effects of soluble impurities boiling and melting points

sgstudent

In my textbook, it says that impurities lower the melting point and increase the boiling point.

But is this only true if the boiling point of the impurity is greater than the boiling point of the solvent, and the melting point is greater than of the solvent too?

So essentially, if greater bp impurity, boiling point of solution will be increased. if lower bp impurity then the boiling point of solution is decreased. If greater mp impurity, melting point is lowered. if higher melting point impurity then the melting point of the solution will be decreased.

then what about alloys? so if i have two metals i want the alloy to have a low melting point, then i put a impurity that has a higher melting point so that the alloy will have a lower melting point? Must the impurity have a smaller percentage in the alloy or the effect will be otherwise where the impurity have a lower melting point which will in turn increase the overal melting point of the alloy?

Borek

Actually it is true when the impurity can be assumed to be non-volatile.

sgstudent

Oh then what about the alloys explanation? Thanks for the help Borek!

aroc91

From what I gather, the same rules don't apply to alloys. I don't have much inorganic experience, but from what I gather, the melting point of an alloy is more like the average of the melting points of the constituents. I'll have to look further into it.

Borek

Low melting alloys are usually eutectic mixtures.

sgstudent

Thanks then when can I use the non-volatile decreases mp increases bp? Because in a workbook question they say the melting point of slag CaSiO3 is 1570 degrees. But when they check the temperature of the slag in a blast furnace the temperature of the slag was slightly lower. The answer I gave was that there are impurities in the slag. Thanks for the help!

aroc91

I would stick to using the decrease mp/increase bp when you're talking about solutions. Alloys are not solutions, so that rule doesn't apply. I did some more reading and it seems that alloys follow a weighted average sort of rule for melting points.

http://www.sewanee.edu/chem/Chem&Art...Cox/index.html

I wouldn't consider slag a solution, so saying that it had an impurity in it with a lower melting point than 1570° would likely be valid.

sgstudent

Oh ok thanks for the help! But then again what can be considered as solvents?

aroc91

More further reading- It seems alloys are, in fact, considered solutions, not that it changes anything. I would consider pretty much all liquids as solvents, with the exception of molten metal, so water and all organic solvents. I get the feeling you wouldn't deal with liquid salts in these types of problems and you wouldn't deal with liquid nitrogen/other gasses, either. You can do bp elevation with gasses, but it involves pressure, not impurities.

Borek

Solution is a solution, doesn't matter what the solvent is - liquid metals are not much different from other liquids. When we talk about boiling point elevation due to impurities, we usually assume constant pressure - and it doesn't matter if the boiling liquid is water, molten metal, or something that is gaseous at room temperature. Laws of physics are universal. At constant pressure boiling point of a liquid "gas" will get up if the impurities dissolved are non-volatile.

I put "gas" in quotes as calling something a gas without stating conditions doesn't make much sense. What we consider to be "gases" on liquid giants is usually a liquid, what we consider a liquid (water) is almost always a solid, so the classification just follows are point of view, not the universal reality. Boiling is boiling and it follows the same rules regardless of whether given substance is solid, liquid or gaseous at an arbitrary temperature that we happen to call "room temp".

sgstudent

Oh, so how does adding say, carbon into molten iron affect the boiling point? Even though carbon is non volatile, its not soluble in molten iron so it won't affect the melting boiling point right? But the melting point will be reduced? Then what if I have an alloy which is 50:50? How will it affect the melting point since there is no indication on which is the impurity. Likewise if I have two metals added together if one is the impurity if will have a different effect if the other one was added as the impurity instead right? Thanks for the help!