Monday, January 9, 2012

Siphoning A Gas

Siphoning carbon dioxide gas.  It's hard to know how long to wait!
Pouring the carbon dioxide.
Testing for the carbon dioxide gas.

No, not gas from someone’s gas tank, although my students now know how to do it.  We are heading into the gas laws unit with my classes.  As a “warm up”, I teach the kids about the states of matter, including some physical characteristics of all three.  One thing that is hard to show to students is that gases can flow, like a liquid.  Using my “go to reaction” of baking soda and vinegar to produce carbon dioxide, I showed my students that you can pour a gas and actually siphon it. 
I actually used HCl instead of vinegar today.

I got this demo from Flinn as one of their monthly chemistry activity emails.  I love those guys!  This particular month was especially helpful because they sent along the video of the demo along with the instructions.  Watching the video got me excited about trying the demonstration.  (ChemFax! Publication No. 91636, Pouring and Siphoning a Gas)

Pouring gas directly onto the lit candle to see what happens.
To start off, I cut the tops off of two 2-liter soda bottles.  In one of the bottles, I poured some baking soda and then added vinegar to create carbon dioxide gas.   We never get tired of watching the fizzing from this reaction.  The problem here is that the carbon dioxide is clear and colorless.  How do we know it’s in there?  I created a “gas detection apparatus” out of a piece of wire coat hanger and tea candle.  By lowering the lit candle into the bottle, it is very obvious that the gas is in there because the candle is extinguished as it touches the carbon dioxide.  What I love about this simple test is that we can now quantify the amount of CO2 in the bottle.

Yes, there is carbon dioxide in there.
Once we established the presence of CO2, I poured the gas into the “empty” bottle (of course, it isn’t empty because it contains air).  Using the gas detection apparatus, we can easily see that the gas filled up the second bottle.  The hard part about the demo is waiting long enough for the gas to flow because there is no way to know when it’s done.  With the candle, we measured the level of the CO2 in the bottle.  Not all the gas was transferred, but we got over half of it.

For the finale, I set the bottle with the CO2 on a block, and put a rubber tube into it leading to another empty bottle at a lower level.  I sucked in some of the CO2 to get it started and then we waited.  Let me tell you that the gas was not tasteless; it had a terrible taste of acid that filled up my mouth.  Once again, it was hard to know how long to wait, but using the gas testing apparatus; we confirmed that the gas flowed through the tube into the lower bottle.  

The taste was terrible.  Don't try this at home!
Thanks to Kelley, my  student photographer for this demo.
Thanks to my student assistance during this part of the demo.

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