Qualitative Analysis and Deductive Reasoning: Identification of an Unknown Salt

Qualitative Analysis and Deductive Reasoning: Identification of an Unknown Salt

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Qualitative Analysis and Deductive Reasoning Part 1: Identification of an Unknown Salt Heather White Group Partner: Robbie Chem 2AC, Evening Instructor: Claron Ridge 10/17/06
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Introduction My group was given an unknown salt containing three ions, either two cations and one anion, or two anions and one cation. We needed to identify one cation and one anion within the salt from a selection of possible options. The possible ions in the salt are listed as follows: chloride, nitrate, carbonate, sulfate, lithium, potassium, sodium, magnesium, calcium, strontium, and barium. We accomplished this task through two experimental techniques. The first, a flame test, focused on the color of light emitted when the salt was heated in the flame of a Bunsen burner. Different metals, when heated, emit different colors of light. Thus, the cation could be determined. The second test focused on precipitation reactions between the aqueous solution of the unknown salt and added ionic compounds. Based upon whether or not a solid was formed when each compound was added to the solution, we were able to determine one anion within our salt, and reinforce our other experimental data on the cation. Theoretical Basis Every element has a characteristic emission spectrum. When an atom is excited, as through heat, it absorbs excess energy and the electrons within the atom reach an excited state. When the electrons fall back to their normal state, the excess energy is emitted as light of various discreet wavelengths. The discreet wavelengths vary from element to element. Thus, the emission spectrum of the atom is produced, and is often observable as a distinct color of light. 1 The metals lithium, strontium, potassium, magnesium, calcium, and sodium each emit a distinct color of light, while the non-metals being tested do not. 2
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When a chemical reaction in a solution results in the creation of a solid substance, the substance is called a precipitate. If an aqueous ionic compound is added to a solution containing ions, the formation of a precipitate is indicative of chemical change. 4 Certain anions and cations bond to create insoluble compounds, while others create soluble compounds. The color and density of the precipitate also vary from compound to compound. Experimental Procedure Solvent and Solution We were informed by the instructor prior to the start of the experiment that our unknown compound was a salt. When added to deionized water, the compound dissolved. Also, the solubility of many ions in water was known, and water was thus the ideal solvent. About 200 milligrams of the unknown salt was dissolved completely in 100 mL of water. All samples of the solution used throughout the project were taken from this original batch of solution. From hereon, this solution of solely the unknown salt in deionized water will be referred to as the “unknown solution.” Flame Test A cleaned wire loop was dipped into the solution. It was then held over the central, blue part of the flame emitted from a Bunsen burner. It was held in place for thirty seconds, and the change in color of the flame was recorded.
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