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JoVE Lab Manual
Lab: Chemistry

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Melting Points

Melting Points

Learning Objectives

At the end of this lab, students should know...

What is the melting point?

The melting point of a compound is measured as a range of temperatures from where the solid phase transitions into the liquid phase. The upper limit of the range is the temperature where the first liquid drops are observed, and the lower limit of the range is the temperature where the solid phase has fully turned into liquid.

Why is the melting point important?

The melting point is a physical property that is unique to a compound. In organic chemistry, the melting point can help identify the compound in the sample being tested.

What are the three main types of intermolecular forces?

The three major types of intermolecular forces are hydrogen bonding, which occurs in compounds containing oxygen and hydrogen, dipole-dipole interactions, and London dispersion forces, which occur in all molecules.

How do intermolecular forces affect the melting point of a compound?

The strength of the intermolecular forces affect the amount of energy required to break apart the ordered structure found in the solid phase. Hydrogen bonds are the strongest type of intermolecular forces, followed by dipole-dipole interactions, and lastly, London dispersion forces. Compounds with hydrogen bonds have higher melting points than nonpolar compounds with only London dispersion forces.

How do impurities affect the melting point?

Impurities cause the melting point of a mixture to be lower than the melting point of the pure compound. This occurs due to the disordered nature of the structure in the solid phase of the mixture, which requires less energy to transition into the liquid phase.

List of Materials

  • Lab stand
  • Thermometer clamp
  • Digital thermometer (60-150°C range)
  • Melting point capillary tube
  • Rubber bands
  • Watch glass
  • 250-mL glass beaker
  • Metal spatula
  • Benzoic acid
    0.5 g
  • Napthalene
    5 mg
  • Urea
    1 g
  • Stir bars
  • Stirring hotplate
  • 2-mL glass vial with cap
  • 50-mL glass beaker
  • Powder funnel
  • Glass tube (2')
  • Mineral oil
    750 mL
  • Weighing boats
    Dependent on lab size
  • Top-loading or analytical balance (at least 1)
    Dependent on lab size
  • Magnetic wand
    Dependent on lab size

Lab Prep

Source: Lara Al Hariri at the University of Massachusetts Amherst, MA, USA

  1. Preparation of the Laboratory

    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

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