### Introduction The determination of melting points is a pivotal technique in organic chemistry that provides valuable insights into the purity and identity of a substance. The physical property of melting point is influenced by various intermolecular forces that dictate the interactions between molecules within a solid substance. These intermolecular forces include London Dispersion forces, Dipole-Dipole interactions, Hydrogen bonding, and ionic bonding. Each of these forces contributes to the strength of molecular attractions, affecting the energy required to transition a solid substance into its liquid phase. If the molecule experiences strong intermolecular forces, then it will have a high melting point compared to molecules with weaker intermolecular forces. When two or more substances are combined to form a mixture, their melting points can significantly differ from those of the pure individual chemicals. The eutectic point is the composition at which the mixture has the lowest possible melting point. This phenomenon arises due to the interplay of the individual chemical components, where their interactions result in a more disordered, and less rigid, arrangement in the solid phase. If two chemicals are mixed, then the melting point will be lower than either of the constituents melting points. ### Materials and Methods ![triangle](melting_point_lab_chemicals_used.png) ![triangle](melting_point_lab_materials_used.png) #### Determination of Urea and Cinnamic Acid Melting Points Samples from each urea/cinnamic acid mixture were collected: pure cinnamic acid, pure urea, 1:4 cinnamic acid:urea, 1:1, and 4:1 cinnamic acid:urea. 2 2mmx4cm capillary tubes were loaded for the pure cinnamic acid by tapping the capillary tube gently on the opening of the tube until about 3mm from the top had the mixture. The tubes were then inverted and tapped on the lab table to get the solid from the open end of the tube to the closed end. One capillary tube was loaded into the Thomas Hoover system, which heats the sample at a rate of approximately 20℃/minute. Once a melting point approximation was obtained from the Hoover system, the Mel-Temp unit was brought to a temperature approximately 20℃ less than that temperature and the second capillary tube was inserted into the Mel-Temp unit. The sample was observed through the viewing lens until the sample began melting, at which point the current temperature of the unit was recorded. This process was repeated for all 5 mixtures. #### Determination of an Unknown by Mixed Melting Point Unknown Chemical #3 was obtained from the stockroom and the melting point was determined using the method above. From this melting point, 3 pure chemicals were selected that were closest to the unknown melting point: 4-nitrophenol, acetanilide, and fluorene. Equal parts of unknown material #3 were mixed with 4-nitrophenol, acetanilide, and fluorene to make 1:1 mixtures and the melting points were tested using the method described above. ### Results #### Urea and Cinnamic Acid Melting Points The pure cinnamic acid began melting at 116℃ and was completely melted at 130℃. The pure urea began melting at 123℃ and was completely melted at 130℃. The 1:4 mixture of cinnamic acid:urea began melting at 99℃ and was fully melted at 107℃. The 4:1 mixture of cinnamic acid:urea began melting at 78℃ and was fully melted at 95℃. The 1:1 mixture of cinnamic acid:urea began melting at 73℃ and was fully melted at 89℃. ![triangle](melting_point_lab_mixture_cinnamic_urea_graph.png)

Figure 1: Melting Points of Cinnamic Acid and Urea Mixtures

#### Unknown Chemical Melting Point Unknown chemical #3 (UC3) began melting at 91℃ and finished melting at 114℃. 3 additional chemicals were selected based on this melting point to try and verify the identity: 4-nitrophenol (MP = 113℃-114℃), acetanilide (MP = 113℃-115℃), and fluorene (MP = 114℃-116℃). 1:1 mixtures of unknown chemical #3 and each of the 3 chemicals was made and tested. The UC3:4-nitrophenol mixture began melting at 66℃ and was fully melted at 80℃. The UC3:acetanilide mixture began melting at 92℃ and was fully melted at 113℃. The UC3:fluorene mixture began melting at 69℃ and was fully melted at 81℃. Therefore the identity of UC3 was acetanilide. ![triangle](melting_point_lab_unknown_melting_point_graph.png)

Figure 2: Unknown Chemical #3 Mixtures and Melting Points

### Discussion The results of the cinnamic acid/urea experiment demonstrated the concept of eutectic points. Cinnamic acid and urea had the same melting point at 130℃ however, once mixed with the other, the melting points began to drop to the lowest point (eutectic point) which was a 1:1 mixture of cinnamic acid and urea. This observation was helpful for determining the identity of unknown chemical #3. As mixtures of chemicals were demonstrated to have lower melting points than their pure counterparts, the identity of unknown chemical #3 was able to be determined. Unknown chemical #3 was determined to have a melting point of 114℃ and 3 other chemicals with known melting points around that temperature were selected. Since a 1:1 mixture would result in a lower melting point, each mixture was evaluated and the mixture that contained the closest melting point to 114℃ would reveal the identity of unknown chemical #3. Based on this melting point, UC3 was determined to be acetanilide as the other chemicals selected had much lower melting points. ### Conclusion The experiment demonstrated merged the concept of liquid-liquid extractions along with the Brønsted–Lowry acid–base theory. The prediction at the start of this experiment was that exposure of the selected organic compounds to an acidic or basic environment will influence the solubility of said compound to the respective environment. This lab demonstrated that the organic compounds initially began as insoluble to water and soluble to ether, but when exposed to an acidic/basic condition that reacted to create the conjugate form, the conjugate molecule became soluble in aqueous phases. Don't forget to check out our [Molecule Namer study tool](/study-tools#namer) and our [Reaction Solver study tool](/study-tools#solver)!