Chembio Lab Midterm

• 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 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 inter-molecular forces of attraction between molecules
• Thiele tube


  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
• Do not handle ethyl alcohol near an open flame since it is highly flammable
• Be careful to avoid contact of chemicals with your skin
• True boiling point


  The boiling point of a pure substance
• True melting point


  The melting point of a pure substance
• True boiling point of ethyl alcohol is 78°
• True melting point of pure benzoic acid is 121°C
• Experimental boiling point


  The point the instant the bubbles cease to come out of the capillary tube and just before the liquid enters it
• Chemicals


  • Benzoic acid
  • Benzoic acid + vanillin mixture
  • Ethyl alcohol
  • Ethyl alcohol + water mixture
  • Cooking oil
• 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.
• Powder about a pinch of benzoic acid using a mortar and pestle.
• The tightly packed sample should occupy a height of about 1 cm in the capillary tube.
• 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
• Suspend the thermometer-capillary tube assembly into a Thiele tube

  containing 20 mL of cooking oil. A beaker maybe used in place of a Thiele tube
• 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.
• In boiling point determination introduce about 20 drops of ethyl alcohol into a culture tube.
• Prepare 20 mL of a water bath in a beaker. Suspend the culture tube- thermometer assembly into the water bath with the use of a clamp.
• Raise the temperature of the water bath until a rapid and continuous stream of bubbles is observed in the alcohol sample