Diffusion in liquid and fick’s law


Diffusion in liquid has been natural occurrences in our daily activities but to our notice we know take cognizance of them. Diffusion can occur in solids, liquid and gases. Diffusion is faster in gaseous state than liquids having the least as the solids. From our basic understanding and definition diffusion is defined as follows:

Diffusion is the movement of molecules of a substance from the region of higher concentration to lower concentration over a period of time. Diffusion in liquids occurs when two or more substances mixed together to attain an appreciable level of equilibrium within the molecular movement. An experiment carried out to determine the rate of equilibrium and to determine the diffusion coefficient using one mole of sodium chloride.

The principle of mass and heat transportation is base on two principles: convection and diffusion. Convection is the process of by which substances or heat is transferred as a result of the mean motion of the carrier fluid bought about by difference in temperature.

Diffusion is the process by which materials are transferred by random thermal motion of the molecules within the fluid. This motion can be macroscopic or microscopic.


Methods of experiment.

A small volume of concentrated solution is placed on one side of the honey comb of a capillary, inside the glass diffusion cell, whilst the other side consist initially of a volume of pure solvent (water). As diffusion of the solute occurs, the concentration within the larger volume increases and is monitored with a conductivity concentration meter within the bulk liquid.

Brownian motion

One of the simplest ways to explain diffusion in liquids is by the famous experiment performed by a botanist called Robert Browns in 1827. detailed history of Brown. He viewed through a microscope and saw how the particles are trapped in cavities inside a pollen grain in water, he observed that the pollen grains were moving randomly but in an irregular form, this he could not explain what mechanism caused the motion.

Before he observed this, theories about the existence of atoms and molecules has been made and Albert Einstein has published a paper in 1905 that explains in precise detains how the motion that Brown had observed was as a result of individual molecules of water interacting with the pollen grains.

Another example of diffusion is found in living organisms, in the aspect of movement of oxygen from the environment to the blood stream. inhalation and exhalation processes in the thoracic and alveoli. The distribution of materials in the body makes use of diffusion principles. Also the trapping of oxygen by plants through the stomata to different region of the plant is also by diffusion principles.

Fick’s law of diffusion.

Fick’s law is one the ways to approach diffusion studies,: phenomenological approach defines diffusion as the movement of substances from region of higher concentration to the region of lower concentration without bulk motion. According to his law, ‘the diffusion flux is proportional to the negative gradient of the concentration’. This law was developed from the concept of thermodynamic s and non equilibrium thermodynamics. Diffusion is results from the random walk of the diffusing particles.

Conductivity of the solution

From diffusion theory and experimental proof of the diffusion of one mole of sodium chloride with water, it is proved that the conductivity of a solution increases with increase in temperature and time. This is because of the different in levels of concentration of the solution in contact. When a graph of conductivity is plotted against time and an accurate line of best fit is ensured, a straight line is obtained.

When a concentration of a solution is increased as a result of increased temperature, this causes dissociation of molecules into ions and the mobility of the ions to different region in the fluid is facilitated.







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