The purity of a substance will often affect the melting and boiling points. This
ID: 479639 • Letter: T
Question
The purity of a substance will often affect the melting and boiling points. This is used by chemists to assess the purity of a product. How does the purity of the substance affect the melting and boiling points? Impurities frequently cause melting point depression and boiling point elevation. Impurities frequently cause melting point elevation and boiling point depression. Impurities frequently cause melting point depression and boiling point depression. Impurities frequently cause melting point elevation and boiling point elevation.Explanation / Answer
Ans: (A) Impurities frequently cause melting point depression and boiling point elevation.
The melting point depression can be explained on the basis of intermolecular forces in the solvent. Most solids have a crystalline structure of repeating atoms, ions or molecules. These units are held together by strong intermolecular forces in the crystal lattice. The presence of impurities in the crystal weakens these intermolecular forces and thus, less heat energy is required to break the crystal lattice. Therefore, the presence of impurities lowers the melting point of the solvent.
The boiling point elevation can be explained from a lowering of vapor pressure of the solvent. Boiling point is defined as the temperature when the vapor pressure of the pure solvent equals the atmospheric pressure. When impurities are present in a solvent, the vapor pressure of the solvent is lower than the vapor pressure of the pure solvent. Thus a higher temperature is needed to attain the atmospheric pressure and therefore, the boiling point is higher than that of the pure solvent.
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