If we look carefully, the smoke from the combustion initially can be seen. After a while, smoke cannot be viewed. Have you used a perfume? Even though you spray perfume in the room, other people who are outside the home can also feel the fragrance of the perfume. Likewise, if the mother cooks delicious and appetizing food in the kitchen, the aroma of cooking can be felt from a neighbor’s house. Why is that?

There are many other examples. If you put a few drops of ink into a glass containing clear water, ink or food coloring will spread evenly throughout the water. This happens automatically. Some previous examples are diffusion events that are often experienced in everyday life. Diffusion is the process of moving substances from a high concentration to a low concentration. What is meant by concentration is the number of molecules/moles of a substance per volume. A high concentration place is a place where there are many molecules of substances per volume. Conversely, low concentrations are places where there are few molecules per volume.

When you burn garbage, usually the concentration of smoke around the trash burning is quite high. Because there are different concentrations, smoke molecules automatically spread from high concentrations to low concentrations. The smoke molecules that initially gathered together eventually dispersed in all directions.

When you spray perfume on the body, the place where the perfume is sprayed has a high concentration. Because there are differences in concentration, perfume molecules move from high concentrations to low concentrations. When you put a few drops of ink or food coloring into a glass of clear water, the part of the water that is first dripped with ink or food coloring usually has a higher concentration. Because there are different concentrations, ink molecules or food coloring molecules spread to all parts of water that have low concentrations. The diffusion process will stop after the concentration of ink molecules in all parts of the water is the same.

Please note that the diffusion process can be explained using kinetic theory. The kinetic theory states that every substance consists of molecules and these molecules move continuously randomly. To understand this, observe the picture below. All parts of the container are filled with water. Because the ink is dripped, the water on the surface of the container has a higher concentration than the water at the bottom.

Diffusion 1C1 is a cylindrical part or part of the water that has a high concentration, while C2 is a cylindrical part or part of the water that has a low concentration. To facilitate analysis, we only look at the movements of ink molecules in the center of the cylinder.

The number of ink molecules in C1 is more (high concentration) than the ink molecules in C2 (low concentration). Because ink molecules move randomly, the ink molecules in C1 have a higher chance of moving towards the center of the cylinder (Δx). On the contrary, the number of ink molecules in C2 is tiny so that it has a little possibility to move towards the center of the cylinder (Δx). Thus, there will be a total flow of ink molecules from C1 to C2. By research conducted by a physiologist named Adolf Fick (1829-1901), it was found that the diffusion rate was directly proportional to the difference in concentration (C2 – C1). The higher the difference in concentration, the higher the rate of flow of the substance molecules.
Conversely, the smaller the difference in concentration, the lower the flow rate of the molecules of substances. This may be by our expectations that differences in concentration also influence the flow rate of molecules. In addition to switching places by diffusion, substances molecules (especially gas substances) can also move from one place to another using wind assistance.

Application of Diffusion in everyday life

If there was no diffusion, your beloved girlfriend could not enjoy your perfume. Without diffusion, the aroma of mother’s cooking in a delicious and appetizing kitchen also cannot disperse your daydream and want to eat nutritious food available at the dining table quickly. Diffusion also has a significant role in the survival of humans, animals, plants, etc.

Plants usually require carbon dioxide (CO2) to carry out photosynthesis. Because there is a difference in CO2 concentration between the inside of the leaf and the air, the CO2 molecules diffuse into the leaf through the stomata. In contrast, water vapor and oxygen diffuse out. In addition to plants, cats, mice, etc. can also suffocate if there is no diffusion. If plants need CO2 to do photosynthesis, then cats, mice, etc. need oxygen for every reaction that produces energy. Oxygen molecules move by diffusion to arrive inside cells.