Unknown facts of Zn-Mg system and NaCl-H2O system

Unknown facts of Zn-Mg system and NaCl-H2O system


  1. In today’s article, we are going to study about ‘Unknown facts of  Zn-Mg systems and NaCl-H2O system’ How beneficial it is and what is the Zn-Mg system and NaCl-H2O system. We will also read about some more topics like eutectic point, phase diagram of Zn-Mg system and NaCl-H2O system, explanations of curves, etc in detail.  So, take your notebooks in you hand and get ready to study physics in an easy and sorted way.

Zn-Mg system

It is an example of a two component system in which two metals form a compound Mg(Zn)2 having congruent melting point. Melting point of pure Zn is 420°C (point A) and that of pure Mg is 650°C (point B) whereas m.p. of compound Mg(Zn)2 is 590°C (point D) which is in between the melting points of two metals. This system has four phases i.e. solid Zn, solid Mg, solid compound Mg(Zn)2 and melt of Zn and Mg. Phase diagram of the system is shown in fig.

  • Zn-Mg system

Important features of the phase diagram are :

Point A : It represent the m.p. of pure Zn (420°C)

Curve AC : When Mg is added to Zn gradually the m.p. of Zn is lowered along the curve AC. Along this curve solid Zn is in equilibrium with melt (liquid). Thus, there are two phases along this curve and on applying the condensed phase rule F= C-P+1=2-2+1=1 we see that it is a monovariant system.

It means only the composition varies along this curve.

Point B : This represent the melting point of pure Mg(650°C).

Curve BE : When Zn is added to Mg gradually, m.p. of Mg is lowered along the curve BE. Along the curve BE solid Mg is in equilibrium with melt (liquid). Again this is also a monovariant Curve having one degree of freedom.

Curve DC : This curve represent the freezing point curve of solid Mg(Zn)2. When Zn is added. The m.p. of compound Mg(Zn)2 lowers along this curve.

Curve DE : This curve represent the melting point Curve of Mg(Zn)2 when Mg is added in it.

Point D : At this point, the liquid and solid have the same composition i.e. Mg(Zn)2. Thus point D is the congruent point and temprature (590°C) corresponds to this point is known as congruent melting point of Mg(Zn)2.

Eutectic point

Point C : Two Curves AC and DC interest at point C. Three phase i.e. solid Zn, compound Mg(Zn)2 and melt are in equilibrium at this point. Thus, on applying phase rule F= C-P+1 = 2-3+1 =0 . We see that point C is non variant point C is known as eutectic point.

Point E : This is the point where curves DE and BE interset. Here also three phases i.e. Solid Mg, compound Mg(Zn)2 and melt are in equilibrium at this point. Again it is invariant and known as eutectic point.


When liquid melt is cooled at point x, it follows the path xy and Mg(Zn)2 separates out at y and two phases i.e. solid Mg(Zn)2 and liquid are in equilibrium and the degree of freedom is one. On further cooling curve follows the path yC and at C, solid Zn also separates out. Thus, there will be three phases at point C. These phases are solid Zn, solid Mg(Zn)2 and melt. Therefore C is monovariant.

NaCl- H2O system

Phase diagram of NaCl-H2O system is shown in fig.

Point A is the freezing point of water (0°C). When NaCl is added gradually in it, its freezing point is lowered along the curve AB. Ice and saturated solution are in equilibrium along AB and the system is univarient. At point B(-21.2°C), a new solid phase NaCl.2H2O separates out from the solution. Point B is the cryohydric point where three phase, viz., Ice, incongruent melting compound (NaCl.2H2O) and the solution are in equilibrium and the system is nonvariant (F=0). (Below this point only ice and NaCl.2H2O exist). On further addition of NaCl, the freezing point rises along BC which is the solubility curve of NaCl.2H2O. point C (0.15°C) is the peritectic point where the dihydrate changes into anhydrous sodium chloride and solution. CD is the solubility curve for anhydrous sodium chloride.

NaCl. 2H2O ⇔ Nacl + solution


Area ABCD consist of saturated solution of NaCl and area ABE has two phases, viz. Ice and saturated solution. Solid NaCl + saturated solution are found in area CDHG and area CBFG have two solid phases NaCl.2H2O + saturated solution.


So our article is finished and after completely reading this article, one can easily tell what an phase diagram of Zn-Mg system and NaCl-H2O is. And how we can explain it.

‌So one can say that they got a detailed information about Zn-Mg system and NaCl-H2O.

Must read : 

Application of phase diagram : water system, Application of phase diagram : carbon dioxide system and Application of phase diagram : sulphur system in earlier articles. You can visit there for more information.

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