Phase Rule Application: FeCl3-H2O System
MP Assistant Professor Exam 2022
Q. In the FeCl3-H2O system, how many phases and components are there?
(A) P = 2 and C = 1
(B) P = 8 and C = 2
(C) P = 5 and C = 2
(D) P = 4 and C = 2
Correct Answer: (B) P = 8 and C = 2
Detailed Explanation:
Components (C = 2):
This is a binary system. Every phase can be defined using two independent chemical constituents: Ferric Chloride (FeCl3) and Water (H2O).
Phases (P = 8):
The Ferric Chloride-Water system is unique because it forms several stable hydrates. The 8 possible phases that can appear in its phase diagram are:
- Liquid Phase: Aqueous solution (1)
- Vapor Phase: Water vapor (1)
- Solid Phases (Ice & Anhydrous): Pure Ice and Anhydrous FeCl3 (2)
- Solid Hydrates: Four distinct stable hydrates (4):
- Hexahydrate: FeCl3 · 6H2O
- 3.5-Hydrate: FeCl3 · 3.5H2O
- 2.5-Hydrate: FeCl3 · 2.5H2O
- Dihydrate: FeCl3 · 2H2O
Note: In this system, the phase diagram shows four congruent melting points (where a solid hydrate melts into a liquid of the same composition) and five eutectic points.
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