To begin with, 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. The fluid does not have an ideal model, which greatly 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 associated with a concentration directly proportional to the dependence. The diffusion flux will change similarly to the concentration gradient.
Varieties
If diffusion in a liquid occurs 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 under the laws of Brownian motion.
Leakage features
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 a maximum speed in gases, and a minimum in solids.
The trajectory of the particle is a broken line, since the direction and speed periodically change. Due to the disordered movement, a gradual removal of the particle from its original position is observed. Its displacement in a straight line is much shorter than the path that takes place along a broken path.
Fick's Law
Liquid diffusion obeys two Fick laws:
- the density of the diffusion flow is directly proportional to the concentration with a diffusion coefficient;
- the rate of change of the diffusion flux density is directly proportional to the rate of change of 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 transmitted to the molecule in a volume sufficient to break the bond with other particles. The average jump does not exceed the distance between the molecules.
Arguing over what diffusion in a liquid is, we note that the process depends on temperature. With its increase, the "loosening" of the liquid structure occurs, as a result of which there is a sharp increase in the number of jumps per unit time.
Diffusion in gases, liquids and solids has some distinctive characteristics. For example, in solids, the mechanism is associated with the movement of atoms inside the crystal lattice.
Features of the phenomenon
Diffusion in a liquid is of practical interest due to the fact that it is accompanied by an equalization of the concentration of a substance in an initially inhomogeneous medium. From areas with a high concentration, particles leave much more.
The experiments
Fluid experiments have shown that diffusion is of particular importance in chemical kinetics. During the course of the chemical process on the surface of the reacting substances or catalyst, this process helps to determine the rate of removal of reaction products and the addition of the starting reagents.
What explains diffusion in liquids? Solvent molecules are able to penetrate through translucent membranes, resulting in osmotic pressure. This phenomenon has found application in chemical and physical methods of separation of substances.
Biological systems
In this case, diffusion models can be considered by the example of air oxygen entering the lungs, absorption of digestion products from the intestines into the blood, and absorption of mineral elements by the root hairs. Ion diffusion occurs during the generation of bioelectric pulses by muscle and nerve cells.
The physical factor that affects the selectivity of the accumulation of certain elements in the cells of the body is the different rate of penetration of ions through cell membranes. This process can be expressed by Fick's law, replacing the value of the diffusion coefficient with an indicator of membrane permeability, and instead of the concentration gradient, use the difference in values on both sides of the membrane. With diffusive 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 several types of this process are distinguished. 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 in which the molecules have small sizes, for example, methyl alcohol, water.
A limited option involves a weak transfer of matter. For example, even small particles are not able to penetrate into the cell.
Pages of history
Diffusion was discovered during the heyday of ancient Greek culture. Democritus and Anaxogor were convinced that any substance consists of atoms. The variety of substances common in nature, they explained by the interconnections of 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. He was given a definition of a molecule, an atom, the mechanism of dissolution was explained.
The experiments
Experience with sugar allows us to understand all the features of diffusion. If you put a piece of sugar in iced tea, gradually a thick syrup is formed at the bottom of the cup. It is visible to the naked eye. After a while, the syrup will be evenly distributed throughout the entire 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 perfume aroma spreads throughout the room.
The experiments presented indicate that diffusion is a spontaneous process of penetration of molecules of one substance into another. The substance spreads in all directions, despite the presence of gravity. A similar process is a direct confirmation of the constant motion of the molecules of a substance.
So, in the above example, the diffusion of sugar and water molecules is carried out, which is accompanied by a uniform distribution of the molecules of the organic substance throughout the volume of the liquid.
The experiments make it possible to detect diffusion not only in liquids, but also in gaseous substances. For example, you can install on the scales a container with ether vapor. Gradually the cups will come to balance, then the glass with ether will be heavier. What is the reason for this phenomenon?
Over time, the ether molecules mix with air particles, and a specific smell begins to be felt in the room. An experiment is considered in a high school physics course, in which a teacher dissolves a grain of potassium permanganate (potassium permanganate) in water. At first, a clear trajectory of the grain is visible, but gradually the entire solution acquires a uniform shade. Based on the experiment, the teacher explains the features of diffusion.
To identify factors that affect the rate 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 and the rate of diffusion.
Conclusion
The experiments carried out with gases, liquids and solids, allow us to formulate the laws of physics, to establish the relationship between the individual quantities.
It was as a result of experiments that a mechanism was established for the mutual penetration of particles of one substance into another, and the randomness of their motion was proved. Empirically, it was found that diffusion in gaseous substances most rapidly occurs. This process is of great importance for wildlife, used in science and technology.
Thanks to this phenomenon, a homogeneous composition of the earth's atmosphere is maintained. Otherwise, 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 this 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 agriculture, foliar top dressing of shrubs and trees is used, which involves spraying the crown with special chemical compounds.
It is during diffusion that a plant receives nutrients from the soil. The physiological processes in living organisms are also associated with this phenomenon. For example, salt balance is not possible without diffusion. Of great importance are such processes in the supply of lakes and rivers with oxygen. Gas goes deep into the reservoir precisely by diffusion. If such a process were absent, life inside the reservoir would cease to exist.
Taking medications that allow a person to protect themselves from pathogens of various diseases and improve well-being is also based on diffusion. This phenomenon is used when welding metals, obtaining sugar juice from their beet chips, and preparing confectionery products. It is difficult to find a branch of modern industry where diffusion is not applied.