diffusion. Such processes are of great importance in supplying lakes and rivers with oxygen. The gas enters the depths of the reservoir precisely by diffusion. If such a process were absent, life inside the reservoir would cease to exist.
The intake of drugs that allow a person to protect himself from pathogens of various diseases and improve well-being is also based on diffusion. This phenomenon is used in the welding of metals, the production of sugar juice from beet chips, and the preparation of confectionery. It is difficult to find such a branch of modern industry where diffusion is not used.
