Diffusion

What is Diffusion?

Open a bottle of perfume across the room and the scent reaches you in a few minutes. Drop ink into still water and the color spreads outward without stirring. Every such observation is diffusion: net movement of particles from high to low concentration, driven entirely by random thermal motion. No pumping — every molecule wanders independently, but because there are more on the high-concentration side, statistically more cross to the low side. Fick's First Law captures it: J = −D · ΔC/Δx, where flux is proportional to the gradient and D depends on temperature, mass, and medium. Add a semi-permeable membrane and you get osmosis — the case where the diffusing species is the solvent crossing in response to solute concentration. This lab puts a two-chamber box on screen with a tunable membrane, lets you set temperature and concentrations, and shows the molecular dance plus the concentration-vs-time curve.

Parameters explained

Temperature(K)

Absolute temperature in kelvin. Temperature controls molecular kinetic energy; KE_avg = (3/2)k_B T. In the liquid Brownian-motion model used here, the Stokes-Einstein relation gives D = k_B T/(6πηr), so doubling T from 300 K to 600 K doubles D if viscosity η and particle radius r stay fixed. Real liquids also change viscosity with temperature, which is why warm tea brews much faster than iced tea.

Left Concentration(mol/L)

Particle concentration in the left chamber, in mol/L. Sets the high-concentration side at start. Fick's First Law J = −D · ΔC/Δx says the flux is proportional to the difference between left and right; bigger gradient means faster initial flow. As particles diffuse rightward, conc_left drops and the gradient shrinks, so the diffusion rate slows over time — that's why concentration vs. time curves bend toward equilibrium exponentially.

Right Concentration(mol/L)

Particle concentration in the right chamber, in mol/L. Together with conc_left this sets the initial gradient. Equilibrium is reached when both chambers have the same concentration (the average of the two starting values, by mass conservation). Equilibrium does not mean particles stop moving — it means equal numbers cross each direction per second, so net flux is zero.

Pore Size(nm)

Effective membrane pore size in nanometers. Bigger pores let more particles through per unit time, so flux scales roughly with cross-sectional area. Smaller pores selectively block larger molecules — this is how cell membranes choose which solutes to admit, and how dialysis filters separate small waste molecules from larger blood proteins. Set pore size below the particle diameter and diffusion stops entirely.

Common misconceptions

• MisconceptionAt equilibrium the molecules stop moving because there's nothing more to do.

  CorrectMolecules keep moving at full thermal speed at equilibrium — KE_avg = (3/2)k_B T doesn't go to zero. What stops is the net flux. Equal numbers cross each direction per second, so concentrations stay constant. Watch the simulation at equilibrium and you'll see particles still bouncing across the membrane in both directions; the net flow is just zero.

• MisconceptionParticles diffuse from high to low concentration because they 'want to' spread out.

  CorrectMolecules don't want anything. Each one moves randomly, independent of all the others. The net flow toward low concentration is statistical: with more particles on the high side, it's simply more likely that one of them is the next to cross. This is the second-law-of-thermodynamics flavor of diffusion — entropy increases because spreading out has way more microstates than staying clumped.

• MisconceptionOsmosis is special diffusion where water is somehow pulled across the membrane by the solute.

  CorrectOsmosis is just diffusion of water through a membrane that blocks the solute. Water diffuses both directions equally if there is no solute. Add solute on one side and that side has slightly fewer water molecules (because solute occupies some volume and interacts with water), so net water flow goes from low solute to high solute concentration. Nothing 'pulls' the water — it just diffuses normally based on its own concentration gradient.

• MisconceptionHeat and temperature mean the same thing in this experiment.

  CorrectTemperature is the average kinetic energy per molecule (intensive); heat is the energy transferred between systems (extensive). Two boxes of gas at the same T have molecules with the same average KE, but the bigger box holds more total thermal energy. Diffusion rate depends on T because that controls per-molecule speed; the total amount of heat in the system doesn't directly enter Fick's law.

• MisconceptionBigger molecules diffuse faster because they're more massive.

  CorrectBigger and heavier molecules diffuse slower. From kinetic theory v_avg ∝ 1/√m, so a hydrogen molecule (m = 2) zips along about four times faster than oxygen (m = 32) at the same temperature. The diffusion coefficient D scales roughly with v_avg, so heavier molecules also have smaller D. That's why oxygen takes longer to diffuse across an alveolar membrane than CO₂ would on its own — even though biology helps speed up the actual gas exchange.

How teachers use this lab

1. Equilibrium prediction: have students compute the final equilibrium concentration before running the simulation. With c_left = 8 and c_right = 2, mass conservation gives 5 in each chamber. Compare with the simulation's final readout to anchor the average rule.
2. Temperature dependence lab: have student pairs run the simulation at 200 K, 400 K, and 800 K with identical starting concentrations, recording time-to-half-equilibrium each time. Plot vs. √T to recover the kinetic-theory prediction.
3. Pore-selectivity discussion: assign teams to find the pore size that just stops the simulated particle from crossing. Connect the result to real cell membranes (water channels are ~0.3 nm, ions move through specific channels) and dialysis filters (let urea through but not albumin).
4. Osmosis tie-in: ask 'why does pickling shrivel a cucumber?' before the lab. Use the high-solute-low-solute logic to predict that water leaves the cucumber's cells, then run a simulation with the right side as 'high solute' to model it visually.
5. Misconception probe: at equilibrium, pause the simulation and ask 'are the particles still moving?' Many students will say no because the concentration isn't changing. Use the still-moving particles on screen to drive home the difference between net flux and microscopic motion.