Introduction:
The MSU Office of Environmental Health & Safety was presented with an unknown substance in the lab. They required the help of the MSU Chemistry department to identity the unknown substance so that it could be disposed of in a proper manner (1). 5 grams of the substance were provided to each group, and multiple identification tests were conducted to discover what the substance was. These tests included solubility, flame, cation, pH, and anion tests. The solubility test determines whether the substance is soluble in water, the flame test indicates which cation is present in the substance, the cation test also determines the presence of cations and also the charge of the substance, the pH test determines whether the substance is basic or acidic, and the anion test indicates which anions are present in the unknown substance. In the experiment, the substance was combined with other compounds with different properties, such as acids. These reactions indicated that having a plan of action for disposal was important to prevent harmful gases, precipitates, and even potential fires in case the substance came in contact with something else in the lab. The main purpose of this lab was to discover the properties of the unknown substance so that is could be disposed of properly.
Experimental Details/ Theoretical Analysis:
Cation Test: The cation test first required a test experiment with another substance so that the individual conducting the test knows what a positive result of cations looks like. First, 0.5g of NH₄+ was combined with 10mL of distilled water in a plastic boat to create a solution. The solution was then combined with 3mL NaOH liquid (an indicator for cations, contains “the sodium salt of an anion”) (5). If cations were present, the reaction would produce an ammonia-like scent. The preliminary test was positive, so we knew what to look for. Then 0.5g of the unknown substance was combined with 10mL distilled water to create the solution. Again, 3mL of NaOH liquid was combined with the solution.
Flame Test: First, a small amount of distilled water was put into a plastic boat and then a small sample of the unknown substance was added to the plastic boat using a spatula. The unknown substance dissolved into the water to create a solution. A wooden splint was then soaked in the solution for 2 minutes, and was then placed in the flame of a Bunsen burner. The color of the flame was recorded. The results of the flame test allowed for the test substances for the anion and solubility to be narrowed down into a more specific group (2).
Solubility Test: For the solubility test, one spatula full of the unknown substance was measured out into a plastic boat along with 20 mL of distilled water (4). It was then stirred until it dissolved. The cation and flame tests allowed the following experiments to be narrowed down into a more specific group of compounds (3). Rather than testing all the provided compounds that would be used to compare to the unknown substance, only substances containing sodium were used in the solubility test. This included Na2SO4, Na2CO3, and NaCl.
pH Test: The pH test indicates whether or not a substance is acidic or basic. First, 0.5g of the unknown substance was combined in a plastic boat with 10mL of distilled water. A litmus paper was then inserted into the solution to see whether the unknown substance is acidic or basic. For pH validation, the solution was then combined with 1mL of 3M HCl in a clear beaker. Results were recorded.
Anion Test: Initially, all testable reference substances were going to be used in the anion experiment. However, like the solubility test, the flame test allowed the anion test substances to be narrowed down to only substances that contained sodium. The testable compounds containing sodium that were used were NaCl (sodium chloride), Na2SO4 (sodium sulfate), and Na2CO3 (sodium carbonate). First, 1 spatula-full of NaCl, Na2SO4, and Na2CO3 were added to three individual test tubes. Then 1mL of 1M HNO3 solution was added to each of the test tubes. If any reaction or precipitate occurred, then results were recorded. A spatula-full of each compound was measured into the test tubes again, and 1mL of 1M HCl and 0.1M BaCl2 solution was added to each test tube. Again, results were recorded. Finally, a spatula-full of each compound was measured into three test tubes, and 1mL of 3M HCl solution was added. Results were recorded. These results provided baseline reactions for the different anions: halides, carbonate, and sulfates. 1 spatula-full of each unknown compound was then added to three individual test tubes and 1mL of each of the solutions (1M HNO3, 1M HCl and 0.1M BaCl2, and 3M HCl) was added. Results were recorded and compared with reactions from the sample substances.
Color Changing Validation Experiment: After preliminary experiments, it was suspected that the unknown substance was Na2CO3 due to its reactions (2). To validate this identity, an experiment was used combining a 10mL solution of 3.3g mL 3M HCl and 6.7 mL distilled water (to produce 1M HCl) and a 1M solution of 3.7 g Na₂CO₃ and 35mL distilled water in a graduated cylinder. Before combining the two solutions, a few drops of universal indicator are added to the 1M solution of 3.7 g Na₂CO₃ and 35mL distilled water. The solution turned bright pink. Record observations. The same experiment was the conducted with the unknown substance. A .429M solution was produced combining 1.59g unknown substance and 35mL distilled water, and this was then added to a 0.429M 10 mL solution of 1.4 mL 3M HCl with 8.6 mL distilled water. Observations were recorded.
Results:
Cation Test: This test indicated whether or not there was a presence of cations in the unknown substance (5). After combing 0.5g of the unknown substance and 10mL distilled water solution with 3mL NaOH liquid, an ammonia-like scent was immediately produced. There was no physical precipitate or reaction, but the scent was produced. This result indicated that there was a presence of cations.
Flame Test: The flame test helped identify which cation was present in the unknown substance. When the wooden splint soaked in the unknown solution was placed in the Bunsen burner, an orange flame was produced (3). This result prompted a sodium chloride test to be conducted. A wooden splint was soaked in a sodium chloride solution and then placed in the Bunsen burner. This also produced an orange flame. Referencing the flame test color meaning table in the “Dealing with Unknowns” section of Lab Archives, an orange flame means there is a presence of sodium. The results from the sodium chloride test and the flame color table help narrow down the unknown substance to something containing sodium (5).
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Solubility Test: Due to the results of the flame test, the testable reference substances were narrowed down to one that only contained sodium. The testable substances containing sodium and the unknown compound were all soluble in water (4). Below is a photo of all the substances tested (L-R): Na2SO4, Na2CO3, NaCl, unknown. Top row is the dissolved substances and the bottom is the dry substances for reference (4).
pH Test: The pH test indicates whether a substance is acidic or basic. When the litmus paper strip was inserted into the beaker containing the unknown substance solution, the paper turned a deep blue. Referencing the litmus package, the color was between a dark blue and purple, showing up between 10 and 11 on the pH scale (6).
To validate whether the substance was an acid or basic, a hydrochloric acid test was conducted. The unknown substance solution combined with 1mL of 3M HCl immediately fizzed, created a slightly vinegar-y scent, and quickly dissipated (6).
Anion Test: When the NaCl was combined with the 1mL 1M HNO3 a white precipitate was formed (2). According the “Dealing with Unknowns” section, this indicated the presence of halide ions. This was a positive test for the halides. Similarly, reference positive tests for sulfate and carbonate ions were both conducted. The sulfate reference test occurred with the combination of Na2SO4 and 1mL of 1M HCl and 0.1M BaCl2 solution. This produced a cloudy precipitate throughout the solution. Lastly, for the positive carbonate test, Na2CO3 and 1mL 3M HCl was combined. This produced fizzing as the reaction. Finally, the unknown solution was tested with 1mL of all the solutions. No result occurred for the halide and sulfate indicators, however fizzing did occur with the HCl. This indicated the presence of carbonate ions.
Color Changing Validation Experiment: The results for the Na2CO3 and the unknown substance in the Color Changing Validation Experiment were the exact same. When the two solutions were combined in the graduated cylinders, fizzing occurred. A line between the solution containing the universal indicator and the HCl solution showed up at the 35mL mark at the beginning of the experiment. This line gradually dropped to 28mL over the course of two minutes due to the neutralization of area between the two solutions (2).
Known Substance Reaction
Unknown Substance Reaction
Discussion:
Many of the preliminary experiments such as cation, flame, and solubility tests all supported that the unknown substance contained sodium. Starting with the cation test, the ammonia-like scent produced by the reaction between the unknown substance and the NaOH solution meant that there was a presence of cations, and that the unknown substance had a positive (+1) charge. After further evaluating the meaning of the presence of cations, it was found that when sodium forms an ion with another substance, it also forms a +1 charge (7). Because the ammonia scent was created, the unknown had a positive charge, and was most likely thought to be sodium at this point in the testing process.
In the solubility test, the testing substances were narrowed down to the group of compounds containing sodium. When Na2SO4, Na2CO3, NaCl, and the unknown substance was tested in 20mL of distilled water, every single one dissolved. This is because all the substances containing sodium (Na) were ionic compounds, and when dissolved in water the ionic compounds “separate into their component ions in the solvent,” in this case the solvent being the distilled water (8). It is also known that salt (NaCl) dissolves in water. Because all the testable substances and the unknown compound dissolved in water, evidence supported that the compound contained sodium.
In the flame test, again, the idea that the unknown substance contained sodium was supported. Referencing the flame color table from Lab Archives allowed the idea that the substance contained sodium to be fully verified.
The pH test helped prove that the unknown substance was basic, as opposed to acidic. The pH test was conducted after the anion test, where it was initially suspected that the unknown substance was Na2CO3, or sodium carbonate. After conducting the litmus test, the blue-ish purple result went under further review. According the site aqion.de, the pH of sodium carbonate is 10.52 which lines up perfectly with the result provided by the litmus paper (9). 10.52 is on the basic side of the pH scale. To verify the result of this test saying that the unknown substance is basic, a HCl acid test was conducted. When the unknown substance combined in a solution with distilled water was added to 1mL of 3M HCl, it fizzed. This “neutralization” reaction verified that the unknown substance was basic due to its reaction with an acidic solution (10).
The anion test that verified the presence of carbonate anions was exactly same as the HCl test conducted in the pH test. The anion test verified that the unknown substance contained carbonate ions, which was a big find, similar to the discovery that the unknown substance contained sodium.
In the final “Color Changing Validation” experiment, the true identity of the unknown substance was suspected to be sodium carbonate, and the results of this experiment supported this claim. When the unknown substance solution and the HCl solution were combined, the line between the two substances eventually dropped due to the production of carbonate gas. The fizzing reaction supported the claim that the unknown substance was sodium carbonate, or Na2CO3.
Conclusion:
Comparing the results from the six tests, the claim that the unknown substance found in the lab is sodium carbonate is supported. Sodium carbonate is a positively charged, basic pH substance that contains sodium. All the chemical properties of the substance and reactions with other substances were the same as the reactions that occurred when the reference substance Na2CO3 was tested. The flame, solubility, and cation tests all supported that the substance contained sodium, while the anion test helped discover that the substance contained carbonate ions.
This experiment was conducted to discover the nature of an unknown substance for the MSU Office of Environmental Health & Safety. Proper disposal of unknown substances is important to prevent dangerous and hazardous accidents. If the unknown substance were disposed of with any acidic substances without being neutralized, hazardous gases could’ve been produced.