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Melting Point and Boiling Point

Melting Point and Boiling Point

The purpose of this experiment was to observe the purity of a compound by testing its melting or boiling point. When a chemical is at a melting point, the vapor pressure of its solid phase is equal to that of the liquid phase. Impurities like H20 will lower the melting point of a compound.

Reagents Tested:

Cinnamic Acid
Urea
Pentane

Analysis:

Boiling Point of Pentane

Pentane boils at about a third of the temperature of boiling water (see Table A below), making it easier to experimentally observe the vapor point. In this test, a hot bath was used to elevate the pentane to its vapor point while an electronic thermometer was secured above the liquid surface. The experimental vapor point recording was 33.5° C. There was a 6.9% discrepancy with the actual vapor point of pentane (see Table A). This deviation is perhaps due to impurities in the reagent or inaccuracy of the electronic thermometer.

Experimental Actual % Discrepancy
Boiling Point of Pentane (°C) 33.5°C 36°C 6.9%

Table A: Boiling Point of Pentane

 

Melting Point of Urea & Cinnamic Acid

The purpose of this experiment was to acquire experience with the Mel-Temp apparatus and to observe the melting points of a couple compounds and mixtures of compounds. Samples used in the Mel-Temp apparatus were prepared as a finely crushed powder and packed to a height of 3-4mm in a capillary tube. The experimental melting points seemed quite erroneous; there was a 44% discrepancy in melting range with cinnamic acid and 17% with respect to Urea (see Table B on next page). This error could be due to impurities but that would lead to an expectation of lower melting points and broader ranges, whereas the opposite was recorded with cinnamic acid (see Table B, melting points). The cinnamic acid was shown to melt at a higher temperature than the accepted value. Considering the data, and that cinnamic acid was purified by crystallization and urea was not, it seems possible that the Mel-Temp apparatus gave temperature readings that were around 1° Celcius above the actual temperature. Regardless, better care must be taken in later experiments to observe the melting point ranges with the Mel-Temp.

Chemical Trial 1 –Melting Point Trial 2 – Melting Point Actual Melting Point % Range Discrepancy
Urea 131.5-133.2°C 131.7-133.6°C 132.5-133°C 17%
Cinnamic Acid 131.9-135.5°C 132-134°C 44%
4 parts Urea, 1 part Cinnamic Acid 97.6-100.1°C

Table B: Melting Point Data

 

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