* Intermolecular forces: Impurities disrupt the regular arrangement of molecules in a pure substance. This disrupts the balance of intermolecular forces (like hydrogen bonding, dipole-dipole interactions, etc.), making it more difficult for the molecules to escape into the gas phase.
* Vapor pressure: The vapor pressure of a liquid is the pressure exerted by its vapor when the liquid and vapor are in equilibrium. Impurities lower the vapor pressure of the liquid, meaning it takes a higher temperature to reach the point where the vapor pressure equals the external pressure (which is the definition of the boiling point).
Exceptions:
* Certain types of impurities: In rare cases, impurities can actually lower the boiling point. This occurs when the impurity forms a mixture that has a lower boiling point than the pure substance. For example, adding a small amount of salt to water can actually lower the boiling point slightly.
* Very small amounts of impurities: If the amount of impurity is extremely small, the effect on the boiling point might be negligible.
In summary:
* Usually: Impurities raise the boiling point.
* Sometimes: Impurities can lower the boiling point, especially if they form a mixture with a lower boiling point.
* Always: It's important to consider the specific impurities and their interactions with the substance to predict the impact on the boiling point.
