UPSC Civil Services (Mains), Chemistry optional · 2023 · Paper I
Q1(e)(ii). Surface tension, contact angle and loss of gravity
Question
The surface tension of liquid A is seven times higher than that of liquid B. (1) Which liquid is expected to have the 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?
Answer in brief
Liquid A has the higher contact angle; without gravity A draws into one nearly spherical drop with least contact with the glass, while B creeps over the glass and coats the beaker.
Key points
- In Young's equation, cos θ = (γSV − γSL)/γLV, the liquid's own surface tension is in the denominator, so a larger γLV gives a larger θ.
- When cohesion exceeds adhesion to glass, θ is above 90° and the liquid does not wet the glass; mercury and water are such a pair.
- Without gravity each liquid takes the shape of lowest total surface energy.
- A (non-wetting) collects into a single nearly spherical drop of radius about 1.3 cm with only a small patch of contact.
- B (wetting) climbs the wall, spreads as a film over the whole inner surface and can creep over the rim.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coverssurface tensioncontact angleYoung equationwettingcohesion and adhesionmicrogravity
All parts of Question 1
- 1(a) Light emitted by a particle in a box; the uncertainty principle10 marks
- 1(b) Lewis structures of [Br₃]⁻ and thiourea; the most polar bond10 marks
- 1(c) Bragg equation and the layer spacing10 marks
- 1(d) Root-mean-square speed of water molecules10 marks
- 1(e)(i) E° of Cr³⁺/Cr from two couples5 marks
- 1(e)(ii) Surface tension, contact angle and loss of gravity5 marks