Question

Which law is used to describe steady-state diffusion in gases?

• A. Henry’s law
• B. Raoult’s law
• C. Fick’s first law
• D. Dalton’s law

Hint:

For non-steady state diffusion (where concentration changes with time), we use Fick's Second Law: \( \frac{\partial c}{\partial t} = D \frac{\partial^2 c}{\partial x^2} \).
Steady state \( \implies \frac{dc}{dt} = 0 \).

Answer 1

The Correct Option is C

Step 1: Explain the idea of diffusion. 
Diffusion refers to the spontaneous movement of particles from a region where their concentration is high to a region where it is lower.
This movement occurs because molecules are in constant random motion.
When the concentration profile remains unchanged with time, the process is called steady-state diffusion.

Step 2: Identify the governing law.
Under steady-state conditions, the rate at which a substance diffuses is directly related to how sharply its concentration changes with distance.
This relationship is expressed by Fick’s first law, written as:

\[ J = -D \frac{dc}{dx} \]

where:

• $J$ is the diffusion flux (amount transferred per unit area per unit time),
• $D$ is the diffusion coefficient,
• $\frac{dc}{dx}$ is the concentration gradient.

The negative sign indicates that diffusion occurs in the direction of decreasing concentration.

Step 3: Distinguish from other physical laws.
Henry’s law deals with gas solubility in liquids.
Raoult’s law relates vapor pressure to mole fraction in solutions.
Dalton’s law concerns the total pressure of gas mixtures.
None of these describe molecular transport due to concentration gradients.
Only Fick’s first law directly explains mass transfer by diffusion under steady conditions.

Step 4: Final conclusion.
The law that governs steady-state diffusion is:

\[ \boxed{\text{Fick’s First Law}} \]