How to Confirm That Benzoic Acid Is Purified?

Test the purity of a compound by verifying its melting point range.

Benzoic acid is a compound often used in food preservation. It is also commonly used in student chemistry labs to demonstrate how to recrystallize a substance from water. After, students are asked to test the purity of the compound. A method of verifying the purity of a substance is to check its melting point range. This is the degree at which the substance begins to liquefy, and the temperature at which no solid remains. An impure sample will have a melting point range that is broader and lower than its pure counterpart.

Things You'll Need

• Sample of benzoic acid
• Aluminum block
• Thermometer
• Bunsen burner or hot plate

Instructions

1. Place a thermometer into a clean aluminum block. This measures the beginning and ending melting points.

2. Sprinkle a thin layer of the sample product onto the surface of the block. Handle the material carefully and always wear safety glasses.

3. Heat the block using a Bunsen burner or hot plate. Perform this procedure slowly.

4. Observe the melting process carefully. Write down the temperature when the first drop of liquid appears and the degree at which the entire sample is liquefied.

5. Verify your results. A pure sample will have a melting range of 244 to 290 degrees Fahrenheit. An impure sample begins to melt at 241 degrees Fahrenheit.

Tips & Warnings

• Never leave a burner unattended.

References

• Pub Chem: Benzoic Acid
• MSDS: Safety Data for Benzoic Acid
• MC Science: Melting Point Determination
• Eckard Academics: Recrystallization and Melting Points