Slide 1 / 34 Slide 2 / 34 New Jersey Center for Teaching and Learning What Makes Hard Water Hard? Progressive Science Initiative Have you ever had difficulty lathering soap or find that the scum in your shower constantly needs to be removed? This material is made freely available at www.njctl.org and is intended for the non-commercial use of These are signs of “hard water.” students and teachers. These materials may not be used for any commercial purpose without the written Soap doesn’t lather well in hard water because metal ions, permission of the owners. NJCTL maintains its such as Ca 2+ , form precipitates, creating “soap scum.” website for the convenience of teachers who wish to make their work available to other teachers, Because this happens, soap is a less effective cleanser in hard participate in a virtual professional learning water. Hard water may contain various metal ions, including community, and/or provide access to course Ca 2+ , Mg 2+ and Fe 2+ . materials to parents, students and others. Click to go to website: www.njctl.org Slide 3 / 34 Slide 4 / 34 What Makes Hard Water Hard? 1 The addition of a water softener that replaces calcium Students type their answers here ions with sodium ions has clear advantages in terms of While the metal ions in water are generally harmless, extending the life of a water heater. What disadvantages hard water has other disadvantages, such as "boiler could result when a household's water supply has an scale". Boiler scale is a scaly buildup of calcium increase in sodium ions? carbonate. This scale can build up on the inside of water pipes and can reduce the operating efficiencies of water heaters. Scale can also result in the failure of boiler tubes as they become clogged. The best way to control the formation of boiler scale is through water pretreatment, such as installing a water softener. Water softeners typically replace the calcium ions with soluble sodium ions. Slide 5 / 34 Slide 6 / 34 What Makes Hard Water Hard? What Makes Hard Water Hard? In order to determine the hardness of water, each ion present in the water must be isolated separately. These ions or analytes can be The goals of this lab are to: isolated by precipitation reactions and analyzed through a process called gravimetric analysis. In completing the analysis the analyte · Analyze water samples for their quantity of is precipitated, purified, dried and massed. From the mass of the hardness through the principles of metal ion known composition of the precipitate, the amount of the analyte in precipitation and separation. the original solution can be calculated using stoichiometry. · Rank samples in order of increasing water hardness.
Slide 7 / 34 Slide 8 / 34 2 A student wants to complete a gravimetric analysis of a water 3 Which salt below could be added to her water sample to sample to determine its quantity of hardness. Specifically, she precipitate out the calcium ion? wants to determine the mass percent of calcium in her sample. The calcium ion in this experiment is called the A Na 2 CO 3 A substrate B NaC 2 H 3 O 2 B solvent C NaNO 3 C analyte D NaCl D specimen Slide 9 / 34 Slide 10 / 34 What Makes Hard Water Hard? 4 The student selects sodium carbonate to precipitate out Students type their answers here her calcium ion. Write net ionic equation for the reaction of aqueous solutions of calcium chloride and sodium Once a determination has been made on carbonate. which substance can be added to precipitate the analyte in question, the next issue to address is which laboratory techniques can be employed to ensure the analyte is free of impurities and sufficiently dried. Slide 11 / 34 Slide 12 / 34 What Makes Hard Water Hard? 5 The student is prepared to isolate the calcium ion from her 20 ml water sample using sodium carbonate. She When completing a gravimetric analysis, an important obtains a 50 ml sample of the 0.50 M sodium carbonate consideration is that the analyte is completely and adds 3 mls then begins to collect her precipitate. precipitated. This can be accomplished by ensuring Was this a good idea? that the analyte acts as the limiting reactant in the Yes precipitation reaction. No
Slide 13 / 34 Slide 14 / 34 What Makes Hard Water Hard? 6 The student obtains another water sample and begins adding sodium carbonate. How will she know when she Students type their answers here fully precipitated out all the calcium ion? Once the analyte is precipitated, it should be collected through a vacuum filtration system or through gravity filtration. All of the impurities should be removed from the precipitate through washing and the precipitate should be thoroughly dried. Slide 15 / 34 Slide 16 / 34 7 Once the student has collected her precipitate onto the 8 In the final step of her analysis, she thoroughly dries and Students type their answers here filter paper, she washes her precipitate with deionized masses her sample. The calcium carbonate precipitate water, what is she attempting to remove? weighs 1.251 g. What was the mass of calcium chloride in her original sample? A Ca 2+ ions B Na + and Cl - ions C Cl - ions only D Na + ions only Slide 17 / 34 Slide 18 / 34 Materials: What Makes Hard Water Hard? Sodium carbonate, Na 2 CO 3 , Calcium chloride,CaCl 2 , weighing paper, balance, scoopula, 2 – 50 ml beaker, 25 ml graduated cylinder, Pre Lab Activity deionized water, stirring bars, filter paper, vacuum filtration system (if available), funnel, permanent marker, watch glass, drying oven (if available), rubbing alcohol, goggles and aprons. Setup: Each group of 3 students is given access to or given the materials. · If available your teacher will provide you with a vacuum filtration · system and a drying oven to dry your precipitate. If these are not available then you can allow your sample to dry overnight.
Slide 19 / 34 Slide 20 / 34 Pre Lab Activity - Procedure Pre Lab Activity - Procedure 1. Weigh 2 g of sodium carbonate in a clean, dry 50 ml beaker. 11. Weigh the watch glass. 2. Weigh 2 g of calcium chloride in a clean, dry 50 ml beaker. 12. Carefully remove the filter paper with the precipitate from the filtering apparatus and set it 3. To each beaker, add about 20 ml of distilled water. Stir each with a glass stirring rod until on the watch glass. The filtrate, the solution remaining in the beaker, can be poured down the drain. each solid is dissolved. 4. Pour a small amount of the sodium carbonate solution into the beaker containing the 13. If a drying oven is available allow the filter paper to dry for 10-15 minutes. If not, allow to dry overnight and continue with step 16. calcium chloride solution. Stir and observe the mixture. Add some more of the sodium carbonate solution while stirring and observing. After all the sodium carbonate solution 14. Carefully remove the warm watch glass. Use a metal scoop to break the precipitate into has been added, continue to stir this mixture for another couple of minutes. small pieces. Set aside to cool. 5. Weigh out a piece of filter paper. 15. Return the watch glass to the drying oven for another 5 minutes. 6. Fold the filter paper following your teacher's directions. 16. Carefully remove the watch glass out of the oven and set aside to cool. 7. Set up th e filtering apparatus. 17. Weigh the watch glass, filter paper and precipitate again. 8. Insert the filter paper into the filtering apparatus. Wet the filter paper with distilled water. 18. Subtract the mass of the watch glass and filter from the mass of the watch glass, filter paper and precipitate. 9. Pour the contents of your beaker slowly into the filter paper inside the funnel. Be careful as you pour so that none of the mixture flows out of the filter paper or the funnel. Use a 19. The precipitate and filter paper may be discarded in the waste basket. Wipe the permanent wash bottle of distilled water to rinse the precipitate out of the beaker with small quantities marker off of the watch glass using some rubbing alcohol. of water. Use a little more water to wash the precipitate that is now collected in the filter paper. 10. Using a permanent marker, label a clean, dry watch glass with your initials. Slide 21 / 34 Slide 22 / 34 Safety Warning: Roles Always wear your goggles and aprons while completing this lab. · Lab teams should consist of 3 students. The students Also wear your goggles and aprons when you're near someone working with should take turns and rotate through the roles listed below. · chemicals. Familiarize yourself with the location of the eyewash and the emergency · shower in the room. If you get any chemicals in your eye, use the eyewash. 1. One person to follow the lab procedure and complete the lab set up and start the reaction. If any of the chemicals come in contact with your skin rinse them off · immediately with lots of water and let the instructor know what you spilled --- on your way to rinsing them off. 2. One person (Collector) records the observations. Never mix chemicals that you haven't been told to mix. · 3. One person leads the completion of the calculations. Tell the instructor of any accidents immediately. · Keep food and drinks out of the laboratory work area. · Slide 23 / 34 Slide 24 / 34 Data Collection What Makes Hard Water Hard? 1. Follow the procedure outlined on your lab sheet and collect mass data on sodium carbonate, Gravimetric Guided Inquiry Lab Activity calcium chloride, the filter paper, the watch glass, the filter paper and the watch glass and precipitate after the first heating and after the second heating. 2. Make observations concerning the formation of the precipitate.
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