Melting Point and Solid Identification

Introduction


Melting point is the temperature or temperature range at which a given substance melts at standard atmospheric pressure. Most pure substances have a very small range of temperature for their melting points, while a substance with an impurity will have considerably larger temperature range. Because of this property, measuring the melting point is a useful tool when one has to isolate or identify an unknown compound or substance. By measuring the melting point of a compound one can compare the experimentally recorded temperature to that of the written scientific data found in various publications. Recorded temperatures range by a few tenths of a degree in some catalogs so the best literature source to reference would be the Merck Index which measures the melting point of a pure compound, element, or substance.



Procedure


The procedure for this lab can be found HERE. Each group in lab was assigned a compound to test the melting point. The species concerned with this phase of the experiment was 3,4-Dimethoxybenzaldehyde (DMB). The melting point was taken three times to increase precision in the data. Part II introduced the species 1-Indanone and for part III of the procedure samples of urea and cinnamic were obtained. Part III required one sample of pure urea and two ratios of urea and cinnamic. The pure urea was put into a capillary tube and two samples with ratios of 1:5 urea and cinnamic and 5:1 urea and cinnamic were also put in capillary tubes. All three were inserted into the Mel-Temp at the same time. The melting points of each sample were only taken once.

Compounds used in the following lab tests:
  • Name : Veratraldehyde
    • IUPAC name: 3,4-Dimethyloxybenzaldehyde
    • CAS Number: 120-14-9
    • Molecular Formula : (CH 3 O 2 ) 2 C 6 H 3 CHO
    • Compound Formula : C 9 H 10 O 3
  • Name: 1-Indanone
    • IUPAC name: 2,3-dihydroinden-1-one
    • CAS Number: 83-33-0
    • Molecular Formula: C 9 H 8 O
    • Compound Formula: C 9 H 8 O
  • Name: Carbamide
    • IUPAC name: Urea
    • CAS Number: 57-13-6
    • Molecular Formula: (NH 2 ) 2 CO
    • Compound Formula: CH 4 N 2 O
  • Name: trans -Cinnamic Acid
    • IUPAC name: (E)-3-Phenylprop-2-enoic-acid
    • CAS Number: 140-10-3
    • Molecular Formula: C 6 H 5 CH=CHCOOH
    • Compound Formula: C 9 H 8 O 2





Data


Part I: Melting point observations

In the capillary tube before melting occurred the substance was off-white, with an orange tint. It was fine crystalline substance. As it began to melt, the crystal to glass contact points began to moisten, this change of state is known as shriveling. As melting continued, the crystalline structure underwent a state change to liquid at an increasingly rapid rate. In the liquid state, 3,4-Dimethoxybenzaldehyde (DMB) was similarly colored as in the solid state, but more vibrant. The liquid appeared to be highly viscous. The three temperature ranges were recorded.

Part II: Melting point ranges of two mixed substances
  • Prior to mixing:
    • 3,4-Dimethoxybenzaldehyde was off-white with an orange tint. It was crystalline and seemed to have a moist appearance.
    • 1-Indanone was off white with a yellow-green tint. The substance had large crystalline structures.

  • Mixing:
Almost instantly upon mixing the crystals began to melt. Ambient room temperature was 23.5ºC. All clumps of solid were broken up after 4 minutes. State change was complete within 5 minutes. Strong cloying odor that resembled napthalene was noted. Final state was liquid, viscous in nature. The beaker was cool to the touch for the duration of the mixing process

  • Addition of NaOH:
    • NOTE: This step had to be repeated due to an undesirable result during the first experiment. The following data is from the second run. The results from the first run can be found in the discussion section.
Approximately 0.05g of the white powder (NaOH) was introduced into the 3,4-Dimethoxybenzaldehyde, 1-Indanone mixture. Within seconds of the addition of NaOH, the aqueous solution transformed into a yellow-green paste. Highly viscous and cloudy. After 2.5 minutes of mixing the solid began to harden and turn a more opaque color. 3.0 minutes after addition the product was completely solidified and mixing proved futile. The substance hardened along the walls of the glass beaker while large clumps accumulated on the bottom.

  • Addition of HCl:
After the requisite 10 minute waiting period, 2.0 mL of 10% HCl (aq) was added to the product and mixed. The substance remained extremely firm and did not dissolve readily. The resultant mixture of product and HCl appeared like oil and water, with a suspension of some solid particles. Color varied from a green to a light brown. The water like portion of the solution was clear and translucent. The pH was tested and was found to be ≈ 1.

  • Isolating the solid:
After isolating the solid from the aqueous solution, its appearance was off white/light beige in color and flaky. It reflected light off of the flat surfaces. After the required 8 minutes in the drying oven the substance was dry and flaky, grouped into larger clumps. The color ranged from dull yellow to more vibrant yellow, with a few spots containing various shades of red.

  • Melting point of the product:
Three samples were placed in the Mel-Temp. Initially the energy was set at 10v, but was increased to 20v, and then 30v, and finally to 50v. All three samples had the same data points. After the meniscus point was reached, the solution darkened significantly to an opaque black. Leaf like particles were still visible. Because of the opaque nature of the substance, the clear point was difficult to detect.

Part III: Pure urea and urea ratio melting pointsObservations:Urea and cinnamic were obtained from the supply counter and distributed on two separate watch glasses. Both were pulverized into a fine powder. The urea was white in color while the cinnamic was pale yellow, both were comprised of fine crystals before being ground. The pure urea was put into a capillary tube and set aside. The ratios of urea and cinnamic were measured out with an approximation. The ratios of 5:1 urea:cinnamic and 1:5 urea:cinnamic were chosen with the thought that they would best reflect the melting ranges of a substance with many impurities and a substance with few impurities.










Analysis


This first graph shows the melting point ranges of the compound DMB average between 43.8-44.9ºC.


Screen_shot_2010-10-08_at_12.02.18_PM.png


The class ranges for DMB show similar numbers with some wide variation. The range discrepancies and thoughts on them can be found in the Discussion section. The average range for the class was 42.9-45.4ºC.
Screen_shot_2010-10-08_at_12.09.14_PM.png
The same can be said about the 1-Indanone ranges. The average range for the class was 39.1-42.5ºC. Merck index melting point for 1-Indanone is 42°C.

Screen_shot_2010-10-08_at_12.09.42_PM.png


The combined mixture of DMB and 1-Indanone show a very elevated range; almost tripled.


Screen_shot_2010-10-08_at_12.12.37_PM.png

For the urea and cinnamic mixture we see a normal range for the pure urea, a slightly depressed range for the 5:1 urea:cinnamic ratio and a considerably depressed range ratio for 1:5 urea:cinnnamic. Merck index has melting points for urea at 133.3°C and cinnamic acid at 133°C respectively.


Screen_shot_2010-10-08_at_12.11.17_PM.png



Conclusion/discussion


Until the addition of the NaOH, both experiments yielded similar if not exact results. Because of the significant difference that occurred upon the addition of NaOH, it is reasonable to surmise this was the step that was done improperly in the first experiment. During the second run a greater mass of NaOH was added resulting in the desired effect. Because of this, it is probable that an insufficient mass of NaOH was added during the first run to catalyze the reaction.

When testing the melting point of the product of the reactions, precision was lost to make up time. Given ample time to complete the experiment, each sample would have been tested independently producing three separate readings and thus increasing precision. With no time left in the lab, all three samples had to be tested simultaneously which resulted in all three having the same data. As seen in the graphs, it can be said that pure substances have a much smaller range of temperature variation than that of a substance with an impurity.

The impure compounds extend over a much wider range than that of the pure compound. The ratios of urea and cinnamic provided insight to a slightly off substance and a substance that contained ample amounts of impurities. The melting point of the 1:5 urea:cinnamic had a lower initial melting point and the final melting point ended just below the final melting point for the 5:1 urea:cinnamic. The same can be said of the 5:1 urea:cinnamic and the pure urea initial and final melting points.

In the end, the melting point of a substance is critical to determining that substance's purity. The amount that the compound is saturated with a foreign substance is directly related to the magnitude of the depression of its melting point.



Post Lab Question


Q: I give you 0.3g of an unknown substance. It is either substance "A" with a melting point of 83.0°C or substance "B" with a melting point of 83.1°C. A and B are available to you. How would you figure out what substance it is?A: It is unknown whether A and B are the same or different substances. Since the melting point for both compounds is given it is not necessary to re-obtain the same information. The recommended procedure would be to mix both compounds together in different ratios and then to obtain the melting point of each ratio. A large melting point depression or expansion would indicate that one of the substances is acting as an impurity on the other. If the melting point stayed the same (with a 50:50 mixture) it would indicate that substance A and substance B are the same compound. If the mixture melts sharply over a short time, it would indicate that the compound is pure.


NOTES
The chemical structures, formulas, and vital statistics of each compound was researched using Wolfram Alpha computational knowledge engine and confirmed using the CRC Handbook of Chemistry and Physics [90th Edition].