The figure shows the partial vapour pressures and the total vapour pressure over an ideal solution of two volatile liquids, components 1 and 2, plotted against the mole fraction at constant temperature.
Which component of this solution is more volatile?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Component 2, because the vapour pressure of pure component 2, p20, is higher than p10.
At the same temperature, the more volatile liquid is the one with the higher vapour pressure in the pure state. In the figure each partial-pressure line reaches the vapour pressure of its pure component where its mole fraction is 1.
Reading the figure — p10 is on the left axis (x1 = 1) and p20 on the right axis (x2 = 1). The point p20 is higher, and the total vapour pressure rises from p10 to p20.
More volatile — component 2, since p10 < p20.
Why the other options are incorrect
Option (1) Component 1, because its partial vapour pressure p1 falls as x2 increases. — Incorrect. p1 falls only because there is less of component 1 as x2 increases. The partial pressure of each component falls as its own mole fraction falls; this says nothing about volatility.
Option (2) Component 1, because the line of p1 starts from the vapour pressure axis on the left. — Incorrect. Where a line starts depends only on how the mole-fraction axis is drawn. Volatility is judged by the height of p10 and p20, and p10 is the lower one.
Option (4) Neither, because the line of the total vapour pressure is straight, so both are equally volatile. — Incorrect. The line is straight because the solution is ideal. It slopes upward from p10 to p20; if the two liquids were equally volatile it would be horizontal.
NCERT (2026-27 edition):
1.4.1 Vapour Pressure of Liquid-Liquid Solutions – “A plot of p1 or p2 versus the mole fractions x1 and x2 for a solution gives a linear plot as shown in Fig. 1.3. These lines (I and II) pass through the points for which x1 and x2 are equal to unity. Similarly the plot (line III) of ptotal versus x2 is also linear (Fig. 1.3). The minimum value of ptotal is p10 and the maximum value is p20, assuming that component 1 is less volatile than component 2, i.e., p10 < p20.”
Fig. 1.3: The plot of vapour pressure and mole fraction of an ideal solution at constant temperature. The dashed lines I and II represent the partial pressure of the components. (It can be seen from the plot that p1 and p2 are directly proportional to x1 and x2, respectively). The total vapour pressure is given by line marked III in the figure.