Science Buddies: "Ask an Expert"

I am adapting "Yeast Busters: Stopping Fungus in its Tracks with Antifungal Medicines" to test the effects of different meds on yeast metabolism. I ran into a problem with the procedure. The tube that connects the bottle with the yeast to the graduated cylinder fills with water when it is placed in the inverted cylinder, which itself is full of water. The Co2 gas, a byproduct of yeast metabolism, which is meant to displace the water in the graduated cylinder, cannot evacuate the water in the tube, and therefore, certainly can't displace the water in the cylinder. I'm sure there is some trick to this procedure keeping the tube clear of water but I can't figure it out. I tried to hold the end of the tube with my finger keeping the Co2 gas in the tube before placing the tube in the cylinder under water but that didn't work. With the tube not clear of water, the CO2 gas cannot displace the water in the graduated cylinder, the volume of which is what I am trying to measure. Please HELP. My experimental results are due this week and the water in the cylinder is not properly being displaced by the gas. What am I doing incorrectly. Thanks, David L.

Hi DavidL,

Is there a leak in your tubing anywhere? Is the seal between the lid on your bottle and the tubing tight, so that gas can't leak out through that junction? I see two possibilities here (although there are probably more). First, you may have a leak in your system so that the gas produced by the yeast is escaping from the container instead of traveling through the tube and into the graduated cylinder. Or, another possibility is that the yeast aren't producing enough gas to displace the water from the tube. In an experiment like this, the volume of the tubing is one of the sources of error, since gas has to displace the water in the tube before it can displace water from the graduated cylinder. That's just the nature of this way of measuring gas production. Is the gas produced by the yeast moving the water in the tube at all? If it is, then you could measure the volume of gas produced by measuring how far the water in the tube is displaced, and converting that to a volume using the inner diameter of the tube and the formula for the volume of a cylinder.

Thanks Terik!

Hi. I am trying to do this project for my Science Fair Project. Unfortunately the second step is missing so I do not know how to properly make the yeast solution. I would really appreciate it if you could help me out. Thank you.

Hello taylorharbin18,

I'm assuming that you are working on the project described here:

https://www.sciencebuddies.org/science- ... p012.shtml

I found the following description of how to mix the yeast solution in the “Procedure” tab under the heading “Testing Different Antifungal Medicines”.

Dissolve ½ teaspoon (tsp.) of sugar in ¼ cup of warm water (110°F to 115°F, or whatever temperature the yeast package recommends). When the sugar is fully dissolved, add 1 tsp. of yeast (this is about the same amount as half a packet of yeast), mix well, and pour into the appropriate bottle. Tip: It may help to use a fork to mix in the yeast. Be sure to note the actual temperature of the water in your lab notebook.

I hope this helps. Please post again if you have more questions.

A. Norman