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Q & A: home-made still

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Most recent answer: 03/12/2015
Q:
My daughter, Abby, and I are doing an experiment. We are creating a distillation process using a stainless steel pot, glass bowl floating inside, and upside down glass lid with ice on top. We have 8cups of distilled water with 2ozs of kosher salt dissolved in the water. This saltwater solution is heated in the stainless steel pot and the glass bowl should collect evaporated water from the inverted glass lid with ice on top. The hypothesis is that over time, 15min at a data mark, the water in the ice bowl has no salinity and the saltwater in the pot increases in salinity as h2o evaporates. I got a tds measurer. I've tested it in distilled water. It works. The experiment is not working though. The salinity in the heated saltwater goes down over time, the glass bowl has tds readings of 45 rather than 4 of the distilled water we started with. Any ideas whats going on?
- Abby and Heather Snyder (age 7)
Ashburn, VA
A:

This is a puzzle.

The tds (total dissolved solids) readings aren't too strange. The tds meter actually just measures the electrical conductivity. If good distilled water gives a reading of 4 (in some units, maybe µmhos/cm) a reading of 45 might just come from picking up some extra CO2 from the atmosphere, since it partly ionizes in solution.  Or maybe there was a little detergent or something on the lid. Or the tds meter sensor could have picked up some salt and brought that contamination into your distilled water. I bet that the reading is still low compared with tap water, and much, much less than the salt water.

What's going on in your saltwater is stranger. In equilibrium, the conductivity of the liquid wouldn't change. But you saw it go down. So I'm clueless. Perhaps with more information we could guess. Do you see some bubbles in the liquid? What's the pattern of how the reading changes over time? If you quit heating that saltwater, what then happens?  

Mike W.


(published on 03/10/2015)

Follow-Up #1: experimental results on distillation

Q:
I was not going to let this simple experiment get the better of us. So after searching and reading lots of Q&A on this site (which thank you so much for doing. It is amazing and a true gift of knowledge to us all), I suspect the water was too close to a boil. Thus the NaCl molecules could kinda hop a ride for a moment on the H2O and get into the collection glass bowl. Also discovered that my particular TDS/EC meter did not appreciate anything other than room temps. Furthermore, salt water is denser and we skimmed the water off the top rather than stirring the water in the pot and then selecting. We did the whole thing again. This time we kept the stove on low. Waited an hour rather than 15 mis to get the same amount of condensed water. We made sure to stir the remaining salt water before sampling. And we let everything cool to room temp before testing the TDS level. It worked!!!! The salinity in the pot rose from a start of 61 ppm, to 64 ppm, to 69 ppm, and finally 71ppm. The three condensed water samples were 24, 7, and 11 respectively. So yes salt water will increase in salinity as h2o evaporates. The condensed evaporated water vapor is distilled. Oh and if you hold the small glass jar you collect your water in the tds levels will rise. I presume from a human's minor electrical current. At any rate the effect goes away if you keep the glass on the countertop and just insert the tds meter.
- Abigail and Heather Snyder (age 7)
Ashburn, VA, USA
A:

Thanks for this great experimental report. You've given an excellent illustration of how people have to fight through a number of details, paying close attention to various side issues, to get real experiments to work. Your daughter has learned the main lesson for first-year physics grad students.

Mike W.


(published on 03/12/2015)

Follow-up on this answer.