The Independent Center for Integrative Education: Learning without Limits

1/24/11

The topic of our fifth class was also how to clean water from salts. Please read the summary below and the homework assignment at the bottom.

Our first question was not how to clean water but how to measure how much salt is dissolved in it. Tasting is one way, of course, but we were looking for something more scientific. We tried to use electricity.

Pure water is not a good conductor of electricity. We found that out trying to measure the electroresistance of distilled water that I bought in the pharmacy. We poured the distilled water into a glass. We weighed the water and found that there was 214 grams of it. We made two electrodes out of aluminum foil and connected them to the universal tester switched to resistance measurement mode. We found that the resistance of the water between the electrodes was about 140 kΩ (140 thousand Ohms; Ohm is a unit of electroresistance named after a great physicist who discovered the laws of electric current).

After we dissolved 2 grams of table salt in this water, the resistance dropped to 4 kΩ, and when we added three more grams of salt, the resistance dropped further to 0.7 kΩ. Using these data, we plotted resistance versus concentration. We were going to measure resistance and calculate the concentration using this plot. Of course, we would need much more measurements to make our calculations more accurate, but we were interested in the principle rather than accuracy. In addition, we were limited by time.

Next, we tried to distill the water to see how its resistance (and therefore concentration of salt) would change. We used a kettle, a Styrofoam cup, and a piece of rubber bellows (tube with ripples). We made a hole in the Styrofoam cup and attached the tube to it. Then we poured our measured water into the kettle and boiled it. The Styrofoam cup was attached (with my hand) to the spout and Adina was holding the other end of the tube inside a plastic cup covered by a Styrofoam cover and submerged into cold water for cooling. Unfortunately, the Styrofoam did not withstand the temperature and melted. We replaced it with a paper cone and eventually collected about a gram of water. It was too little to measure conductivity with our coarse technology, so we just tasted it. In comparison with the initial solution, we found the distillate quite fresh.

HOMEWORK ASSIGNMENT:

Next time, we are going to try to clean water from ammonia. Please read the NASA Educator Guide about ammonia.

Dear parents, I would highly encourage you to help you children with this assignment.