Which of the following curves is in accordance with Freundlich Adsorption Isotherm?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Straight line of log x/m against log p with a positive intercept
The Freundlich adsorption isotherm is
\(\displaystyle\frac{x}{m} = k\,p^{1/n}\quad (n > 1)\)
Taking logarithms:
\(\log\displaystyle\frac{x}{m} = \log k + \displaystyle\frac{1}{n}\log p\)
This is the equation of a straight line of slope 1/n and intercept log k on the log x/m axis. So the plot of log x/m against log p is a straight line that cuts the y-axis above the origin, as in graph (3).
Why the other options are incorrect
Option (1) — Incorrect. A curve that rises and levels off is the shape of x/m against p (or a Langmuir-type plot), not of the log–log plot.
Option (2) — Incorrect. A curve bending upwards does not fit a linear equation in log x/m and log p.
Option (4) — Incorrect. A straight line through the origin would mean log k = 0 (k = 1), which is not the general case; the intercept gives log k.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Adsorption Isotherm – …
