Rewrite of Melting Points Lab:

Introduction:

The purpose of this lab was to identify the melting points of two pure chemicals, as well as some mixtures. This information was then used to extrapolate information regarding the pattern of melting point behavior or pure versus impure substances. The melting point of a solid is the range of temperatures at which there is a transition in the physical state of the chemicals from solid to liquid form. This desired range data is achieved by recording the temperature (in degrees Celsius) at which the sample begins to sweat and also the temperature at the point that the sample has fully transitioned into a liquid. In order to measure the melting point of each sample, a MelTemp apparatus was used. A capillary tube containing each substance is inserted into the apparatus and then observed through the lens as it is heated. It is known that pure samples will melt at higher temperatures than mixtures. This information can be useful in determining the purity of a known sample, for instance, if a given pure substance has a noted melting point of 162 degrees Celsius but in experiments a sample of the same substance melts at 145 degrees Celsius, you can deduce that some impurities are present. This lab measured the melting points of 1-indanone, 3,4-dimethoxybenzaldehyde, HCl, NaOH, urea, and cinnamic acid.

Procedure:


The procedure for this lab can be located at the web address listed below:
http://web.cocc.edu/chigginbotham/O_CHEM/FallOchem/MeltingPoints09.html

Additional Procedure:
1. Based on your experience, what can you say now about how the melting point of a mixture of two substances compares to the melting point of either substance, in pure form?
2. Test the melting points of (a) pure urea, and (b) urea with cinnamic acid in any two ratios of your choosing: 1:5, 2:5, 1:1, 5:2, 5:1. If you would like to make additional measurements, you are welcome to do so provided you have time during your lab period.


Data:


Substance
Melting Points (Sweating-Melting)
Measured in Degrees Celsius
Observations Made

Trial 1
Trial 2
Trial 3

1-indanone
40.7-46.0
39.6-43.6
40.0-46.1
NA
Mixture of 0.20g 1-indanone w. 0.25 3,4 Dimethoxybenzaldehyde
NA
NA
NA
· Melted point lowered to below room temperature
· Melted within a minute
· Translucent yellow
· Almond Odor

Add 0.05g powdered NaOH to above mixture



· Cloudy substance
· Color change to Olive green
· Slightly thickened

Add 2mL of 10% HCl



· pH paper showed high acidity
· color change to tan
· gritty texture, almost crystal-like

Powdered product of (mix, NaOH, HCL) mixture
113.3-156.8



100% Pure Urea
124.3-130.8



Urea/Cinnamic Acid
(1:1)

101.7-106.0



Urea/Cinnamic Acid
(1:5)

134.4-138.4




Graph:

graph.png


% of Urea
Sweat
Melt
50.0%
101.7*C
106.0*C
16.6%
134.4*C
138.4*C
100.0%
124.3*C
130.8*C

The experimenters found that the melting range of pure urea was 124.3 to 130.8 degrees Celsius. As cinnamic acid was added to urea the melting temperature increased. In the 1:1 mixture the melting point ranged from 101.7 to 106.0 degrees Celsius. In the 1:5 mixture the melting temperature ranged from 134.4 to 138.4 degrees Celsius. All of these points are shown in the graph displayed above.


Analysis/Conclusion:


The melting point temperature range of 1-indanone was observed to be between 39.6 to 46.1 degrees Celsius. These ranges take into account the three trials of pure 1-indanone. When mixed with 3,4-dimethoxybenzaldehyde (DMB) the mixture melted within a minute and remained a liquid at room temperature. This result indicates that impure substances experience lowered melting points when compared to their pure counterparts. By simply adding another substance to 1-indanone, the melting point of the mixture dropped to below room temperature. When NaOH was added, the substance thickened, became cloudy, and the color changed to an olive green. When HCl was added the pH test paper showed the mixture had a high acidity, the color changed to tan, and had a gritty texture. A precipitate formed as the result of combining 1-indanone, 3,4-dimethoxybenzaldehyde, NaOH, and HCl. Adding 3,4-DMB to 1-indanone dropped the melting temperature range to below room temperature which was further altered by adding NaOH and HCl. When all substances were mixed, the melting temperature range climbed to 113.3-156.8 degrees Celsius. When all four substances were combined, not only was this a mixture but a brand new substance. This new substance had a completely different melting point than the mixtures alone.

Possible Errors:
  1. The samples placed in the capillary tubes that we supposed to represent a "pure" substance may have not been pure.
  2. When heating the substances and recording the melting temperatures the MelTemp apparatus' temperature setting was not kept consecutive possibly skewing the results.
  3. The mixture isolated in the end may have not been fully dried.


Post-Lab Question:

I give you 0.3g of unknown. It is either substance "A" (melting point 83.0*C) or substance "B" (melting point 83.1*C), both are available to you.
How would you figure out which it is?

Answer:
With full availability of all three substances, the unknown, "A", and "B", it is possible to test all three simultaneously but the temperatures would be to close to tell if there was any difference. Another solution to this problem would be to mix the unknown with equal amounts of substances "A" and "B" individually. If the melting point is lowered in any of the combination, such as the unknown with substance "A", then the two substances are different. If the melting point stays the same then the two substances of the mixture are identical.