Boiling and Melting point

• Boiling point and Melting point are important physical properties that can be 

  used to characterize organic compounds.
• The range of melting point and boiling point of substances is also indicative of the degree of purity of the substance.
• An impure compound melts or boils over a wide range of temperatures whereas a pure compound boils at a constant temperature.
• Boiling points and melting points depend on the molecular weights of substances as well as on the type of intermolecular forces of attraction between molecules.
• REAGENTS

  • Benzoic acid
  • benzoic acid + vanillin mixture 
  • ethyl alcohol
  • ethyl alcohol and water mixture
  • cooking oil
• SAFETY PRECAUTION

  1. The Thiele tube is a melting point apparatus which uses mineral oil as a heat transfer medium. Heating the oil above 100 °C causes the oil to splatter if it is contaminated with even a few drops of water. Examine your Thiele tube for evidence of water droplets in the oil. If there are any, change the oil or exchange tubes.
• PROCEDURE

  Wear protective glasses and mask.

  Melting Point Determination

  1. Seal one end of two capillary tubes. Place the tip of the tube to the edge of the flame. The tube is sealed when the tip is smooth and rounded.
• PROCEDURE

  Wear protective glasses and maskMelting Point Determination

  2. Powder about a pinch of benzoic acid using a mortar and pestle. 
• PROCEDURE

  Wear protective glasses and mask.

  1. Melting Point Determination
• A.

  3. Insert the powdered sample lightly into the first capillary tube. Push the open end of the tube into the powder and tap the closed end on the table top. The tightly packed sample should occupy a height of about 1 cm in the capillary tube.
• A.

  4. Add another pinch of benzoic acid and a pinch of vanillin into another mortar. Powder the mixture smoothly using circular motion and introduce into the second capillary tube.
• A.

  5. Attach the capillary tube to the thermometer by means of a string. Position the capillary tube such that the sample and the thermometer bulb are adjacent.
• A.

  6. Suspend the thermometer-capillary tube assembly into a Thiele tube containing 20 mL of cooking oil (See Figure 3, p. 21). A beaker maybe used in place of a Thiele tube.
• A.

  7. Heat the oil bath evenly.
• A.

  8. Record the temperature at which the sample starts to melt and the temperature at which the sample has completely melted. The difference between these two values is the melting point range while the average of the two values is the experimental melting point.
• A.

  9. Determine the experimental melting points of pure benzoic acid and impure benzoic acid (benzoic acid + vanillin) allowing the oil bath to cool in between determinations.
• A.

  10. Calculate the % error for pure benzoic acid if its true melting point is 121°C.