Lab 6 Properties of Ionic and Molecular Compounds in Solution.pdf

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Lab 6: Properties of Compounds in Solution 1 Properties of Ionic and Molecular Compounds in Solution Learning Objectives: Describe how ionic and molecular compounds dissolve in water Use electrical conductivity to classify solutions as containing electrolytes or nonelectrolytes Compare the electrical conductivity of substances in the pure solid or liquid state to their conductivity in solution Establish a relationship between electrolyte concentration and conductivity Quantitatively prepare a solution of a given concentration and use it to perform a series of dilutions Background: When two or more elements chemically combine, they form a compound . Compounds are generally classified by what type of bond holds together their constituent atoms. Ionic compounds contain ionic bonds , which form when one or more electrons is transferred from one species to another. Ionic compounds always contain one type of cation (positively charged species) and one type of anion (negatively charged species). These cations and anions combine in such a way so that the total positive charges cancel out the total negative charges, resulting in a neutral compound overall. Ionic compounds often consist of a metal cation and a nonmetal anion, though one or both of these may be replaced by a polyatomic ion . An ionic compound is often referred to as a salt . (Note that this is a broad term that does not just refer to the compound we know as table salt, a particular ionic compound.) Molecular compounds contain covalent bonds , which form when one or more pairs of electrons is shared between atoms. Covalent bonds generally occur between two or more nonmetal atoms. Ionic and molecular compounds can be distinguished by their differences in several physical properties including melting point, boiling point, solubility, and electrical conductivity. In this lab we will focus on using electrical conductivity measurements to differentiate between ionic and molecular compounds. To illustrate the difference between ionic and molecular compounds in solution, let us consider two familiar examples. Table salt and sugar are both compounds that dissolve fairly easily in water. Despite their similarities in appearance, however, there are some important differences between the solutions that result when these two compounds dissolve in water. Salt, or sodium chloride (NaCl), is an ionic compound that consists of sodium cations (Na + ) and chloride anions (Cl ). When sodium chloride dissolves in water, it separates into individual cations and anions. This process is called dissociation . The individual ions are free to move around in the solution, and since electricity is the movement of charged particles, the solution therefore conducts electricity. The more ions in solution, the better it will conduct electricity. Compounds that dissociate nearly 100% and produce a large number of ions in solution are called strong electrolytes . Common types of strong electrolytes include soluble salts, strong acids, and strong bases.

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