Testing Unknown Solids with Water

In the last lesson, students explored some of the physical properties of the unknown solids. Over the next three lessons, they will continue this exploration by investigating the property of solubility. They do this by creating mixtures with each of the five unknowns and increasing amounts of water. This experience shows students that combining one substance with another can reveal new properties of each. 

1 Green l Talc Does not mix easily water beads up skin forms on water surface  

Blue Baking soda Dissolves liquid may be slightly cloudy 1 

When they have finished testing, students may observe that an unknown has melted or seems to have disappeared. After they have explained these observations In their own words, you may want to introduce the term dissolve. When a material is dissolved (for example, sugar dissolved in water), it is spread evenly throughout another substance and is not visible. 

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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. 

Add about 50 mg of the unknown solid (2 mm or 3 mm off the end of a spatula) or 5 drops of the liquid unknown to 1 mL of water. Stir the mixture with a spatula so that as much as possible of the unknown dissolves in water. Add several drops of a 2.5% aqueous solution of ferric chloride to the mixture. Most water-soluble phenols produce an intense red, blue, purple, or green color. Some colors are transient, and it may be necessary to observe the solution carefully just as the solutions are mixed. The formation of a color is usually immediate, but the color may not last over any great period. Some phenols do not give a positive result in this test, so a negative test must not be taken as significant without other adequate evidence. 

The laboratory procedures associated with identification comprise a process known as qualitative analysis. In Part A of this experiment, you will examine 11 household products to determine some specific physical and chemical properties and to establish a qualitative analysis scheme for their identification. The physical properties include characteristics associated with their physical appearance, including color, particle size, and texture, as well as their solubility in water, rubbing alcohol, and hot water. The chemical properties include the manner in which the white solids react chemically with various other chemicals. In Part B, you will have three to five unknown household products (taken from those tested in Part A) and your work will involve using your qualitative analysis scheme to identify them. The flow chart that follows the procedure (Figure 3.6) should be filled out while performing Part A and should help when you perform Part B. 

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. 

If the unknown is insoluble in a particular solvent, warm the test tube in the hot-water or steam bath. Stir or swirl the contents of the tube and note whether the unknown is soluble in hot solvent. If the solid is soluble in the hot solvent but only slightly soluble or insoluble at room temperature, allow the hot solution to cool to room temperature slowly. If crystals form in the cool solution, compare their quantity, size, color, and form with the original solid material and with those obtained from other solvents. 

On a white spot plate place 2 drops of water, or 1 drop of water plus 1 drop of ethanol, or 2 drops of ethanol, depending on the solubility characteristics of the unknown. To this solvent system add 1 drop (10 mg if a solid) of the substance to be tested. Stir the mixture with a thin gjass rod to complete dissolution. Add 1 drop of 2.5% aqueous ferric chloride (FeQa) solution (light yellow in color). Stir and observe any color formation. If necessary, a second drop of the FeCls solution may be added. 

In a small test tube, place 1-2 drops of the unknown (—20 mg if a solid) and add 1-2 mL of water. Check the pFl of the solution with pFl paper. In the HOOD hood, add saturated bromine water dropwise until the bromine color persists. A precipitate generally forms. 

Solubility ia Water and in Ether.—A 0.10 g. portion of a solid unknown is treated with successive 1 cc. poi-tions of water until 3 cc. have been added. If the compound does not dissolve in the ratio of 1 20 or 25, it is designated insoluble in water. The substance if solid must be finely powdered so as to eliminate the possibility of a verdict of insoluble when in reality a mechanical difficulty is responsible for the decision. If the substance appears to be insoluble, the suspension may be warmed genth. If solution occurs, the test portion is again cooled and shaken vigorously to prevent supersaturation upon cooling. 

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