Question

Which one of the following is NOT correct regarding diffusion?

• A. In diffusion, concentration gradient is a driving force
• B. Diffusion is a physical flow of matter
• C. Diffusion occurs at a definite velocity
• D. A diffusion component leaves no space behind

Hint:

Unlike bulk fluid flow (which has a definite velocity vector), molecular diffusion is a slow, statistical process.
Its rate is characterized by the diffusion coefficient (\(D\)), and the mean square displacement of molecules is proportional to time (\(x^2 \propto t\)), not velocity.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Molecular diffusion is the net transport of individual molecules from a region of higher concentration to a region of lower concentration.
This mass transfer process is governed by the random thermal motion of molecules (Brownian motion).

Step 2: Detailed Explanation:
Let us evaluate each statement:
- In diffusion, concentration gradient is a driving force: Correct. According to Fick's first law, the mass flux is directly proportional to the concentration gradient.
- Diffusion is a physical flow of matter: Correct. It represents the physical migration of chemical species through a medium.
- Diffusion occurs at a definite velocity: Incorrect. Because molecular diffusion is driven by random, spontaneous thermal collisions (Brownian motion), individual molecules move in a random walk. Thus, molecular diffusion does not occur at a single, fixed, or definite velocity, unlike bulk convection flow.
- A diffusion component leaves no space behind: Correct. As solute molecules diffuse out, the surrounding solvent or fluid molecules immediately fill the space, maintaining the continuous liquid/gas phase density.

Step 3: Final Answer:
Therefore, the statement "Diffusion occurs at a definite velocity" is not correct, which corresponds to option (C).