[BARAJAN]

hi all
why impurities influence on physical properties of compounds?
 for example a solid compound with a little impurities changes to a viscose liquid but purification process convert it to the solid phase again. can we compare this phenomenon to preparation ion liquids?

[fledarmus]

The phenomenon you describe is called freezing point depression. The reason that a material freezes to form a solid is that the interactions between the molecules holding them together in an ordered structure are stronger than the entropic forces that move them around. When you add impurities, you remove some of the interactions between the molecules of the solid.

I'm not sure I understand the last part of your question...

can we compare this phenomenon to preparation ion liquids?

If you are referring to this type of ionic liquid http://en.wikipedia.org/wiki/Ionic_liquid, then yes, one of the ways to get a room temperature or low temperature ionic liquid is with a mixture of two higher melting ionic compounds.

[BARAJAN]

The phenomenon you describe is called freezing point depression. The reason that a material freezes to form a solid is that the interactions between the molecules holding them together in an ordered structure are stronger than the entropic forces that move them around. When you add impurities, you remove some of the interactions between the molecules of the solid.

thank you very much, that seems interesting but one more question  so by the mentioned phenomenon why some of compounds are liquid in their pure form in room temperature?

[fledarmus]

why some of compounds are liquid in their pure form in room temperature?

I'm not sure I understand exactly what you mean by this question. Are you asking why all pure compounds are not solids at room temperature?

[BARAJAN]

why some of compounds are liquid in their pure form in room temperature?

I'm not sure I understand exactly what you mean by this question. Are you asking why all pure compounds are not solids at room temperature?

yes that is right 

[fledarmus]

Because molecules move, and the higher the temperature the faster they move. The surprise is not that some compounds are solid at room temperature, the surprise is that there are any solids or liquids at all! If all molecules are moving, why aren't they all gases?

The answer is that there are forces between molecules that hold them together. When the forces holding the molecules together are very strong, they are locked into a tight lattice and the only movement they are able to make is small vibrations around their center of motion. Slightly weaker intermolecular forces give you liquids, where the molecules are able to move past each other but still stay in contact with each other. Still weaker intermolecular forces give you gases, which go shooting around everywhere and don't have to stay in contact with each other.

Since molecular motion increases with temperature, when you heat a solid, the molecules will move faster and faster until the speed of their motion is stronger than the intermolecular forces holding them together in a lattice. That is when the solid melts to form a liquid. Then as you increase the temperature, the motion of the molecules gets faster still, until they are moving fast enough to escape the surface of the liquid. That is when the liquid evaporates to form a gas.

The concepts of heat and disorder are expressed in thermochemical terms as enthalpy and entropy, usually given the variables H and S. If you are interested enough in physical properties to do some research, this would be a good starting point for you.

[BARAJAN]

thank you very much