Boiling Points

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

1. Boiling point determination of acetone and ethanolExpand

   The temperature at which a pure organic substance changes from the liquid phase to the gas phase is known as the boiling point. A liquid's boiling point can be determined using the capillary method, where an inverted capillary is placed in the liquid of interest and the liquid is heated. As the temperature increases, the air in the capillary escapes and is replaced by the vapor of the liquid. The vapor pressure in the capillary increases with temperature. Once it exceeds the atmospheric pressure, the vapor escapes the capillary in a stream of bubbles. When the heat is removed, the liquid cools, and the vapor pressure in the capillary decreases. When the vapor pressure reaches the atmospheric pressure, the liquid begins to fill the capillary. The temperature at which this occurs is the boiling point.

   • Put on a lab coat, splash-proof safety glasses, and nitrile gloves. This experiment must be conducted in a hood.
   • Attach a small test tube to the thermometer using a rubber band, and secure the thermometer in the clamp.
   • Obtain acetone and bring it back to your hood. Using your glass pipette and a bulb, measure 1 mL of acetone and transfer it into the small test tube.
   • Align the thermometer end so that it is level with the acetone in the test tube, keeping them close to each other. Invert a capillary tube and place it in the test tube so that the open end is facing downwards.
   • Use a 250-mL beaker to create a water bath. Add approximately 180 mL of water to the beaker, and place it on the hotplate.
   • Lower the thermometer and test tube into the water bath. Turn on the hotplate to the lowest setting, about 30°C.
   • Slowly increase the temperature on the hotplate by 10 – 20°C every 10 min, and closely observe the liquid in the test tube. When you start to see occasional bubbles in the liquid, increase the heat setting by only 5°C every 10 min.
   • Closely observe the capillary tube inside the test tube. Report the temperature at which a rapid and continuous stream of bubbles comes out of the capillary.

     Table 1: Boiling point of acetone and ethanol

     	Boiling point temperature (°C)
     	Bubbles	Liquid in capillary
     Acetone
     Ethanol

     Click Here to download Table 1
   • Turn off the heat and allow the water bath to cool. Keep observing the capillary as the production of bubbles decreases until no bubbles emerge from the capillary. The liquid will start to rise in the capillary. Record the temperature at which this occurs.
   • Label a clean 25-mL beaker as ‘organic waste’. Once the water bath has cooled to about 35°C, remove the thermometer and test tube from the bath. Detach the test tube from the thermometer and pour the acetone into the waste beaker.
   • Use a clean test tube and capillary and repeat the experiment using 1 mL of ethanol.
   • After you have measured the boiling point of ethanol, allow the water bath to cool. Once it has cooled sufficiently, remove the test tube and thermometer from the water bath, detach the test tube from the thermometer, and pour the ethanol into the organic waste beaker.
   • Dispose of the water from the water bath down the sink and place the capillaries and test tubes in the glass disposal. Dispose of the organic waste in the container provided by your instructor.
   • Wash all of your glassware with detergent and water.
2. ResultsExpand

   In this experiment, we measured the boiling point of acetone to be 56°C, which compares well to the reported value. Similarly, the boiling point of ethanol was measured to be 78°C. Errors in the boiling point measurement can be attributed to many experimental errors, such as heating the water bath too rapidly, or poor alignment of the thermometer and sample.

   The boiling point of an organic substance is directly related to its structure, where stronger intramolecular forces result in a higher boiling point as molecules are able to hold onto each other and remain in the liquid phase longer. The higher boiling point for ethanol is observed due to the OH structure that causes hydrogen bonding between the molecules. Acetone has a polar CO double bond, which results in dipole-dipole forces. Since hydrogen bonding is stronger than dipole-dipole forces, ethanol has a higher boiling point.

   Additionally, ethanol has a lower molecular weight than acetone. However, molecular weight has less of an impact on the boiling point than the molecular structure. For example, butane is a gas at room temperature and pressure, as it has a boiling point lower than 25°C. Ethanol has a slightly lower molecular mass than butane, but it is liquid at room temperature and, therefore, has a boiling point higher than room temperature. This is due to the hydrogen bonding between the ethanol molecules, which is stronger than the van der Waals forces between the butane molecules.