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Boiling Point of an Organic Compound

Materials Required

The Procedure

Real Lab Procedure

  • First fill two-thirds of the small test tube with the given liquid whose boiling point needs to be determined.
  • Fix this test tube to the thermometer with a rubber band in such a way that the bottom of the tube is at the middle of the thermometer bulb. The rubber band should be fixed near the mouth of the tube so that it remains outside the acid bath.
  • Fill half of the beaker with Con. sulphuric acid and place it over a wire gauze placed over a tripod stand.
  • Clamp the thermometer carrying the test tube to an iron stand through a cork. Lower the thermometer along with the tube into the acid bath.
  • Adjust the thermometer so its bulb is well under the acid and the open end of the tube with the rubber band is sufficiently outside the acid bath.
  • Take the capillary tube and seal at it about 1 cm from one end by heating it in flame and giving it a slight twist.
  • Place the capillary tube in the test tube containing the  given liquid so that the sealed part of it stands in the liquid.
  • Start heating the acid bath slowly and stir the bath gently. Keep an eye on the liquid and the test tube and also on the thread of the mercury in the thermometer.
  • At first a bubble or two will be seen escaping at the end of the capillary tube dipped in the liquid, but soon a rapid and continuous stream of air bubbles escapes from it.  At this stage the vapour pressure of the liquid just exceeds the atmospheric pressure.
  • Note the temperature (t1) when continuous stream of bubbles starts coming out.
  • Remove from the flame and note the temperature (t2) when the evolution of bubbles from the end of the capillary tube just stops.
  • The mean of these two temperatures gives the boiling point of the liquid.
  • Allow the temperature to fall by 10oC and repeat the heating and again note the boiling point.

Simulator Procedure (as performed through the Online Labs)

  • You can select the compound from the ‘Select the unknown compound’ drop down list.
  • To start the experiment, click on the ‘Start’ button.
  • The temperature begins to rise.
  • Note the temperature ‘t1’ at which a continuous stream of air bubbles appear from the end of the capillary tube.
  • Now turn off the burner by clicking on the ‘Knob’ of the burner.
  • Note the temperature ‘t2’ at which the air bubbles completely disappear.
  • Enter the values in the respective text boxes.
  • The boiling point of the compound is shown in the text box.
  • You can select the actual compound from the ‘Select the actual compound’ drop down list.
  • You can verify your result by clicking on the ‘Show chart’ button.
  • To redo the experiment, click on the ‘Reset’ button. 

Observations

Record your observations in the table given below.

Note the temperature when; Boiling Point of the given organic liquid (t1 + t2 / 2 (oC)
Rapid and continuous evolution of gas bubbles occurs t1(0C) The evolution of gas bubbles just stops t2 (0C) (after removing from the flame)
     

Inference

The boiling point of the given organic liquid = ... oC.

Precautions

  • If on placing the sealed capillary tube in the test tube, the liquid is seen rising in the capillary tube, it indicates that the capillary tube is not properly sealed. Reject this capillary tube and use a new one.
  • The seal point of the capillary tube should be well within the liquid.
  • The acid bath must be heated very slowly and the acid is stirred to ensure uniform heating.