How can a reading of melting point analysis tell a compound is pure?
The correct answer and explanation is:
A melting point analysis is a common method for determining the purity of a compound. The melting point is the temperature at which a solid turns into a liquid. Pure compounds typically have a sharp and specific melting point, while impure compounds show a depression (lowered melting point) and broadened melting range.
When a pure substance is heated, it undergoes a smooth transition from solid to liquid, and the temperature at which this occurs is consistent. If the compound is pure, the melting point will be very precise, typically falling within a narrow range, indicating the absence of impurities.
On the other hand, the presence of impurities disrupts the orderly arrangement of molecules in the solid phase, which lowers the energy required to break these interactions. This causes the substance to melt at a lower temperature and over a broader range. As impurities interfere with the crystal structure, the melting point decreases, which makes it easier for the substance to melt.
For example, if a sample of a known pure compound has a melting point of 100°C, a sample of the same compound that is impure might melt between 90-95°C. This change in the melting point is an indicator of contamination.
The more impurities present in the sample, the greater the melting point depression and broader the melting point range. Therefore, by comparing the observed melting point to the known value for the pure compound, the purity of the substance can be assessed. In practice, pure compounds will melt within a tight temperature range, and any deviations suggest that the compound is not pure.
In summary, a sharp and specific melting point is a strong indicator of purity, while a lowered and broadened melting point suggests the presence of impurities.