How to Determine the Boiling Point of a Liquid

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Chemists characterize and identify liquids by, among other physical properties, their boiling point. Chemists usually measure boiling points during the process of distillation. During distillation -- used as a process for separating liquids -- the chemist boils a solution and the vapor condenses on the inside of the glassware that’s far enough away from the heat source to remain relatively cool . Thus, the measured temperature actually represents that of the liquid that has been vaporized and then condensed back to the liquid phase. Chemists measure boiling points this way because any impurities in the liquid affect the temperature at which it boils, but not the temperature at which it condenses back to liquid. The side-arm distillation flask represents the simplest and least expensive piece of glassware for carrying out boiling-point determinations. It consists of a round-bottom flask with a long neck and a side arm attached to the neck at a downward angle.

Things You'll Need

  • Side-arm distillation flask
  • Unknown liquid
  • Boiling chips
  • Clamp
  • Ring stand
  • Iron ring
  • Heating mantle and temperature controller
  • Thermometer
  • Thermometer adapter
  • Fill a side-arm distillation flask approximately half full with the liquid being investigated and add two or three boiling chips -- small pieces of limestone that help solutions develop smooth, rolling boils. Camp the distillation flask by its neck in a clamp attached to a ring stand.

  • Attach an iron ring to the ring stand beneath the distillation flask about 3 inches from the countertop or lab bench and place a heating mantle designed for round-bottom flasks on top of the iron ring.

  • Insert a thermometer into a thermometer adapter designed to fit the opening of the distillation flask, then insert the thermometer through the opening in the top of the flask and secure the thermometer adapter to the flask’s opening.

  • Adjust the thermometer so that the entire bulb sits below the point in the neck where the side arm is attached. The correct positioning of the thermometer is critical to obtaining an accurate boiling point.

  • Lower the flask into the heating mantle. Attach the power cord of the heating mantle to a temperature controller -- sometimes called a rheostat -- and plug the controller into an electrical outlet. Position a beaker or bowl beneath the side arm to catch any liquid that distills over and drips from the side arm.

  • Turn on the temperature controller and set the power level to no more than 50 percent. If the liquid does not begin to boil within 10 minutes, increase the power level in increments of 10 percent until it boils. If the liquid boils in less than 10 minutes, reduce the power until the liquid exhibits a smooth, rolling boil.

  • Monitor the temperature reading of the thermometer and note the temperature after the liquid has achieved a rolling boil for about 5 minutes. This temperature represents the boiling point of the liquid. After you have determined the boiling point, turn off the temperature controller.

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