Testing Unknown Solids with Heat

Overview and In Lesson 9, students analyzed their data from five tests. These tests involved 

Objectives adding water and other chemical liquids to the five unknown chemical solids. In 

Background Many students are fascinated by the heat test—because of both the heating process 

As one safeguard, students will rotate through the heat station in manageable teams of five. Because you will repeat the test several times, you may want to spread it out over two days. You may also want to enlist the help of an adult volunteer. 

While each team is performing the test, engage the others in activities such as making safety posters about the use of heat, reading mystery books, or illustrating unit activities they have enjoyed. If students remain distracted by the activity, that is fine it will help them understand what to do when it is their turn at the station. 

---

Your students have carried out two physical tests (water drop and water mixtures) and four chemical tests (vinegar, iodine, red cabbage juice, and heat) on the unknown solids. In this lesson, they review and analyze all the data they have collected as a result of performing these tests to determine the distinctive properties of each unknown. This process provides students with the information and skills they need to solve the mystery in Lesson 12. 

Partially melt the naphthalene in the 25 x 200-mm test tube from part A so that you can remove the stopper with the thermometer and stirrer. Transfer the weighed sample of unknown solid to the test tube containing the naphthalene, being careful not to lose any. Tap the test tube so that all of the unknown solid is down in the naphthalene. Replace the test tube in the beaker of water, and heat the water sufficiently to cause all the naphthalene to melt. When all of the naphthalene is melted, stop heating the water. Insert the thermometer into the test tube to the same depth as in part A. When the temperature has dropped to 82°C, record the temperature of the solution of unknown in naphthalene every half-minute in TABLE 20.1, until stirring becomes impossible. 

To identify the anion present in an unknown sample, silver, barium or lead(II) solutions are generally added to determine whether a precipitate forms and whether the precipitate reacts with acids (carbonate or sulfate(IV)). The unknown sample can also be given in a solid form and students can be asked to heat the substance. When adding acid or when heating, students must be prepared to test for the appropriate gases as discussed in section 9.3. 

Place 0.5 mL of a 2M stock solution of semicarbazide hydrochloride (or 0.5 mL of a solution prepared by dissolving 1.11 g of semicarbazide hydrochloride [MW =111.5] in 5 mL of water) in a small test tube. Add 0.15 g of the unknown compound to the test tube. If the unknown does not dissolve in the solution or if the solution becomes cloudy, add enough methanol (maximum of 2 mL) to dissolve the solid and produce a clear solution. If a solid or cloudiness remains after adding 2 mL of methanol, do not add any more methanol and continue this procedure with the solid present. Using a Pasteur pipette, add 10 drops of pyridine and heat the mixture in a hot water bath (about 60°C) for about 10-15 minutes. By that time, the product should have begun to crystallize. If the product does not crystallize, evaporate h. the volume of methanol. Collect the product by vacuum filtration. The product can be recrystallized from ethanol if necessary. 

In a 1.0-mL conical vial place 0.5 mL of 2% ethanolic AgNOs solution and 1 drop of unknown ( 10 mg if a solid). A positive test is indicated by the appearance of a precipitate within 5 min. If no reaction occurs, add a boiling stone and equip the vial with an air condenser. Heat the solution at gentle reflux for an additional 5 min using a sand bafh. Cool the solution. 

In a 3.0-mL conical vial containing a boiling stone and equipped with an air condenser place 1 drop of a liquid unknown ( 10 mg if a solid), followed by 0.5 mL of 1 M hydroxylamine hydrochloride-propylene glycol solution. Heat the resulting mixture to reflux temperature (—190 °Q using a sand bath, and reflux for 3-5 min. Cool the solution to room temperature, and add 2 drops of 5% aqueous FeCh solution. The formation of a red-to-purple color is a positive test. 

In a 1.0-mL conical vial containing a boiling stone, and equipped with an air condenser, place 1-2 drops of the unknown liquid ( 10 mg if a solid) and 0.5 mL of 20% aqueous NaOH solution. Heat this mixture to gentle reflux on a sand bafh. Moisten a strip of filfer paper with 2 drops of 10% aqueous copper sulfafe solution and place it over the top of fhe condenser. Formation of a blue color (copper ammonia [or amine] complex) is a positive test. 

---