report : Identification of a Compound using Melting and Boiling Points

Identification of a Compound using Melting and Boiling Points

Introduction

 One of the primary methods used to characterize a new compound is the physical determination of its normal melting and boiling points.  The “normal” melting and boiling point is the temperature at which a substance melts or boils when the barometric pressure is 760 mmHg or 1 atm.  In this experiment we will first calibrate our thermometers using ice and water, whose normal melting and boiling points are well characterized as 0.0 °C and 100.0 °C, respectively[1].  Following this, we will measure the normal melting and boiling points of an unknown compound.  We will use this data to determine the identity of our unknown from a list of possible unknown samples and physical data from the Chemical Handbook[2].

 Experimental Procedure

 As described in the lab manual,[3] ice was placed in a beaker and warmed until approximately 50% had melted.  The temperature of the ice/water mixture was then measured with a thermometer. This was followed by a similar measurement of our solid unknown.  In part II, water was heated until boiling and the temperature of the liquid/gas mixture measured with a thermometer.  This was followed by a similar measurement using our unknown compound.  To get the best results possible, the procedure in the manual was modified by repeating each trial three times. 

Data & Results

The Barometric pressure in the lab was measured to be 761.2 mmHg.

Table One – Experimental Data 

Trial	Water Melting Pt.	Water Boiling Pt.	Unknown 7 Melting Pt.	Unknown 7 Boiling Pt.
1	0.7 °C  *	101.2 °C	80.2 °C	272.7 °C
2	0.1 °C	101.1 °C	80.7 °C	272.8 °C
3	0.0 °C	100.9 °C	80.4 °C	273.0 °C
4	0.1 °C	n/a	n/a	n/a
Averages:	0.15 °C	101.1 °C	80.4 °C	272.8  °C
Standard deviation (s):	± 0.06	± 0.15	± 0.15	± 0.06
95% confidence limits:	± 0.14	± 0.4	± 0.4	± 0.14

 * This trial was eliminated because the thermometer was broken (there was a bubble of air in the mercury).  A new thermometer was obtained from the stockroom and used for all other data.

 Observations: The unknown was yellowish-orange in color and had a fruity smell.

As can be seen from our water data the experimental values for the melting and boiling points of water differed from the theoretical values by +0.15 °C and +1.1 °C, respectively.  These differences were used to calibrate the average data for the unknown.  Thus the corrected values for the unknown boiling and melting points are given in Table 2.

Table Two – Corrected Temperatures  

	Unknown 7 Melting Pt.	Unknown 7 Boiling Pt.
Measured value	80.4 °C ± 0.4 (95%)	272.80 ± 0.14 °C (95%)
Correction	+0.15 °C	+ 1.1 °C
Corrected value	80.5 °C ± 0.4 (95%)	273.90 ± 0.14 °C (95%)

 These values were used to identify our unknown.  Table Three below lists possible unknowns and the melting and boiling points for these compounds found in the Chemical Handbook.²

Table Three – Reference Data from Chemical Handbook 

Compound	Melting Point	Boiling Point
Blabber Gas	-15.8 °C	17.2 °C
Freezer Gel	82.7 °C	456.1 °C
Silly Putty	57.2 °C	121 °C
Billgatesium	1000 °C	unknown
Farsel Juice	80.8 °C	274.0 °C
Shampoo	-1.2 °C	108.7 °C

  

Based on these data we conclude that our sample was probably “Farsel Juice” since both the melting and boiling points fall within the confidence limits of our average melting and boiling points.  Additional evidence to support our conclusion is that Farsel Juice is described in the Chemical  Handbook as having a yellowish-orange in color and has a “peach-like” smell.  Our unknown was this color and one of our group members observed a “fruity” smell when she opened the bottle.

              Although our measured melting and boiling points differed from the theoretical data by a few percent, this difference was very small leading us to believe that our results were quite good.  While there is still room for error in our results due to the change in boiling and melting points as a function of atmospheric pressure this difference should be very small.  Other factors such as contaminates in the water used may have affected the results, but again every effort to minimalize such effects was made by using only deionized water.  Finally we did encounter some problems with our thermometer in the first trial, but this was fixed by replacing it at the stockroom.  Thus our careful work, our additional color and smell observations, and the fact that the corrected average of data exactly matched only one of the choices with 95% confidence, all suggest that our unknown was in fact Farsel Juice.

  

Conclusions

 In this lab we determined the identity of our unknown to be Farsel Juice using normal melting and boiling points.  A future experiment might include an additional calibration using the barometric pressure and/or inclusion of other chemical properties such as reactions of the compounds with acids and stuff to further test the nature of the chemicals and more positively identify the chemicals.

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[1] Agenius, I.M., General Chemistry for College, 2nd Ed., Overcharge Publishing House, Beverly Hills California, 1999, page 12.

[2] Dr. Joe Scientist, Ed., Chemical Handbook, 578th Ed., Big Chemical Press Inc., Bigtown, USA, 1999.

[3] Wizard, Mr., “Don’t try this at home” – Experiments for General Chemistry, 1st Ed., Explosive Info Co., Ground Zero, 1978, Experiment 2, pp. 10-15.