Diffusion is a fundamental passive transport process where particles move from a higher concentration to a lower concentration due to random molecular motion, influencing diverse phenomena from biological respiration to industrial semiconductor doping.

Diffusion is the spontaneous movement of particles from an area of higher concentration to an area of lower concentration, driven by the random motion and collisions of molecules in gases or liquids. It is a passive transport process that does not require external energy input.

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Important factors affecting diffusion include:

• Concentration gradient: The greater the difference in concentration, the faster the diffusion rate. As equilibrium approaches, diffusion slows down.
• Mass of molecules: Lighter molecules diffuse more quickly than heavier ones.
• Temperature: Higher temperatures increase molecular energy and speed up diffusion; lower temperatures slow it down.
• Solvent density: Denser solvents slow diffusion because molecules have more difficulty moving through the medium.
• Surface area and membrane thickness: Larger surface areas increase diffusion rate, while thicker membranes reduce it.
• Distance traveled: Longer distances slow diffusion, which limits the size of cells and biological structures relying on diffusion.

Diffusion results in dynamic equilibrium where molecules continue moving but with no net change in concentration.

Applicative Examples of Diffusion

• Respiration: Oxygen diffuses from the lungs (high concentration) into the blood (low concentration), while carbon dioxide diffuses in the opposite direction to be exhaled.
• Perfume dispersal: When perfume is sprayed, its molecules diffuse through the air from a high concentration near the spray to lower concentrations throughout the room, allowing the scent to be detected.
• Tea brewing: Tea molecules diffuse from the tea bag into hot water, coloring and flavoring the water without stirring.
• Helium balloons: Helium slowly diffuses out of balloons because of the concentration difference, causing the balloon to deflate over time.
• Food preservation: Osmotic dehydration uses diffusion where water moves out of food into a high concentration sugar or salt solution, preserving the food by reducing water content.
• Carburisation in metallurgy: Carbon diffuses into the surface of steel at high temperatures to harden it by forming carbides, improving wear resistance while maintaining toughness inside.
• Semiconductor doping: Dopant atoms diffuse into silicon wafers at high temperatures to modify electrical properties, essential for making electronic devices.
• Water purification: Diffusion principles underlie processes like reverse osmosis, where water molecules move through membranes to separate impurities.