Fick's law of diffusion
Summary
Fick's law says the net flux of a solute by diffusion is proportional to the concentration gradient and points from high to low concentration. Testable point: doubling the gradient doubles the flux, and a larger diffusion coefficient D (smaller particle, higher temperature, less viscous medium) also raises it.
Detail
Meaning: Particles move randomly, but more of them start in the concentrated region, so the net movement runs down the concentration gradient. The flux grows with the steepness of the gradient and with the diffusion coefficient D. D is larger for smaller particles, at higher temperature, and in less viscous media. Across a membrane the same idea is often written as rate ∝ (area × concentration difference) ÷ thickness, which is how gas exchange in the lungs is usually described in biology.
Equation: J = −D(ΔC/Δx), where J is flux in mol/(m²·s), D is the diffusion coefficient in m²/s, and ΔC/Δx is the concentration gradient in mol/m⁴ (concentration in mol/m³, distance in m). The minus sign means flux points toward lower concentration.
On the MCAT: A passage describes two compartments or a membrane with different concentrations. You predict how flux changes if the gradient doubles (flux doubles), the membrane thickness doubles (flux halves), the temperature rises (D increases), or the solute is larger (D decreases). You may also read a concentration versus distance graph, where a steeper slope means a larger flux.
Common confusion: Flux depends on the gradient, which is a difference over a distance, not on concentration alone. At equilibrium the gradient is zero, so net flux is zero, but particles keep moving randomly. Diffusion is not pushed by a force; it is a statistical result of random motion.
Sources
- Physical Chemistry (Atkins)
- Physics: Principles with Applications (Giancoli)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.