Melting Points

In this experiment, you will be measuring 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. Before you start the lab, be sure to wear the appropriate personal protective equipment, including a lab coat, safety goggles, and gloves.

To begin, first, weigh roughly 1 milligram of naphthalene using the analytical balance. Carefully, bring the weigh boat of naphthalene 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.

Now, carefully pick up one capillary tube and tap the open end on the powder to force some of the solid into the tube. Next, 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 bench top 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 to 2 milliliters of powder are packed at the bottom of the capillary.

Next, 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. Make sure that the rubber band is at least 8 to 9 centimeters 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. Now, lower the thermometer and capillary setup into the oil bath until the sample is about 5 centimeters deep in the bath.

Make sure that the rubber band is above the mineral oil. Next, increase the hotplate temperature to 60 degrees Celsius, which is about 20 degrees below the melting point of naphthalene in the literature. It will take about 10 minutes for the entire oil bath to reach the same temperature as the hotplate.

Once the bath reaches 60 degrees, heat it to 75 degrees at 5 degrees per minute. Now that the bath is close to the melting point of naphthalene, which is about 80 degrees Celsius, heat the bath by 1 to 2 degrees per minute 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. Then, carefully remove the capillary and thermometer from the oil bath and set them aside to cool.

Now, you will test the melting point of the next compound, which is urea. Weigh roughly 1 milligram of urea and transfer it to a clean watch glass. Crush the urea to a powder.

Then, load the powder into a capillary tube as before. Attach the capillary to the thermometer with the rubber band. Now, heat the oil bath to 110 degrees Celsius, which is about 20 degrees below the reported melting point of 132 degrees for urea.

Then, insert the thermometer and capillary tube into the oil bath. Heat the bath to about 125 degrees at 5 degrees per minute while watching the capillary. Then, heat the bath by 1 to 2 degrees per minute.

Record the temperature when the first drop of liquid appears and when the last bit of solid melts. Then, turn off the hotplate and remove the capillary and thermometer from the oil bath to allow them to cool. Finally, you'll test the sample of urea mixed with an unknown impurity which was prepared by your instructor.

You will perform two runs of this sample, so weigh 2 milligrams of the sample on the balance and transfer it to a watch glass. Now, 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 degrees Celsius, turn on the stir motor and set the hotplate to 80 degrees.

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 degrees per minute 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.

Then, turn off the hotplate and allow it to cool. Once the oil bath has cooled to at least 80 degrees Celsius, 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 degrees Celsius.

For the second test, heat the bath by 5 degrees per minute until you are close to the rough melting point range, and then continue heating by 1 to 2 degrees per minute 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, who will store it as needed. Then, clean all of your glassware using detergent and water. First, let's take a look at the melting point range of naphthalene.

The reported melting point range of naphthalene is 80 to 82 degrees. However, it is not unusual for the experimental range to be 79 to 83 degrees. These differences may be caused by experimental errors, such as heating the sample too quickly, or by impurities in the sample.

Next, let's check the results for the pure urea sample. The literature value for the melting point range is 132.5 to 133 degrees, but the range obtained in this lab is about 133.5 to 134.9 degrees. So why is urea's melting point so much higher than naphthalene's?

If we think about the structure of naphthalene, we can tell that the intermolecular forces must be primarily London dispersion forces. Meanwhile, urea is capable of hydrogen bonding, so the intermolecular forces in solid urea are much stronger than the forces in solid naphthalene. Finally, let's take a 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 to 130 degrees, which is lower than the range for pure urea.