Essay on Purity and Purifications of Solids Using Melting Points.

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Purity and purifications of solids using melting points.
Tatyana Aleksandrova
CHE 337, Section 001
Department of Chemistry
Portland State University, Portland, OR

Abstract
Melting points of Naphthalene/Biphenyl mixtures differing in their percent compositions were observed and plotted on a graph that indicated eutectic point of the mixture to be at 50 mole percent Naphthalene. Using melting point technique to identify mixture Unknown H was determined to be 3-ethoxy-4-hydroxybenzaldehyde.
Introduction
Melting point is a technique used by chemists to identify unknown substance. Compounds depending on their chemical structure have
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Based on the fact that mixture with 3-Ethoxy-4-hydroxybenzaldehyde had relatively close range of (76.1 °C - 78.1 °C) with 2.5 °C difference in ending values, which can be considered as a sharp point, unknown H is 3-Ethoxy-4-hydroxybenzaldehyde.
Conclusion
Melting point is a technique used to identify pure substances by observing ranges of melting points as it was done in mixtures of Naphthalene and Biphenyl. Eutectic point, however, should be kept in my when testing for pure substances, for eutectic compositions can mislead results of the test if nothing else is considered. Biphenyl’s on this technique unknown H tested was determined to be 3-Ethoxy-4-hydroxybenzaldehyde based on its sharp MT.

Questions:

1. a) Examples in which a pure substance could give a broad melting range:
5 mole percent of Naphthalene to 95 mole percent of Biphenyl (which is good percentage of purity) has a broad melting range.

b) Example of a situation in which an impure substance melts sharply:
Mixture of 60 mole percent of Naphthalene and 40 mole percent of Biphenyl. Mixture has eutectic composition.

2. Eutectic mixture – mixture in which proportions of its constituents allow a uniform melting point for that mixture. MP range is sharp, despite the fact that mixture is impure.

3. Effects of the impurities on melting behavior of benzoic acid: MP Benzoic Acid – 122 °C a. Fragments of crushed glass – MP of crashed glass is around 1500 °C,

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