This document describes the solid-gas equilibria of the CuSO4-H2O system. It discusses how copper sulfate forms different hydrates - monohydrate, trihydrate, and pentahydrate - at increasing vapor pressures when water vapor is added at a constant temperature of 55°C. The phase diagram plots vapor pressure against water vapor composition, showing invariant equilibrium lines where two solid phases coexist with vapor, and monovariant lines where a single solid phase coexists with vapor. It also describes the dehydration of copper sulfate pentahydrate upon decreasing vapor pressure, passing through trihydrate and monohydrate before forming anhydrous copper sulfate.

1. Solid-Gas equilibria
of
CuSO4.5H2O system
-Mohamed Minas
20052201017

2. There are such system in which two component forms a
compound and at a particular temperature it decomposes into
another solid and liquid.
The composition of liquid state is different here then the solid
state.
At this point; Compd with Incg MP Original solid + solution
These compounds are called as compound with Incongruent
melting point.
Actually the compound dissociate at this point and the process
is called as peritectic reaction. The Incongruent melting point is
called as meritectic or peritectic temperature.
Incongruent melting
point

3. Copper sulphate-water vapour
system
This is a two component system in which one component is solid and other component is water
vapour.
This phase diagram is drawn between vapour pressure v/s water vapour composition in such a way
that by the addition of water vapour to the system at constant temp of 55°C copper sulphate forms
three different hydrates at a certain vapour pressure level.
The three hydrates are
Copper sulphate monohydrate (CuSO4.H2O)
Copper sulphate trihydrate(CuSO4.3H20) and
Copper sulphate pentahydrate(CuSO4.5H20)

5. From Point O to A only Copper sulphate is present so there is an increase in
vapour pressure of the system. Point A to B: Now monohydrate starts
forming and system remains in equilibrium ;
CuSO4 + H20 CuSO4.H20 at 4.5 mm of Hg.
From B to C only monohydrate is present so again there is an increase in
vapour pressure of the system.
Point C to D: Now trihydrate starts forming and system remains in
equilibrium ; CuSO4.H20 + 2H20 CuSO4.3H20 at 30.9 mm Hg.
From D to E only trihydrate is present so again there is an increase in vapour
pressure of the system.
Point E to F: Now pentahydrate starts forming and system remains in
equilibrium ; CuSO4.3H20 + 2H20 CuSO4.5H20 at 45.5 mm Hg.

6. From F to G only pentahydrate is present so only increase in vapour pressure
of the system.
At point O anhydrous CuSO4, has very low vapour pressure hence no
reaction between CuSO4 and water, so by the addition of water vapour the
vapour pressure of the system starts increasing.
On line OA,BC,DE and FG two phases are present. One is either copper
sulphate or its hydrate and other is water vapour so on applying phase rule;
F’=C-P+1 ; F=2-2+1=2 i.e. system is monovariant at these lines.
On line AB,CD and EF three phases are present. Two solids and one water
vapour. There is an equilibrium between these three phases, on applying
phase rule; ; F’=C-P+1 ; F=2-3+1=0 i.e. system is invariant at these lines.

7. Dehydration of Copper sulphate pentahydrate

8. Here the temperature is kept constant and the pressure over the salt is continuously
decreased, till the dehydration of the salts and the dissociation equilibrium is set up.
From Point A to B only Copper sulphate pentahydrate is present so there is an decrease in
vapour pressure of the system. Point A to B: Now pentahydrate starts decomposing and system
remains in equilibrium.
From B to C only trihydrate is present so again there is an decrease in vapour pressure of the
system. Point C to D: Now monohydrate starts forming and system remains in equilibrium.
From D to E only monohydrate is present so again there is an decrease in vapour pressure of the
system. Point E to F: Now anhydrous CuSO4 starts forming and system remains in equilibrium.
The degree of freedom of the curve AB,CD,EF is zero i.e, invarient and degree of freedom of the
curve B,D,F is 1 i.e, monovarient.