Diffusion
Section: 1. States of Matter | Syllabus: Cambridge AS Level Physics 9702
What is Diffusion? Diffusion is the natural spreading of particles from areas of high concentration to areas of low concentration. It provides strong evidence for kinetic theory and particle motion. Diffusion The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of random particle motion.
How Diffusion Works Diffusion occurs because particles are in constant random motion: Particles constantly move in random directions In high concentration areas, more particles move randomly outward than inward In low concentration areas, fewer particles are present to move randomly The net effect is movement from high to low concentration Diffusion continues until particles are evenly distributed (equilibrium) Even at equilibrium, particles continue to move randomly, but there's no net movement Key Concept Diffusion is a passive process - it doesn't require energy input.
It happens naturally due to random particle motion and continues until concentration is equal throughout. Examples of Diffusion In Gases Perfume in a room: Perfume particles spread throughout the room until evenly distributed Cooking smells: Food aromas diffuse from the kitchen to other rooms Gas leak detection: Natural gas (with added smell) diffuses to warn of leaks Air fresheners: Fragrance particles gradually spread through the air In Liquids Food coloring in water: Dye particles spread throughout the water Tea bag: Tea particles diffuse from the bag into hot water Sugar dissolving: Sugar particles spread evenly through the liquid Ink in water: Ink particles gradually disperse throughout In Solids Diffusion in solids is extremely slow because particles can only vibrate in fixed positions Example: Lead and gold blocks pressed together for years show minimal diffusion at the interface Diffusion in solids generally only occurs at very high temperatures Rate of Diffusion Several factors affect how quickly diffusion occurs: 1.
State of Matter State Rate of Diffusion Reason Gases Very fast Particles far apart, move rapidly, few collisions slow them down Liquids Moderate/Slow Particles close together, more collisions, slower movement Solids Extremely slow Particles vibrate in fixed positions, cannot move past each other Important Diffusion is fastest in gases because gas particles are far apart and move at high speeds with few obstacles.
In liquids, particles are closer together and collide more often, slowing diffusion. 2. Temperature Higher temperature increases the rate of diffusion: Higher temperature → particles have more kinetic energy More kinetic energy → particles move faster Faster movement → particles spread out more quickly Example: Tea diffuses faster in hot water than cold water 3.
Particle Mass (Molecular Mass) Lighter particles diffuse faster than heavier particles at the same temperature: At the same temperature, all particles have similar kinetic energy Lighter particles move faster to have the same kinetic energy as heavier ones Faster movement → quicker diffusion Example: Ammonia (NH₃, mass 17) diffuses faster than hydrogen chloride (HCl, mass 36.5) Kinetic Energy = ½ × mass × velocity² If KE is constant, lower mass means higher velocity 4.
Concentration Gradient A steeper concentration gradient (bigger difference in concentration) leads to faster net diffusion initially: Large difference in concentration → faster initial diffusion rate As particles spread out, concentration gradient decreases Diffusion rate slows as equilibrium is approached Classic Diffusion Experiments Ammonia and Hydrogen Chloride Experiment This classic experiment demonstrates that diffusion rate depends on molecular mass: Cotton wool soaked in ammonia solution (NH₃) placed at one end of a glass tube Cotton wool soaked in hydrochloric acid (HCl) placed at the other end Both gases diffuse along the tube Where they meet, they react to form a white ring of ammonium chloride (NH₄Cl) The white ring forms closer to the HCl end because NH₃ (lighter) diffuses faster Exam Tip In the ammonia-HCl experiment, remember: ammonia is lighter (mass 17) so diffuses faster than HCl (mass 36.5).
The white ring of ammonium chloride forms closer to the HCl end. Potassium Manganate(VII) in Water Purple crystals of potassium manganate(VII) placed in water gradually turn the entire solution purple as particles diffuse throughout the water.
This is faster in hot water than cold water. Bromine Diffusion in Air Brown bromine gas placed in a sealed container gradually becomes evenly distributed as bromine particles diffuse throughout the air.
Diffusion and Kinetic Theory Diffusion provides evidence for kinetic theory: Proves particles exist: We can observe the effects of invisible particles spreading Proves constant motion: Diffusion wouldn't occur if particles were stationary Proves random motion: Particles spread in all directions, not in an organized way Temperature effects: Faster diffusion at hig…
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