• Observe students during the initial oral presentation and ask questions of students as needed about the types of solutes.
• Assess student performance and provide feedback on the completed Learning Activity Worksheet on Solutes (S-C-7-2_Solutes Worksheet and KEY.doc) and the completed Practicing Your Skills Lab on Solutes (S-C-7-2_Solutes Lab and KEY.doc).
• Listen to or collect reports on what students learned from the discussion on the Web site at http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/premium/0073511080/student/171024/4_1_Stg_Wk_Nonelelytes.swf::Strong%20Weak%20and%20Non%20Electrolytes
• Evaluate student experiences at Frankfort Mineral Springs and scientific reports about the mineral spring water. http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/electroChem/conductivity-4.swf

• carbonic acid (dissolved CO₂ / H₂CO₃)
• iron(II) carbonate (FeCO₃)
• magnesium carbonate (MgCO₃)
• hydrogen sulfide (H₂S)
• sodium chloride (NaCl)
• bitumen

Now say, “More recently, scientists have found that the springs contain an average of 163 mg of calcium and 76 mg of magnesium / liter of water. The pH is 7.02 and the mean temperature of the spring water is 54°F (12°C).”

Tell students, “We are going to do an experiment on a simulated sample of water from the mineral springs, but first we are going to talk about two types of solutes.” Below the names and formulas already given, write the following:

• Ionic solute—
• Molecular (covalent) solute—

After the two solute descriptions, write these short descriptions, shown in red:

• Ionic solute—Substance that consists of ions held together by electrostatic attraction. When dissolved in a solvent such as water, the ions dissociate. This type of substance is also called a salt.
• Molecular (covalent) solute—Substance whose atoms are held together by intermolecular forces. Bonds between atoms are not broken when dissolved in a solvent such as water, so no ions are formed. The substance is usually called a covalent compound.

Explain the definitions and give examples. You might say, “Ionic solutes consist of ions that are held together within a crystal. These ions dissociate in water. For example sodium chloride or ordinary table salt dissolves in water giving off sodium ions (Na⁺) and chloride ions (Cl⁻). Molecular or covalent solutes exist as neutral molecules in water. For example, when oxygen dissolves in water it exists as neutral molecules. It does not form ions.”

Now share with students that the types of bonds between atoms affects how the substance will act in solution, either as an ionic solute or a molecular solute. Say, “Ionic solutes contain ionic bonds. These bonds form when one atom transfers its valence electron (or electrons) to another atom. The atom that gave up its electron (or electrons) becomes positively charged. The atom that received the electron or electrons becomes negatively charged. For example, when a sodium (Na) atom combines with a chlorine (Cl) atom, the sodium atom donates its one valence electron to the chlorine atom creating a Na⁺ ion and a Cl⁻ ion. These ions are held together by electrostatic forces. The forces holding the ions together are called ionic bonds. Usually metals combine with nonmetals to form ionic bonds.”

Continue, “Molecular solutes contain covalent bonds. These bonds form when two atoms share valence electrons. Neither atom obtains a formal positive or negative charge; electrons are simply shared between the atoms. For example, when a chlorine (Cl) atom combines with another chlorine (Cl) atom to form Cl₂, both atoms share a valence electron with each other so that each forms an octet of electrons. The attraction caused by shared electrons is called a covalent bond. Usually nonmetals combine with other nonmetals to form covalent bonds.”

Now explain that there are some other names for these types of solutes. Say, “Strong electrolytes are made of ionic compounds. Since these compounds give off ions in water, they make solutions a good conductor of electricity. Sodium chloride (NaCl) and potassium hydroxide (KOH) are examples of strong electrolytes because their ions in solution conduct electricity really well. Strong acids like sulfuric acid (H₂SO₄) are also strong electrolytes. Even though sulfuric acid is a molecular substance, it dissociates in water forming hydronium (H₃O⁺), hydrogen sulfate (HSO₄⁻), and sulfate (SO₄²⁻) ions. These ions make the solution an excellent conductor of electricity. Sulfuric acid forms these ions because the hydrogen atoms in H₂SO₄ are held loosely by their covalent bonds causing them to be easily ionized by water molecules.

Weak electrolytes can also ionize but not as much as strong electrolytes. In acidic, weak electrolytes, the hydrogen atom or atoms are held more strongly in their covalent bonds than in strong electrolytes and are not as easily ionized. Thus a weak electrolyte only partially ionizes in water to form hydronium ions (H₃O⁺). Basic, weak electrolytes, partially ionize in water to form hydroxide (OH⁻) ions. The partial ionization of weak electrolytes makes solutions a moderate to poor conductor of electricity. Acetic acid (CH₃COOH), a weak acid, and ammonia (NH₃), a weak base, are examples of weak electrolytes. Organic acids containing carboxylic acid (–COOH) groups and organic bases containing amine (–N<) groups are generally weak electrolytes.

Nonelectrolytes are molecular solutes that have no ionizable hydrogen atoms. As a result they do not form ions in solution. Ethanol (CH₃CH₂OH) is an example of a nonelectrolyte. It exists solely as molecules when dissolved in water; the solution is a nonconductor of electricity.”

Tell students, “Before we start the experiment, I am going to give you a worksheet so you can evaluate your understanding of what we have discussed so far.” Hand out the Learning Activity Worksheet on Solutes (S-C-7-2_Solutes Worksheet and KEY.doc). After students have finished marking their worksheets, go over the answers with them, and answer any questions.

Instruct the class to divide into groups of three or four people. Tell the class, “We are now going to do a lab in which you identify the type of solute you have, whether it is a strong electrolyte, weak electrolyte, or nonelectrolyte.” Hand out the worksheet entitled Practicing Your Skills Lab on Solutes (S-C-7-2_Solutes Lab and KEY.doc). Just before the class meets, you will need to prepare 30 test tubes: six labeled Unknown 1, six labeled Unknown 2, six labeled Unknown 3, six labeled Unknown 4, and six labeled Unknown 5:

• Place 10 mL of carbonated water in six test tubes labeled Unknown 1, cork them, and refrigerate.
• Place 10 g of iron(II) chloride (FeCl₂) in each of the six test tubes labeled

Unknown 2.

• Place 10 g of Epsom salt (MgSO₄ · 7H₂O) in each of the six test tubes labeled Unknown 3.
• Place 10 g of sodium chloride (NaCl) in each of the six test tubes labeled Unknown 4.
• Place 10 g of sucrose (C₁₂H₂₂O₁₁) in each of the 6 test tubes labeled Unknown 5.

Give a set of five test tubes to each group and have students conduct the conductivity tests found in the Practicing Your Skills Lab on Solutes (S-C-7-2_Solutes Lab and KEY.doc). After groups have completed the lab, reveal the identity of the unknown solutes and have students evaluate their results.

Extension:

• Students who might need an opportunity for additional learning can log on to a computer and go to the Web site, “Strong Electrolytes, Weak Electrolytes, and Nonelectrolytes” at http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::100%::100%::/sites/dl/free/0072512644/117354/07_Strong_Weak_Nonelectrolytes.swf::Strong%20Electrolytes,%20Weak%20Electrolytes,%20and%20Nonelectrolytes

You may wish to have this site bookmarked for the student.

• Students may do a series of simulations with acids, bases, salts, and unknowns and observe their conductivity using a light bulb, described in “Conductivity of Solutions” found at http://www.chem.iastate.edu/group/Greenbowe/sections/projectfolder/flashfiles/electroChem/conductivity-4.swf
• Students who may be going beyond the standards can enjoy the outdoors or traveling. They may go on a field trip to do “earthcaching.” In earthcaching, participants use a GPS (global positioning system) device to visit sites that offer significant insights into earth processes. Students can follow the directions for an earthcache by visiting the Web site “Frankfort Mineral Springs Earthcache” at http://www.geocaching.com/seek/cache_details.aspx?guid=c81dbfd7-dcb1-407f-8b0e-3a62f4c829b5. Students may also seek permission from you to borrow a conductivity apparatus to test a small sample of water while at the springs to observe how well the water conducts electricity. After their visit, students can write about their experience and present their scientific findings to the class.
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