Impurities Impact Melting & Freezing: Explained

[Jadaav]

"For a pure substance, melting occurs at the same temperature as freezing."

Does it means that if impurities are added, then the melting point of the substance is not at the same temperature as freezing ?

If yes, how you would explain this ?

 

[Studiot]

Does it means that if impurities are added, then the melting point of the substance is not at the same temperature as freezing ?

If yes, how you would explain this ?

Fair question.

Ask yourself this first.

You can add an impurity to a pure liquid and freeze the result.

But how would you add an impurity to a solid and melt it?


OK so say we take pure water and add a shovel of salt.
We can measure the depression of freezing point (and the elevation of boiling point) and define a new lower freezing temperature - dependent upon the amount of impurity added.

So now we have an impure solid.

Say we now reheat this.

We will find that the impure solid does not melt at a constant temperature but over a range and no clearly defined melting point can be established.

This emphasizes that cooling/heating curves with stationary temperatures at transition work for pure substances only.

Does this help?

 

[Jadaav]

OK.

So why does the cooling/heating curves with stationary temperatures only apply for pure substances ? Why does impurities make the substance melt over a range of temperatures ?

What makes impurities do that ?

 

[Studiot]

You need to study colligative properties.

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch15/colligative.php

 

[Jadaav]

Thanks again. That's really going to be helpful to me.

 

[Jadaav]

I'm going through the Colligative properties from the site above. It mentions Vapor pressure. I've searched on Wikipedia and it says that it "is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system."

What does it mean ?

At boiling point, the vapor pressure and the environmental pressure acting on the liquid equals each other. Same here!

 

[klimatos]

"For a pure substance, melting occurs at the same temperature as freezing."

This is not true for water. In the free atmosphere, ice melts at 0.01°C. Pure atmospheric water does not usually freeze at that temperature. Water containing icing nuclei and/or "icing ions" may or may not freeze at that point, and we don't really know why it does or doesn't.

Pure water has been found in the atmosphere at temperatures as low as -42°C. Droplets of pure water can vaporize, condense, collide, merge, and fragment at temperatures well below the triple point without freezing. Most wintertime clouds are composed of water droplets at temperatures far below the triple point.

Contrariwise, in pressured pipelines, water may freeze to form stable clathrates at temperatures as high as 18°C. In plant tissues, at normal atmospheric pressures, ice may form at temperatures as high as 4°C.

There is no single freezing point for pure atmospheric water.

Water is weird.

 

[Jadaav]

I think it due to the difference in pressure ?