Melting Points

Here, we show the laboratory preparation for 10 students working in pairs, with some excess. Please adjust quantities as needed.

• To set up for this lab experiment, wear the appropriate personal protective equipment, including a lab coat, chemical splash goggles, and gloves.
• Prepare a mixture of urea and benzoic acid in a mass ratio of 4 to 1. Tare a weighing boat and measure 0.48 g of urea. Use a powder funnel to transfer the urea to a glass vial.
• Weigh 0.12 g of benzoic acid and add it to the vial. Mix the solids thoroughly by stirring with a spatula and shaking the closed vial. Label the vial as ‘urea with unknown’.
• Obtain enough naphthalene for each group to have 1 mg.
• Label a 50-mL beaker for solid waste and set it along with the naphthalene, the bottle of pure urea, and the mixture of urea and benzoic acid near the balance.
• For each group, pour about 150 mL of mineral oil into a 250-mL beaker. Distribute the baths to the student’s workstations before the lab.
• Place a 2-ft section of glass tubing on a communal bench for students to use during the lab.
• Set out the following glassware and equipment at each student lab station (we suggest that students work in pairs):

  1    Stir bar

  1    Stirring hotplate

  1    Lab stand

  1    Thermometer clamp

  1    Digital thermometer (pre-calibrated, 60 – 150°C)

  4    Melting point capillary tubes

  3    Metal spatulas

  3    Watch glasses

  1    Rubber band