Melting Points

Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA

1. Determining the Melting Points of Naphthalene, Urea, and an Unknown MixtureExpand

   In this experiment, you will measure the melting point range of two known substances, naphthalene and urea, by observing the melting phenomenon during heating. You'll then analyze a mixture of urea and an unknown substance to observe how the unknown impurity affects the melting point range.

   • Put on a lab coat, splash-proof safety glasses, and nitrile gloves.
   • Weigh ~1 mg of naphthalene using the analytical balance, and carefully, bring it back to your bench.
   • Transfer the naphthalene onto a watch glass, and then crush it using the metal spatula until it becomes a fine powder.
   • Carefully pick up one capillary tube and tap the open end on the powder to force some of the solid into the tube.
   • Obtain a long glass tube from the common bench and hold it upright over your bench. Pick up the capillary with the open end upward and then drop it through the glass tube. Allow the capillary to bounce off the benchtop to pack the solid at the bottom.
   • Retrieve the capillary and tap its open end on the naphthalene powder again to force more of it into the capillary. Continue loading naphthalene into the capillary and dropping it through the glass tube until 1 – 2 mL of powder is packed at the bottom of the capillary.
   • Use a rubber band to attach the capillary to a digital thermometer, with the bottom of the capillary just above the bottom of the thermometer. Note: Make sure that the rubber band is at least 8 – 9 cm above the solid in the capillary.
   • Set up a stirring hotplate and a lab stand in a fume hood and clamp the thermometer above the hotplate.
   • Place the provided mineral oil bath on the hotplate and add a magnetic stir bar. Start stirring the bath at a moderate speed over low heat.
   • Lower the thermometer and capillary setup into the oil bath until the sample is about 5 cm deep in the bath. Note: Make sure that the rubber band is above the mineral oil.
   • Increase the hotplate temperature to 60°C, which is about 20° below the melting point of naphthalene in the literature. Note: It will take about 10 min for the entire oil bath to reach the same temperature as the hotplate.
   • Once the bath reaches 60°C, heat it to 75°C at a rate of 5°C/min. When the bath is close to the melting point of naphthalene (80°C), heat the bath by 1 – 2 °C/min while carefully observing the solid in the capillary.
   • At the first drop of liquid in the capillary, record the temperature of the oil bath. Keep watching the capillary tube as you slowly increase the temperature. Record the temperature when the last piece of solid melts.

     Table 1: Melting Points of Naphthalene, Urea, and an Unknown Mixture

     	Melting point temperature (°C)
     Substance	Start	End
     Napthalene
     Urea
     Urea + Unknown (trial 1)
     Urea + Unknown (trial 2)

     Click Here to download Table 1
   • Carefully remove the capillary and thermometer from the oil bath and set them aside to cool.
   • Weigh roughly 1 mg of urea and transfer it to a clean watch glass. Crush the urea to a powder. Load the powder into a capillary tube as before.
   • Attach the capillary to the thermometer with the rubber band. Heat the oil bath to 110°C, which is about 20° below the reported melting point of 132°C for urea.
   • Insert the thermometer and capillary tube into the oil bath. Heat the bath to about 125°C at 5°C/min while watching the capillary.
   • When the bath reaches 125°C, heat the bath by 1 – 2°C /min. Record the temperature when the first drop of liquid appears and when the last bit of solid melts.
   • Turn off the hotplate and remove the capillary and thermometer from the oil bath to allow them to cool.
   • Test the sample of urea mixed with an unknown impurity, which was prepared by your instructor. Perform two runs of this sample, so weigh 2 mg of the sample on the balance and transfer it to a watch glass.
   • Crush the sample to powder and load two capillaries by tapping them on the powder and dropping them through the glass tube like before.
   • Once the oil bath has cooled to at least 80°C, turn on the stir motor and set the hotplate to 80°C. Attach one of the loaded capillaries to the thermometer and clamp them in the oil bath.
   • In the first test run, increase the temperature by 10°C/min until the sample has melted to determine the rough melting point range. Record the temperature when you see the first drop of liquid form and the last drop of liquid form to determine the rough melting point range.
   • Turn off the hotplate and allow it to cool. Once the oil bath has cooled to at least 80°C, attach the second sample to the thermometer and clamp them in the bath. Turn the stir motor back on and set the hotplate to 80°C.
   • For the second test, heat the bath by 5°C/min until you are close to the rough melting point range, and then continue heating by 1 – 2°C/min to accurately capture the melting point range.
   • When you have finished measuring the melting point of the urea mixed with the unknown substance, turn off the hotplate and detach the capillary tube from the thermometer. Dispose of your capillary tubes in the glass waste container.
   • Return the mineral oil to your instructor, and clean all of your glassware using detergent and water.
2. ResultsExpand
   • First, look at the melting point range of naphthalene. These differences may be caused by experimental errors, such as heating the sample too quickly, or by impurities in the sample.
   • Next, check the results for the pure urea sample. Why is urea's melting point so much higher than naphthalene's? Looking at the structure of naphthalene, the intermolecular forces must be primarily London dispersion forces. Urea is capable of hydrogen bonding, so the intermolecular forces in solid urea are much stronger than the forces in solid naphthalene.
   • Finally, look at the results from the urea sample with the impurity. The melting point range should be broader and lower than the range for pure urea because of freezing point depression. As expected, the melting point range obtained was 127 – 130°C, which is lower than the range for pure urea.