Examples of Diffusion in Chemistry

Diffusion is the movement of atoms, ions, or molecules from an area of higher concentration to one of lower concentration. The transport of matter continues until equilibrium is reached and there is a uniform concentration through the material.

Examples of Diffusion

1. Perfume is sprayed in one part of a room, yet soon it diffuses so that you can smell it everywhere.
2. A drop of food coloring diffuses throughout the water in a glass so that, eventually, the entire glass will be colored.
3. When steeping a cup of tea, molecules from the tea cross from the tea bag and diffuse throughout the cup of water.
4. When shaking salt into water, the salt dissolves and the ions move until they are evenly distributed.
5. After lighting a cigarette, the smoke spreads to all parts of a room.
6. After placing a drop of food coloring onto a square of gelatin, the color will spread to a lighter color throughout the block.
7. Carbon dioxide bubbles diffuse from an open soda, leaving it flat.
8. If you place a wilted celery stick in water, water will diffuse into the plant, making it firm again.
9. Water diffuses into cooking noodles, making them bigger and softer.
10. A helium balloon deflates a little bit every day as helium diffuses through the balloon into the air.
11. If you place a sugar cube in water, the sugar will dissolve and evenly sweeten the water without having to stir it.

Simple Diffusion Experiment

See diffusion for yourself with this simple experiment.

• 2 water glasses
• Baby oil or vegetable oil
• Water
• Food coloring

1. Fill a glass mostly full of water.
2. In a second glass, add a bit of oil and some drops of food coloring. You can use multiple colors of food coloring, but take care to avoid mixing them.
3. Stir together the oil and food coloring so that you break the drops into smaller ones.
4. Pour the oil and food coloring into the water glass. The food coloring drops into the water and diffuses into it.

Expand upon this project by comparing the rate of diffusion in hot water versus cold water. If you use different colors of food coloring, explore color theory and see what you get when two different colors mix. For example, red and blue make purple, yellow and blue make green, and so on. Can you explain why food coloring diffuses in the water, but no into the oil?

Diffusion vs Other Transport Processes

Diffusion, together with osmosis and facilitated diffusion, are types of passive transport processes. What this means is that energy is not required for these processes to occur. They are thermodynamically favorable and driven by chemical potential or Gibbs free energy.

In contrast, active transport processes require the input of energy to occur. Active transport includes primary (direct) active transport and secondary (indirect) active transport. The first uses energy molecules as transport mediators. The second couples molecule movement with a thermodynamically favorable transport.

Types of Diffusion

There are several types of diffusion, including:

• Anisotropic diffusion enhances high gradients.
• Atomic diffusion occurs in solids.
• Bohm diffusion involves plasma transport across magnetic fields.
• Eddy diffusion involves turbulent flow.
• Knudsen diffusion is diffusion of a gas through long pores where wall collisions occur.
• Molecular diffusion is movement of molecules from high concentration to low concentration.
  

Sources

• Barr, L.W. (1997). "Diffusion in Materials". DIMAT 96. Scitec Publications. 1: 1-9.
• Bromberg, S.; Dill, K.A. (2002). Molecular Driving Forces: Statistical Thermodynamics in Chemistry and Biology. Garland Science. ISBN 0815320515.
• Kirkwood, J.G.; Baldwin, R.L.; et al. (1960). "Flow equations and frames of reference for isothermal diffusion in liquids". The Journal of Chemical Physics. 33(5): 1505–13.
• Muir, D. C. F. (1966). "Bulk flow and diffusion in the airways of the lung". British Journal of Diseases of the Chest. 60 (4): 169–176. doi:10.1016/S0007-0971(66)80044-X.
• Stauffer, Philip H.; Vrugt, Jasper A.; Turin, H. Jake; Gable, Carl W.; Soll, Wendy E. (2009). "Untangling Diffusion from Advection in Unsaturated Porous Media: Experimental Data, Modeling, and Parameter Uncertainty". Vadose Zone Journal. 8 (2): 510. doi:10.2136/vzj2008.0055