Salt: The Essence of Life
Multidisciplinary Curriculum for Students
Unit 4: Chemistry
Water Conditioning
1 Class Period
Overview
This module focuses on how different factors can influence the softness of water. Students will discover that water comes in both a hard and a soft form. Through experimentation, they will learn about the different characteristics of hard and soft water, and the benefits of soft water.
Background
Water is considered hard when it contains calcium and magnesium (hardness ions). Most water in the U.S. is hard, but the degree of hardness varies. Moderately hard (60-120 ppm) to hard (121-181 ppm) water has 60 to 181 parts per million [parts per million converted to grains per U.S. gallon is ppm # divided by 17.1 equals grains per gallon] of hardness minerals. The average hardness is 121 parts per million. Over eighty-five percent of the homes in the U.S. and Canada have water that is at least this hard. Hard water requires more soap and detergent for laundering, cleaning and bathing because suds do not form as well in hard water. The reaction between soap and hard water results in a greasy, curd-like, deposit which makes fabrics feel harsh and leaves water spots on dishes and utensils. Mineral scale builds up in hot water appliances and industrial boilers, reducing energy efficiency and shortening appliance and equipment life.
Water is softened by removing the calcium and magnesium ions from hard water and replacing them with sodium ions. Water softeners use cation exchange resin to exchange sodium ions for calcium and magnesium ions. As supply water flows through the resin bed, the exchange takes place and the water becomes soft. Water softener cation exchange resins are regenerated with a 10% salt brine solution made by dissolving water softener salt.
Getting Ready
Time: 1 class period for Activity A, and 1 class period for Activity B
Materials
Activity A and Activity B:
Per Student Group: 6 Volt battery, electrodes, 10% salt solution, beaker
Instructions
Activity A:
Gather equipment and provide background information
Activity B:
Provide background information and materials
Activities
Activity A - Ionic Properties
1. Review with students the ionic bonds involved in the formation of NaCl. Determine the ionic charge of Na and Cl in solution. Explain what the symbols + and - signify.
2. Discuss with students why NaCl is a stable compound and, when dissolved in water, NaCl dissociates into Na+ and Cl_ ions. To demonstrate this process an electrolysis experiment can be designed. Before beginning the experiment explain electrolysis to students. Develop a definition.
3. Connect the electrodes to the + and - terminals of a 6 volt battery. Place the other ends of the electrodes in an aqueous solution of about 10% salt. Students should observe the reactions and develop a hypothesis that explains the reaction.
4. Write the following ionic equations for the electrolysis of NaCl solution on the blackboard:
2Cl- à Cl2 + 2e-
2H2O + 2e- à H2 + 2OH-
2Cl- + 2H2O à Cl2 + H2 + 2OH-
Ask students to explain in their own words how this equation explains the electrolysis reaction. How does NaCl play a role?
5. Summarize the discussion by reviewing the electrolysis of an aqueous solution of sodium chloride. Explain to students that electrolysis is commonly used in industry to produce metallic sodium and chlorine from molten sodium chloride.
Activity B - Ion Exchange for Softer Water
1. Begin the activity by discussing the difference between hard and soft water. You should also address the problems created by hard water in homes, business and industry.
2. Explain to the students that the ionic properties of a salt solution enable hard water to be treated, or softened. Ask students how the ionic properties of salt can soften water containing hardness as calcium and magnesium.
3. Explain that water softening systems allow water to pass through a bed of special resin which exchange sodium ions for calcium and magnesium ions. The resins are periodically recharged by a brine solution derived from salt pellets. Have students develop a chemical diagram and drawing of the process.
4. To enhance student understanding of this process, a graphic model represented by different color elements will help. Cut out elements for calcium/magnesium, water, sodium, and chloride from different colored construction paper. Label the different color elements with the corresponding chemical symbols. Begin by creating a salt compound and a water-calcium/magnesium compound. Based on the dissociation of NaCl in water, show how calcium and magnesium ions are replaced by sodium ions. Ask students why the salt needs to be periodically replaced in the water softening unit.
Assessment
Students can develop a laboratory report explaining the electrolysis process they observed. In a supervised laboratory lesson, students can also conduct electrolysis experiments on different ionic solutions.
Conclusion
To demonstrate their knowledge of the ion exchange process in softening water, students should create a diagram of a water softening system, illustrating the exchange of ions. You may also want to discuss the properties of hard and soft water, and the factors that influence their development. For further information read Softened Water Saves for Homes, (Salt Institute, Alexandria, Virginia), 1982.