LAB+1+Melting+points


 * Melting Points**

**Introduction:** Determining the melting point of a compound is a method of classifying the characteristics of a given solid. A melting point occurs when a crystalline solid becomes liquid at atmospheric pressure. The onset to clear point range is useful in determining the purity of a compound. Pure substances have a sharp melting point range of 1 - 2 degrees Celsius. When compared to pure substances, melting points of mixedcompounds show a depression in temperature and increased range. Depressions can be useful in identifying an unknown substance since the characteristic will occur until the unknown is mixed with the same compound. Application of the capillary method technique is utilized for melting point determination. During experimentation, thin glass capillary tubes containing compact amounts of substances are introduced to increase temperature. Accuracy of melting points can be attributed to tight packaging of the materials and maintaining fixed levels for all samples tested to gain precise results. **Procedure:** General procedure can be found here - [|Melting Points] **Data:**

**Part 1.** Description - Melting point of 1-indadone: During the melting procedure of Inadone, the substance presented as a white crystalline powder with no transparency in the crystals prior to heat applied with the Mel Temp apparatus. With the application of higher temperatures, small amounts of condensation built up in the capillary tubes. Crystals began to appear more diaphanous shortly following the buildup of moisture. Melting from a solid to liquid ensued as total melting time from onset to clear point range fell between 2-4 seconds. **Part 2.** Null hypothesis: The melting point of mixed substances will not decrease compared to the pure substances and the range of starting temperature to finishing temperature will not increase. Description - Melting point of mixed substance containing (.202g) of 1-indadone and (.252g) 3,4-dimethoxybenzaldehyde: Upon the addition of two substances mixed at room temperature, the onset of a melting point occurred immediately. Both substances began to condensate then liquify while stirring together in an open beaker. Total melting occurred in under two minutes. Upon adding .05g of NaOH, initially the liquid substance turned a more green and white color then changed entirely to a prominent green. The substance became more viscous and thickened as stirring continued. Eventually, the mixture solidified on the walls of the beaker with a light green appearance. After letting the mixture of .05g NaOH and 1-indanone stand for approximately ten minutes, 2 ml of 10% HCl was added. The HCl began to break apart the solidified product and also changed the color from a light green to white. It was then testd to determine if enough HCl was added to yield an acidic PH. After acidity was confirmed, the aqueous solution was flushed through an aspirator vacuum system to remove as much liquid possible. Once it appeared that all the liquid had been removed, the mixture was placed in a drying oven for approximately ten minutes to complete the drying process in preparation for another melting point calculation. Prior to the application of heat, the product of 3,4 dimethoxybenzaldehide, 1-inadone, NaOH and HCl was observed to be a fine, yellow orange crystalline substance. Once heat was applied, transformation took place moving from a yellow orange hue to a smooth caramel pigment. Phasing time between the onset and clear points for this substance was significantly longer than other mixtures which made the transformation much more visual. During this time, condensation developed on the sides of the capillary tubes clearly demonstrating the change between the onset and clear point. Transformation from powdery solid crystals to a lucid fluid was also easily observed due to the time span between phases. **Part 3**. Description - Melting point of pure urea, 1:1, and 1:5 urea to cinnamic acid: In the final process of the lab, melting points of urea were tested. To asses purity of the sample 1:1 and 1:5 ratios of urea and cinnamic acid were measured and mixed. Before loading the capillary tubes, white crystalline compounds were crushed to a fine consistency insuring accuracy in melting. One-millimeter samples were added to the capillary tubes and placed in the Mel-Temp apparatus. Onset of 1:1 and 1:5 urea and started within 4 degrees Celsius of each other. Clear point of the 1:1 substance happened within 2-4 seconds. White crystals quickly became clear in the 1:1, 1:5, and pure urea substances. Onset of the pure compound did not begin until after both the 1:1, and 1:5 samples reached clear point.

Temperature in degree C



**Analysis/ Discussion:** Evidence from broad versus small melting point temperature ranges are significant to the identification of purity in a compound. From three of the experimental procedures performed the melting point of pure 1-Indadone, the 1:1 ratio mixture of Urea and Cinnamic Acid, and pure Urea displayed the smallest shift in temperature ranges for solid conversion to liquid. Contrary to this pattern, during experiments with different amounts or weights of products combined resulted in substantial increases in temperature ranges. Secondly, after witnessing the onset of melting points evidence suggests when two substances are mixed there will be a substantial drop in melting point temperature. This was evident after witnessing the instant onset of melting of mixed 1-Indadone and 3,4 dimethyloxybenzaldehyde. Additionally, comparing the melting points of pure urea, 1:1 and 1:5 ratio mixtures of urea and cinnamic acid those combined ratio mixtures yielded significant temperature decreases in melting points than pure urea. Potential sources of inaccuracy could occur during the increase of temperature on the Mel-Temp apparatus, packing the capillary tubes, and conducting several melting point trials. In order to obtain accurate results, the temperature on the Mel-Temp apparatus should be adjusted gradually to ensure the thermometer and capillary tubes will remain in thermal equilibrium during the trials. It is acceptable to increase the temperature more rapidly until reaching a level approximately 15-20 degrees Celsius below the estimated melting point temperature. Once this lower temperature is attained it is imperative that the settings on the apparatus be decreased to allow heat to increase slowly at about two degrees Celsius per minute. Such careful technique will make the melting process more easily observed and accurate. Variability in accuracy of results could also occur during loading of the capillary tubes with product. The amount of substance tested in the tubes could cause variance from the onset to clear point ranges during the melting process. If more product then needed is tested, this period of time between the onset and clear points could be skewed. A consistent measurement to ensure accuracy of melting point ranges in tubes is approximately (2-3)mm. During melting point trials, duplicate amounts of substances tested will deliver more comparative results. Finally, conducting multiple trials of the melting process procedure yields more data results for comparison. Multiple trials will give the most accurate results due to possible error in the testing. Additionally, this will allow the removal of any outlier results when comparing numerical data figures.

**Post-Lab Questions:** I give you 0.3 grams of an unknown. It is either substance "A" with a melting point of 83.0 degrees C, or substance "B" with a melting point of 83.1 degrees C. Substance A and B are available to you. How do you figure out Which one it is? In order to define the unknown compound given, combine this substance with both "A" and "B" separately. After combining the unknown with the given substances, test the melting point of the newly created mixtures to identify your compound. Since melting point determination is a known physical property of a pure compound, the degree at which an unknown compound melts can help identify the substance. Normally, the melting point of a pure substance can be reproduced to within one degree Celsius. To test the temperature at which a substance melts, the following are utilized: a Mel-Temp apparatus, thermometer and capillary tube. To accurately identify the unknown create two new 1:1 mixture ratios. Each ratio should contain one part substance "A" or "B" and one part of the unknown given. A 1:1 mixture ratio that generates a melting point temperature lower than that of the pure substances available contains two different parts in the mixture. If one of the mixtures generated has a melting point of 83.0 degrees Celsius it can be confirmed that the unknown is substance "A". If one of the created ratios has a melting point of 83.1 degrees Celsius it can be determined that the unknown substance is compound "B". It is therefore possible to determine the identity of the unknown substance by testing one-to-one mixtures that yield a melting temperature equal to that of either pure compound.