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Introduction When soap is added to “hard water”, curds form, causing a greasy ring around sinks and bathtubs. It is “hard” (difficult) to get a lather to form because the soap molecules combine with dissolved ions (calcium, magnesium, iron) in the water. These ions are dissolved into the water as it comes into contact with rock materials. For example, water that drains a limestone canyon would probably contain an abundance of calcium ions(limestone is primarily made up of CaCO3). To get a lasting lather, these curd-forming ions must be removed by adding more soap molecules to combine with them. Eventually a lather will form. The more soap that is needed to cause a lather, the “harder” the water is. Purpose: To compare the hardness of several samples of water, including tapwater, well water, bottled water, distilled water, etc.
Pre-Lab Questions 2. Which three ions are usually to blame if water is “hard”? 3. How do these get into surface water and groundwater? 4. How is distilled water different from other water? 5. Most people prefer to have some minerals dissolved in their water. . . especially drinking water. But why is it undesirable to have water that is too hard? Materials: 5 test tubes (in a cup), a small bottle of soap solution, and an eye dropper
Procedures 2. Add 3 drops of soap solution to the test tube, cover the end of the tube with a finger, and then shake vigorously for 5 seconds. Check to see if there is a “lasting lather” (still present after 5 seconds). If not, add another 3 drops of soap, and repeat the shaking. Keep doing this until a lasting lather appears. On the data table, record the number of drops of soap needed to cause the lather. 3. Repeat procedures 1 & 2 for nine more water samples. If you use a test tube more than once, be sure to rinse it between trials. 4. Make a bar graph to show your results.
Follow-Up Questions 2. According to your results, which water sample was the “hardest”? 3. Which sample was the “softest”? 4. Why would the distilled water be considered the “control” in this experiment? 5. Which is typically “harder”, groundwater, or surface water? Explain. 6. Would you expect water in a limestone cavern to be hard, or soft? Explain. 7. If you spilled some really hard water on a dark countertop, and then let it evaporate, what would the spill area look like after the evaporation? |