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Effect of impurities on melting and boiling points
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Effect of impurities on melting and boiling points
All generalities are false (as is this one). Though impurities usually lower the melting point (m.p.) by disrupting crystallization on the atomic order, consider the phase diagram of the binary alloy (amalgam) HgxAg1- x: Pure Hg melts ~-39°C, and adding even a little bit of impurity raises the melting point considerably!
And without resorting to entropic explanations, consider that a high boiling point (b.p.) solute would "hold onto" the solvent, decreasing vapor pressure. $\ce{CaCl2}$ will absorb $\ce{H2O}$ from the air, driving down the vapor pressure (and humidity) even after the $\ce{CaCl2}$ liquefies.
That said, the answer they expect on the exam is that impurities decrease the m.p. and increase b.p.
All generalities are false (as is this one). Though impurities usually lower the melting point (m.p.) by disrupting crystallization on the atomic order, consider the phase diagram of the binary alloy (amalgam) HgxAg1- x: Pure Hg melts ~-39°C, and adding even a little bit of impurity raises the melting point considerably!
And without resorting to entropic explanations, consider that a high boiling point (b.p.) solute would "hold onto" the solvent, decreasing vapor pressure. $\ce{CaCl2}$ will absorb $\ce{H2O}$ from the air, driving down the vapor pressure (and humidity) even after the $\ce{CaCl2}$ liquefies.
That said, the answer they expect on the exam is that impurities decrease the m.p. and increase b.p.
Hi DrMoishe Pippik I can see why that would be the case if the solute binds more strongly to the solvent molecules than the solvent molecules bind to each other, but I dont understand why that would be the case if the impurity binds more weakly to the solvent molecules then the solvent molecules bind to each other? Also, are you saying that impurities in non-crystaline solids do not lower their melting points? Many thanks for your help DaveMore
When boiling water on the stove, take note of the bubbles coming up from the bottom of the pan. And do not confuse kinetic effects (such as nucleation and surface area) with thermodynamics.More
Thanks for letting me know the mistake.. But I have a doubt, when we boil, it becomes gas ..so evapoartion is surface phenomenon right? Then why cant the impurities give hindrance to the evaporation - which leads to increase in boiling point?More
Hi DrMoishe Pippik, No. I am asking why a solute would "hold onto" the solvent, decreasing vapor pressure, if the solute bonds more weakly to the solvent than the solvent molecules bond to each other. I also asked whether you are saying that impurities in NON-crystaline solids do not lower their melting points? Thanks for your help. Dave PS - I tried to use "@DrMoishe Pippik" above but it deleted it every time I saved, so I had to remove the "@" symbol and use "Hi" instead of "@" - any idea why?More
Boiling is not a surface phenomena, it is a bulk thermodynamic phase transition. The rest of the answer similarly ignores thermodynamics of mixtures.More
All generalities are false (as is this one). Though impurities usually lower the melting point (m.p.) by disrupting crystallization on the atomic order, consider the phase diagram of the binary alloy (amalgam) HgxAg1- x: Pure Hg melts ~-39°C, and adding even a little bit of impurity raises the melting point considerably!
And without resorting to entropic explanations, consider that a high boiling point (b.p.) solute would "hold onto" the solvent, decreasing vapor pressure. $\ce{CaCl2}$ will absorb $\ce{H2O}$ from the air, driving down the vapor pressure (and humidity) even after the $\ce{CaCl2}$ liquefies.
That said, the answer they expect on the exam is that impurities decrease the m.p. and increase b.p.
All generalities are false (as is this one). Though impurities usually lower the melting point (m.p.) by disrupting crystallization on the atomic order, consider the phase diagram of the binary alloy (amalgam) HgxAg1- x: Pure Hg melts ~-39°C, and adding even a little bit of impurity raises the melting point considerably!
And without resorting to entropic explanations, consider that a high boiling point (b.p.) solute would "hold onto" the solvent, decreasing vapor pressure. $\ce{CaCl2}$ will absorb $\ce{H2O}$ from the air, driving down the vapor pressure (and humidity) even after the $\ce{CaCl2}$ liquefies.
That said, the answer they expect on the exam is that impurities decrease the m.p. and increase b.p.
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