Diffusion is the movement of a substance or quality from an area of high concentration to an area of low concentration. It plays a key role in physics, chemistry, and biology, including the transport of nutrients in cells. Molecular diffusion occurs due to the random motion of molecules, and it can be observed in the mixing of gases and liquids. Diffusion is a form of passive transport and can also refer to the diffusion of heat or light, as well as the diffusion of ideas or innovations in society.
Broadly speaking, diffusion simply means the diffusion of a substance or quality from an area of relatively high concentration to an area of lower concentration. The term is most commonly used to describe the process by which molecules of different types reach a uniform concentration, after initially being unevenly distributed, through their random motions. This is sometimes called molecular diffusion and plays a key role in many areas of physics, chemistry and biology. It is closely related to how cells absorb nutrients, so life could not exist without it. The term can also refer to the diffusion of heat or light or, in the context of the social sciences, the diffusion of ideas or innovations through society.
Molecular diffusion
The molecules are in constant motion, due to the presence of heat; even at extremely low temperatures, some thermal energy is present, which gives thermal motion to the particles. In solids the particles are held in a relatively rigid structure and cannot move too far from their starting positions, but in liquids and gases they are free to move in a less restricted way. This motion is random, as the molecules are constantly bouncing off each other, so that there is no overall pattern. This random movement is what allows for diffusion. The rate at which the phenomenon occurs increases with temperature, as the speed at which the particles move increases.
If two different gases at the same temperature are brought into contact with each other then, over time, the random motion of their molecules will cause them to mix until they are uniformly distributed, without the influence of any external factors. A similar phenomenon can often be observed with two different liquids, but sometimes, due to chemical factors, the liquids do not mix. For example, adding vegetable oil to water will not cause it to spread. When a solid is dissolved in a liquid, its molecules diffuse throughout the liquid.
In mixing liquids, diffusion may be observable if the liquids are of different colors. This phenomenon should not, however, be confused with convection or advection, which involve the mass movement of fluids by currents that require an energy source. Diffusion is a form of what scientists call passive transport — motion that occurs without any additional energy being needed. In the oft-quoted example of a drop of colorant being added to a glass of water, much of the mixing that occurs is actually due to the currents created by the initial motion of the colorant drop by gravity. In experiments that eliminate this, and similar, effects, true diffusion can be observed; for example, colored water can be seen spreading through a gel, but this is a much slower process.
Importance for life
Diffusion also plays a role in the processes involved in the life of a cell, especially the transport of nutrients, amino acids and other essential substances from one place to another. Osmosis, the process by which molecules are transported across the walls and membranes of individual plant and animal cells, is a form of diffusion. A cell wall is a membrane that is selectively permeable, or through which certain molecules can pass but others cannot. If a highly concentrated solution is present on one side of a membrane, while on the other side is the same solution with a lower concentration, the solvent will tend to move to the area of higher concentration until the two solutions reach equilibrium. This can only occur if the molecules of the solute – the dissolved substance – are larger than those of the solvent. Smaller solvent molecules can pass through the membrane, while larger ones are too big to do so and have to stay on the other side.
Other forms of dissemination
Diffusion can also mean the diffusion of heat through a solid, although this is usually referred to as conduction, and the diffusion of light through a translucent substance, for example the ground glass in a light bulb used to provide “diffused” light which is more pleasing to the eye. Outside the context of physics, the term can be used to describe the diffusion of an innovative idea or technology through a population through people-to-people contact, within a group conformism, or through the observation of benefits to others.
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