The presence of impurity has two effects on melting and freezing point
1. lowers melting/freezing point ( If it is in range, it lowers end of range. For example, if the melting point is 188-189C, the mixture with impurity will be lower than 188C.
2. Melting point/freezing point will be in a range now instead of a sharp peak. For example, if the melting point were 188C, now it could be in a range of temperatures.
Why does this happen?
Adding impurity to the solution basically messes up the lattice (the structure of the molecule). This causes the structure of the molecule to break easily, meaning you need LESS temperature to melt it now. If it requires 180C to melt the solid, now it requires 100C. Pretty cool isnt it?
The opposite is true for freezing point
To form a structure of molecule from liquid, the correct lattice or structure is needed. Impurity basically impedes the formation of the correct structure due to difference in size, shape or charge. To allow the formation of solid to happen, you need lower entropy = low temperature. Therefore, when you add salt to water, it will remain as liquid at temperature at or below 0C, which is the freezing point of water.
Quoted from my previous post at this link
"Many people use salt to lower the freezing point of the ice. What does it mean? Imagine the temperature for today is 0 C. This is also the temperature when ice forms. Adding salt to the ice will lower it to -6 C. This means liquid will be frozen into solid at now -6 C (used to be 0 C). Since today's temperature is 0 C, the ice on the road after addition of salt is going to melt because we haven't reached -6 C (the new freezing point) yet. Until then, everything will be stayed at liquid."
This still remain true.
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