Liquid State – Chemistry Optional PYQs (2000–2026)

Here are all 33 Liquid State questions asked in UPSC Chemistry Optional Paper I from 2000 to 2026, with the latest questions first.

Each question ends with a code such as [2024/15m/3b] – the year, the marks, the word limit (where UPSC set one) and the question number in the paper.

33 questions

1. The surface tension of liquid A is greater than that of liquid B. Which liquid will show larger (i) vapour pressure, (ii) cohesive force, (iii) boiling point, (iv) capillary action and (v) wetting property? Explain. [2026/5m/1b]

2. Two immiscible liquids A and B have surface tension values ${\gamma }_A$ and ${\gamma }_B$, respectively. When both the liquids are taken in a container, then what would be the interfacial tension between the two liquid phases? (Given: ${\gamma }_A>{\gamma }_B$) [2026/5m/2b(i)]

3. Calculate the pressure differential across the surface of ethanol spherical droplet of radius 220 nm at 20 °C. The surface tension of ethanol at that temperature is $22.39mN{m}^{-1}$. [2026/10m/2b(ii)]

4. Why do liquids become superheated before boiling? Explain using Kelvin equation. [2025/5m/1e]

5. Suppose that 10.0 J of work is required to create droplets of uniform size from a mole of water in bulk at 25°C and 1 atm pressure. (i) Assuming that surface tension is independent of area, calculate the radius of the droplets. (ii) Calculate the number of water molecules in a droplet. Given: Surface tension of water $=0.072J/m^2$ [2025/15m/3b]

6. Which of the following liquids has greater surface tension: Ethanol or Dimethyl ether Explain the answer with reasons. [2024/5m/3b(i)]

7. Calculate the difference in pressure across the liquid-air interface for a water droplet of radius 150 nm. [2024/5m/3b(ii)]

8. The surface tension of liquid $A$ is seven times higher than that of liquid $B$. (1) Which liquid is expected to have higher contact angle with glass? (2) 10 mL of each of these liquids are placed in separate 100 mL glass beakers. How do these liquids respond if the gravitational field is switched off? [2023/5m/1e(ii)]

9. A 15 cm drinking straw is inserted in a glass of water at an angle 45°. What pressure difference (in torr) must be generated across the length of the straw to drink water? The density of water is $1.0gc{m}^{-3}$. [2023/10m/4b]

10. A capillary tube of radius $r$ is inserted into a liquid to blow a bubble of the same radius $r$. If the excess pressure required to blow the bubble is 2.16 torr, then what is the diameter of the capillary tube in cm? The surface tension of the liquid is $0.072N{m}^{-1}$. [2023/10m/4c]

11. Consider two liquids $A$ and $B$ such that $A$ has half of the surface tension and twice the density of $B$. If liquid $A$ rises to a height of 2.0 cm in a capillary, what will be the height to which liquid $B$ will rise in the same capillary. [2022/10m/4c]

12. A drop of water, 0.4 cm in radius, is split up into 125 tiny drops. Find the increase in surface energy. [${\gamma }_{water}$ (surface tension of water) = 72 dynes/cm]. [2021/10m/3a]

13. Explain the phenomenon of capillary action with showing relation between the angle of contact and the nature of liquid and derive the expression, $\gamma =\frac{1}{2}h\rho gr$ (Symbols have usual meanings). [2020/10m/4b]

14. A spherical soap bubble of volume $\frac{\pi }{6}c{m}^3$ stands suspended in air. If the interfacial tension for soap solution-air interface is 27 dyn $c{m}^{-1}$, calculate the excess pressure inside the bubble. [2020/10m/6c]

15. Calculate the vapour pressure of a water bubble of radius ${10}^{-8}$ m at 20°C. Vapour pressure of pure water at 20°C is 23.76 Torr. Surface tension of water is $72.76\times {10}^{-3}N{m}^{-1}$; density of water is $0.9982gc{c}^{-1}$. [2019/15m/4b]

16. Explain the significance of Kelvin equation. [2018/5m/2b(i)]

17. Discuss the effect of temperature on the viscosity of liquids and gases. [2018/5m/2b(ii)]

18. Draw diagrams showing the capillary action of (i) water and (ii) mercury in two glass tubes of different radii. Explain their behavior also. [2018/10m/3d]

19. Find the change in surface area and energy when two identical mercury droplets of diameter 2 mm merged isothermally to one drop. Given that surface tension of mercury is $490dynec{m}^{-1}$ at experimental temperature. [2017/10m/2b]

20. In terms of the kinetic molecular model of matter, explain why– (i) a liquid in an open container will eventually vapourize even if the temperature is below the boiling point; (ii) the temperature of a pure, boiling liquid substance does not change even though heat is continually flowing into the container. [2015/10m/2b]

21. How high will a liquid rise in a capillary tube of 0.1 mm diameter, if its density is $1.3\times {10}^{3}kg{m}^{-3}$ and surface tension $0.065N{m}^{-1}$? Assume $\theta \approx 0$. [2015/10m/2c]

22. The viscosity of gases increases with temperature, but the viscosity of liquids decreases with temperature. Explain. [2015/10m/3d]

23. Sodium lauryl sulphate can decrease the surface tension of water, but sodium chloride increases the surface tension. Explain. [2015/5m/4b(i)]

24. On gradual increase of surfactant from zero concentration to a higher value, there is a sharp change in surface tension after some value. What is the reason for this? [2015/5m/4b(ii)]

25. The density of acetone at 30 °C is $0.8310gm{L}^{-1}$. Calculate the surface tension of acetone, given that the parachor values of C, H, O and the double bond are 8.2, 16.3, 19.5 and 21.5 respectively. [2014/10m/1d]

26. How high will a liquid rise in a capillary tube of 0.4 mm diameter, if its density is $1.3\times {10}^{3}kg{m}^{-3}$ and surface tension $8.5\times {10}^{-2}N{m}^{-1}$? ($g=9.8m{s}^{-2}$) [2014/10m/2c]

27. The surface tension of water at 20 °C is $72.75\times {10}^{-3}N{m}^{-1}$. A 33.24 vol% solution of ethanol has $\gamma =33.24\times {10}^{-3}N{m}^{-1}$ at the same temperature. Given $\rho =0.9614\times {10}^{3}kg{m}^{-3}$ for the solution and $0.9982\times {10}^{3}kg{m}^{-3}$ for water. How much less will alcohol solution rise in the same capillary tube? [2013/10m/2c]

28. Using the Kelvin equation calculate $P_s/P$ at 25°C, where $P_s$ is the vapour pressure for a very small droplet of water of radius $r=1\times {10}^{-3}m$ and P is that of bulk water. Given that for water at this temperature, $\gamma =71.97\times {10}^{-3}N{m}^{-1}$, $\rho =0.9970\times {10}^{3}kg{m}^{-3}$ and $R=8.314J{K}^{-1}mo{l}^{-1}$. [2011/15m/2c]

29. Derive an equation for surface tension exerted by a liquid in a glass capillary tube placed in the liquid when there is a rise. [2009/20m/3b]

30. Derive the Kelvin equation for the vapour pressure of droplets and bubbles. [2008/10m/3b(i)]

31. What are nucleation centres? Indicate their importance. [2008/10m/3b(ii)]

32. What are liquid-crystals? Mention their characteristics. Which substances generally behave as liquid-crystals? [2007/20m/1d]

33. How liquid crystals are different from solid crystals? Compare their characteristic features. [2006/20m/7c]

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Last updated: 30 October 2026