Adsorption arises due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk. The extent of adsorption increases with the increase of surface area per unit mass of the adsorbent at a given temperature and pressure. Another important factor featuring adsorption is the heat of adsorption. During adsorption, there is always a decrease in residual forces of the surface, therefore, it is invariably an exothermic process or \(\Delta H\) and \(\Delta S\) are always negative. There are mainly two types of adsorption of gases on solids. In physisorption, the attractive forces are mainly van der Waals forces while in chemisorption, actual bonding occurs between the particles of adsorbate and adsorbent. Generally, easily liquefying gases are adsorbed more easily on the surface of a solid as compared to the gases which are liquefied with difficulty. Freundlich gave an empirical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure, at a particular temperature.
Which two parameters from the following are plotted to give a straight line on Freundlich adsorption isotherm?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(log \frac{x}{m}\text{ vs }log P\)
The Freundlich adsorption isotherm is
\( \displaystyle\frac{x}{m} = k \cdot P^{1/n} \) (n > 1)
Taking logarithms on both sides:
\( \log \displaystyle\frac{x}{m} = \log k + \displaystyle\frac{1}{n} \log P \)
This has the form y = c + mx, so a plot of log (x/m) against log P is a straight line with slope 1/n and intercept log k.
Why the other options are incorrect
Option (1) x/m vs 1/P — Incorrect. x/m varies as P1/n, not as 1/P, so this plot is not linear (and x/m does not fall as P rises).
Option (2) log x/m vs P — Incorrect. Log (x/m) is linear in log P, not in P itself.
Option (3) x/m vs P — Incorrect. This is the adsorption isotherm itself, a curve that rises steeply and then levels off.
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):
Freundlich Adsorption Isotherm – “… Taking logarithm of eq. (5.1): log x/m = log k + (1/n) log P. The validity of Freundlich isotherm can be verified by plotting log x/m on y-axis (ordinate) and log P on x-axis (abscissa). If it comes out to be a straight line, the Freundlich isotherm is valid, otherwise not.”