Aspirin, a common pain reliever, is produced from the chemical reaction between salicylic acid and acetic anhydride, with phosphoric acid added to speed up the reaction. Melting impure aspirin crystals is part of the procedure in the making of aspirin, and this teaches students about how aspirin is made and the chemical reaction among the compounds used.
The process of making aspirin involves isolating the newly formed aspirin from the rest of the remaining chemicals and purifying this through a process called recrystallization, in which the impure aspirin is dissolved in warm ethanol. This is where aspirin melting happens.
The specific process of melting aspirin is used to identify the resulting compound and also to estimate its purity. Chemistry proves that generally, an impure compound will have a melting point that is lower than that of a pure compound. If the aspirin melts at a temperature below the melting point, then it is impure or it is not aspirin at all. Taking note of temperature changes during aspirin melting teaches about the importance of being observant and critical of experimental results.
To be able to carry out a successful science fair project, students learn to identify the proper tools to be used. Aspirin melting makes use of scientific instruments that are prepared beforehand, making sure they are clean to avoid contamination. The instruments used in melting aspirin are a 100-ml beaker, a dropper, a watch glass, a vial to hold the aspirin sample, a lab thermometer that can go up to 110 C, a capillary melting point tube and a melting point apparatus.
In addition to the main chemical components to make aspirin, salicylic acid and acetic anhydride, the experiment also uses concentrated sulfuric acid, phosphoric acid, ethanol, water, and ice. The chemicals are prepared by weighing the exact amounts needed for the experiment. The preparation of the instruments and chemicals needed for the project teaches the student discipline and diligence.
The scientific method entails following a step-by-step procedure for experiments. To remove the salicylic acid from impure aspirin crystals, the crystals go through the process of recrystallization through aspirin melting. First, the impure aspirin crystals are placed in a 100-ml beaker. A mixture of 8 mL ethanol and 25 mL water is added. The beaker is warmed in a 60 C water bath on a hot plate or a hot water bath until the aspirin melts or dissolves. The beaker is then covered with a watch glass and removed from heat. It is set aside to cool slowly.
When it has cooled, the beaker is placed in an ice bath. At this point, needle-like crystals of acetylsalicylic acid---aspirin---begin to form. The aspirin is collected by vacuum filtration, washed, and allowed to dry on the watch glass.
To determine the melting point of the aspirin sample and also verify its purity, a capillary melting point tube is filled with 0.2 cm of the recrystallized aspirin and then placed in the melting point apparatus. The melting point of pure aspirin is 135 C, so if the sample melts at a temperature below the melting point, then it is still impure. Taking note of temperature changes during aspirin melting teaches about the importance of being observant and critical of experimental results.
Following the instructions ensures that the students as well as the audience or observers of the science fair project learn step by step about the process safely.
Melting aspirin as a science fair project is different from melting aspirin in the chemistry lab---for one, there are more people milling about who may not be well-versed in the safety measures needed when doing chemical-based experiments. Therefore, the students who conduct aspirin melting must themselves be strict about taking precautions and inform observers to keep a safe distance. Laboratory gowns, gloves, and goggles must be worn at all times because the salicylic acid and aspirin may cause irritation to skin and eyes.
Students also learn about the proper disposal of excess chemicals. The excess salicylic acid and aspirin can be disposed of in the sink. If some chemicals are spilled, a wet paper towel is used to wipe them up, and the towel is thrown in the trash. The acetic anhydride and phosphoric acid are more dangerous because they can cause burns, and therefore students must wear safety gloves and goggles while handling them. The excess chemicals are disposed of in a plastic tub of water to dilute the phosphoric acid and convert the acetic anhydride to vinegar.
Instruments used in the experiment are washed with soap and water. The work area is also cleaned with disinfectant soap and water.
Science fairs are great opportunities to express students' creativity in presenting their work. The experiment on aspirin melting demonstrates how different scientific instruments are utilized and also gathers data on the temperatures at which aspirin melts. The students doing the experiment may put up a poster that illustrates the process, including a challenging problem such as the comparison of the melting points of impure and pure aspirin, and even build three-dimensional models of chemical compounds using objects such as erasers and gumballs.