How do different types of mixtures affect phase equilibria?

[Urmi Roy]

I found that this thread isn't being displayed in the chemistry forum..can anybody see it?

 

[Borek]

Hardly surprising - it is in Other Sciences, not in Chemistry.

 

[PhaseShifter]

1. Why does calcium freeze so drastically at the beginning (of the freezing process of the liquid melt)?

Anorthite has a higher melting point than albite, and will freeze first as the temperature is lowered.

2. While initially the rate of crystallization of calcium is greater, why does sodium incorporation (into the forming crystal) start increasing as the crystallisation continues?

According to the phase diagram, anorthite and albite are soluble in one another. Only one solid phase is formed as the mixture cools.
The freezing point decreases as the sodium ion concentration increases in the solid--therefore in order to be in equilibrium with the melt, the solid must incorporate more sodium as the temperature is lowered.

3.Why will the first crystal formed due to solidification of calcium consist of any sodium at all -sodium has a much lower freezing point?

Again, albite and anorthite don't form two separate solid phases. Rather, they represent the two extremes in possible compositions of a solid solution. If any sodium exists in the melt, it will also exist in the solid.

4.Is the liquidus curve in the albite-anorthrite curve in any way similar to that of the simple two component eutectic mixture (Like lead and tin)?

It's actually much simpler than the eutectic mixture, since there is only one solid phase. (and only two curves here, where the eutectic phase diagram has seven)

In this case you have two compounds that are completely miscible in both the solid and liquid phases, where in the eutectic mixture a miscibility gap exists between two solid phases.

5. Why are there two curves-solidus and liquidus--(in the diagrams for the simple eutectic systems,the liquid phase and forming cryatal phase were represented on the same diagram)?

The area between the liquidus and solidus curves is thermodynamically unstable.
A mixture inside this region will spontaneously separate out into a liquid phase on the liquidus curve) and a solid phase (on the solidus curve). Given the overall composition of the mixture along with the compositions indicated by the liquidus and solidus curves at a given temperature, you can apply the lever rule to determine the relative amounts of solid and melt.

Incidentally, this is related to the technique of recrystallization used for purification in chemistry. Since the liquid phase has a higher-than-average concentration one compound and the solid phase has a higher-than-average concentration of the other compound, you can separate the phases and then cool the liquid (or heat the solid) and repeat until you reach a desired level of purity.

6.What does it mean to move down a vertical line crossing the liquidus line, solid + liquid region and the solidus line?

Above the liquidus curve and below the solidus curve, moving down a vertical line just reduces the temperature of the existing phase.

The interesting part is when the system enters the thermodynamically unstable region. When the mixture cools below the liquidus curve, it begins to form a solid phase.
As the mixture cools through the thermodynamically unstable region, the composition of the solid phase will follow the solidus curve while the composition of the liquid will follow the liquidus curve. The relative amounts of each phase present can be determined by the lever rule.

 

[Urmi Roy]

Thanks PhaseShifter. As you can understand,my exams have already begun,so I'll take some time to go through the post,sometime in between my exams.

Thanks again.

 

[sp105]

How we can learn from the given transition if it is first order or second order phase transition? e.g. paramagnetic to ferromagnetic phase transitions are ...?

 

[Urmi Roy]

Well, I'm encountering something very much related to the eutectic mixtures stuff that I did in freshman year now in 3rd year...phaseshifter did a lot of hard work in explaining what goes on in an eutectic mixture and how we get at the eutectic composition etc etc...but is there any similar theory or explanation relating to eutectoid, peritectic and peritectoid mixtures..? In most texts, I find these terms just explained as simple definitions...can anyone suggest any website where they are explained thoroughly?

Thanks,
Urmi.