How to Determine Benzoic Acid Melting Point

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Melting point analysis provides easy identification of solid substances, such as benzoic acid, in the lab. Using a melting point device such as a Mel-Temp, students can observe the sharp melting points of pure substances and the different melting ranges that appear in impure substances.

Things You'll Need

  • Melting point device
  • Mortar and pestle or ceramic spot plate and test tube
  • Benzoic acid sample
  • Glass melting point capillaries
  • Goggles
  • Lab notebook
  • Pen or pencil

Determining a Melting Point for Benzoic Acid

  • Make sure the melting point device is close to room temperature before beginning a melting point analysis.

  • Using a spot plate and the butt end of a test tube or a mortar and pestle, grind the sample of benzoic acid into a fine powder. Pack it into a glass capillary tube so that the level of powder is about 2 mm high, as recommended by Boston College.

  • Turn the capillary so that the closed end is down. Pack it tightly by dropping it through a long tube over the lab bench or tapping it on the bench directly. This prevents uneven heating, which can interfere with accuracy.

  • Set the melting point device to a temperature that will create relatively slow heating. Mel-Temp devices have a setting that reaches maximum temperature at 1,000 degrees C and another that reaches maximum at 200 degrees C. If using a device that has these settings, set it to 200 degrees C for benzoic acid.

  • Find the correct setting on the heating rate dial. Determining this setting can be complicated, but a good rule of thumb is to keep the rate of heating to a slow enough pace to avoid missing valuable data. According to Maricopa Community College, you should lower the heating rate as the expected melting point comes closer.

  • Observe the benzoic acid's melting point in the device. Look through the eyepiece, wearing goggles, and glance away briefly to check and record the temperature in your lab notebook every time a change is noted. Material might "sweat" before its melting point.

  • Record the temperature at which the first drop of liquid emerges and the temperature at which the sample has liquefied. That is the observed melting point or melting range of the substance - in this case, benzoic acid.

Identifying a Substance Using Melting Point Analysis

  • To check the identification of a product believed to be benzoic acid - a sample produced by an experiment, for example - first test its melting point alone. Verify that the product has a melting point similar to that of benzoic acid.

  • Mix some of the experimental sample with a known sample of benzoic acid, using equal amounts of both samples. Grind the mixture into a powder, carefully avoiding contamination, and load a melting point capillary as described previously.

  • Place the capillary into the melting point device and begin heating. Watch carefully. If the sample really is benzoic acid, the melting point will remain the same. If it exhibits contamination, the mixed sample will melt at a lower point and over a wider temperature range, according to the University of West Florida.

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