Q4(b)(i). Incongruent melting and azeotropy
Draw and label (1) a two-component T–composition solid–liquid diagram with one incongruently melting compound AB₂ and negligible solid solubility; (2) a constant-temperature composition liquid–vapour diagram with an azeotrope at XB = 0.333 and complete miscibility. Identify phases and intersections.
Model answer
The free solution for 2025 answers questions 1, 2, 3, 5 and 6, the ones picked in its time plan. Question 4 is answered in full in the IFoS Chemistry Paper I, 2021 to 2025 book, with every part of every paper.
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UPSC Indian Forest Service (Main) Chemistry Paper I: Model Answers 2021 to 2025
- All eight questions of each paper, with the question printed above its answer
- Physical chemistry numericals worked with units; inorganic structures drawn
- Topic-wise weightage and a topic-wise index
Coversincongruent meltingperitectic pointazeotropebinary phase diagramliquid–vapour diagram
All parts of Question 4
- 4(a)(i) Boiling point at reduced atmospheric pressure5 marks
- 4(a)(ii) Chemical potential of supercooled water5 marks
- 4(b)(i) Incongruent melting and azeotropy5 marks
- 4(b)(ii) Testing the stated activity-coefficient result10 marks
- 4(c)(i) Hydrogen and propane fuel cells10 marks
- 4(c)(ii) Competition between zinc deposition and hydrogen5 marks