question on chemistry?

Question

why do impurities lower freezing point but increase boiling point?

Answer 1

To understand this you have to look at what happens with the molecules when you dissolve for instance salt in water. The Na+ and Cl- ions of the salt and become free to roam about in the water. Because they are charged they attract water molecules, which are molecules are tiny dipoles, with a positive and a negative side.

The result is that each salt ion is surrounded by a number of water molecules. The result is a complex ion, which is much heavier than each individual water molecule.

If the water cools down ice crystals, can only form if the complex molecules are broken down first, and a lower temperature than normal is needed to do that. Similarly, when water starts to boil, a higher temperature than 100 C is needed before the complexes let go of the water molecules.

Answer 2

Due to thermodynamic problems the free energy of amixture is changed when you put a solute in a solvent

This phenomenon takes the name of colligative properties

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