Science Buddies: "Ask an Expert"

Posted: **Tue Jan 22, 2008 6:20 pm**

I'm actually feeling stupid right now,
http://www.sciencebuddies.com/mentoring ... l?from=TSW
^^^^^^^^^^^^^^^^^^^^^^ my experiment that i will be doing.

i got all of the materials i need for my experiment. I just don't know what the procedure means by this...

You will be collecting CO2 from the yeast by displacing water trapped in an inverted graduated cylinder. Here's how to set it up:

1. Fill your plastic tub (or bucket) about one-third full with water.
2. Fill the graduated cylinder with water.
1. If your tub is big enough, fill the graduated cylinder by tipping it on its side inside the tub. Allow any bubbles to escape by tilting the cylinder up slightly, while keeping it under water. Keeping the opening of the cylinder under water, turn it upside down and attach it to the side of the tub with packing tape.
2. If your tub is not big enough, fill the graduated cylinder completely and cover the top tightly with plastic wrap. Quickly invert the cylinder and place the opening in the tub, beneath the surface of the water. Remove the plastic wrap. Attach the cylinder to the side of the tub with packing tape.
3. The graduated cylinder should now be upside down, full of water and with its opening under the surface of the water in the tub. It is ready to trap CO2 produced by your yeast.

Can anyone be able to explain it to me? in a simple way? THANKS

Posted: **Wed Jan 23, 2008 6:35 pm**

Hi,

This step is for constructing a "gas collecting cylinder" -- the main idea is that the yeast will be generating CO2 gas, which you will direct into the cylinder using a tube. These parts will come together in step 9. The tube will go from a closed bottle (containing the growing yeast) down into the water bucket (or other large container) and up into the graduated cylinder. When you start this means that the end of the tube will be in the water, but inside the cylinder. As the yeast generates CO2, it will bubble out of the tube and rise to the top of the cylinder. When you start, the cylinder will be filled with the water. As the CO2 collects inside the cylinder, it will cause the water in the cylinder to go down. You will measure this lowering (called displacement) and thus be able to measure the amount of CO2 generated.

When you start, the open end of the cylinder will be below the surface of the water and the bottom of the cylinder will be sticking out of the water. The cylinder will be taped to the side of the bucket.

Does this help? Please feel free to ask for more clarification! This looks like a fun experiment.

Bill

Posted: **Thu Jan 24, 2008 9:45 am**

Hi soccerreena55,

Hopefully Bill was able to explain the gas collecting cylinder to you. Other students have also performed this project, so you might want to try using the search feature of this site and see other comments that have been posted.

Here's an edited response that I made to another student on this subject:

Basically, the principle you are using in order to determine the amount of carbon dioxide generated is a form of water displacement. The fact that you are using a graduated cylinder will allow you to actually measure the volume of the carbon dioxide (a gas) that is produced during each experiment. This is actually a very clever way of measuring the volume of a gas!

Now, as you already know, carbon dioxide is a gas. Simply put, the gas is less dense than water, which causes it to rise through the water as visible bubbles. These bubbles, once they reach the closed top of the cylinder, will collect and exert a pressure on the water already present in the container - and consequently force the water down and out of the cylinder. The more gas that is collected, the more water that is displaced - and you can read the volume simply by reading the side of the graduated cylinder.

(Incidentally, carbon dioxide is also partially soluble in water, but hopefully for the purposes of this experiment, the amount of gas that will dissolve will be unimportant when you compare the "+air" and "-air test" results.)

If you need a particular reference for this information, you may want to try searching for some of the terms I used, using your favorite search engine. Also try some alternate words and see what you can find.

Please keep us informed of your progress.

Hope this helps, and please be sure to post back if there is anything that you do not understand or if you have more questions.

Good luck!
Geoff.

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