Melting Point:
* Disruption of Crystal Lattice: Impurities disrupt the regular arrangement of molecules in a solid's crystal lattice. This weakens the intermolecular forces holding the solid together, making it easier for the molecules to transition to the liquid state.
* Freezing Point Depression: The lowered melting point is also known as freezing point depression. This is a colligative property, meaning it depends on the concentration of the solute (impurity) rather than its identity.
Boiling Point:
* Intermolecular Interactions: Impurities can create new intermolecular interactions with the solvent molecules, making it harder for them to escape into the vapor phase. This increases the energy required for boiling, hence raising the boiling point.
* Vapor Pressure Lowering: Impurities lower the vapor pressure of the solvent. This is also a colligative property, and the lower vapor pressure means a higher temperature is required to reach the boiling point.
Important Notes:
* Type of Impurity: The effect of impurities on melting and boiling points can vary depending on the type of impurity and its interaction with the substance.
* Concentration: The magnitude of the change in melting and boiling points is directly proportional to the concentration of the impurity.
* Exceptions: There are exceptions to these general rules. Some impurities can actually increase the melting point or decrease the boiling point of a substance. This usually occurs when the impurity forms a compound with the substance that has a higher melting point or lower boiling point.
Examples:
* Salt in Water: Salt (NaCl) is an impurity that lowers the freezing point of water, which is why we use salt to melt ice on roads in winter.
* Sugar in Water: Sugar (C12H22O11) is an impurity that raises the boiling point of water, which is why it takes longer to boil a pot of water with sugar in it.
By understanding the effect of impurities, we can use them to our advantage. For example, we can use salt to lower the freezing point of water in antifreeze, or use sugar to raise the boiling point of water in cooking.
