Impurities and melting point

[petrushka.googol]

Why do impurities lower the melting point of solids in molecular terms ?

 

Can a solid be doped with a material which will reverse this process ?

[studiot]

Melting means breaking the bonds that hold the solid together.

To do this you have to put energy in.

 

The bonds that hold foreign atoms in the solid matrix (impurities) are in general weaker than the bonds to the 'proper' atoms.

Thus requiring less energy to break.

[Enthalpy]

Though, many salts produce heat upon dissolving but lower the solution's melting point.

[Bill Angel]

The melting point of copper is apparently RAISED by adding nickel to it.

[studiot]

Here is the copper nickel system phase diagram

 

 

It shows that liquid copper and nickel are miscible in all proportions.

 

That is they form a continuous system of alloys (new compounds that are neither copper nor nickel) in all proportions.

 

That is neither is an impurity to the other.

 

This is the simplest form of phase diagram and the melting point curve is a straight line along the solidus line.

Edited April 17, 2015 by studiot

[pavelcherepan]

I agree with studiot, but also like to note that there are a lot of variables in this situation. Firstly, there is the particular crystal structure you're dealing with. For example, phyllosilicates (such as clay minerals) have relatively big gaps between adjacent sheets which cations of impurity can populate. Since these gaps are quite big a whole range of cations can sit there and these wouldn't cause any significant change to overall energy levels of the structure and as a result melting point won't change appreciably.

 

Secondly, it depends a lot on what position an impurity will take in the crystal structure. It can take empty gaps, like in the example above, but also it can replace some of the atoms in their positions. Then, depending on the size of the atom of impurity compared to the original atom the total energy configuration of the structure can increase. This will result in lowering of the melting point.

 

 

 

Can a solid be doped with a material which will reverse this process ?

 

Generally speaking, crystal structure is supposed to be the lowest energy state for the set of atoms that comprise it and any change will most likely result in an increase of the total energy and thus will lower the melting point so the answer is most likely no. That is unless you're talking about alloys or solid solutions.

[studiot]

Melting and other phase changes are only simple events for pure substances and some special mixtures (eutectics).

 

I don't know if the OP has been back or is still interested in the subject ???????

[John Cuthber]

Here is the copper nickel system phase diagram

 

fig3.gif

 

It shows that liquid copper and nickel are miscible in all proportions.

 

That is they form a continuous system of alloys (new compounds that are neither copper nor nickel) in all proportions.

 

That is neither is an impurity to the other.

 

This is the simplest form of phase diagram and the melting point curve is a straight line along the solidus line.

Since copper and nickel are fairly similar the eutectic mixture will contain very little nickel.

It won't show up on a diagram that includes the whole 0 to 100 % range of Ni.

the fact remains that the addition of a small quantity of nickel to copper will lower the mp.

[Bill Angel]

Since copper and nickel are fairly similar the eutectic mixture will contain very little nickel.

It won't show up on a diagram that includes the whole 0 to 100 % range of Ni.

the fact remains that the addition of a small quantity of nickel to copper will lower the mp.

In a few cases, the melting point of the alloy can be worked out approximately by arithmetic. For instance, if copper (melting point 1,083C) is alloyed with nickel (melting point 1,454C) a fifty-fifty alloy will melt at about halfway between the two temperatures. Even in this case the behaviour of the alloy on melting is not simple. A copper-nickel alloy does not melt or freeze at one fixed and definite temperature, but progressively solidifies over a range of temperature. Thus, if a fifty-fifty copper-nickel alloy is liquefied and then gradually cooled, it starts freezing at 1,312C, and as the temperature falls, more and more of the alloy becomes solid until finally at 1,248C it has completely solidified. Except in certain special cases this 'freezing range' occurs in all alloys, but it is not found in pure metals, metallic, or chemical compounds, and in some special alloy compositions, all of which melt and freeze at one definite temperature.

See http://www.uefap.com/reading/exercise/ess3/alex2.htm

[John Cuthber]

See http://www.uefap.com/reading/exercise/ess3/alex2.htm

So?