Science Buddies: "Ask an Expert"

I am performing the experiment "Effects in Carbon Dioxide Output:changes in CArbon Dioxide Output" and I seem to be having trouble. After blowing into the respirometer for ten minutes, the solution would not change from a bluish green to a yellow. I stopped and tried to return it to its original color by using the aeration pump. The directions said this process would only take ten minutes but after 40 minutes there was no change at all. When i tried to use distilled water, the solution turned yellow right away and would not turn the greenish blue as it was supposed to. What can I do? is this normal?

Hi,

Reasons that the indicator change takes so long include slow CO2 input or large volume. In terms of CO2, try exercising (so that the body generates lots of CO2) before blowing into the water. Cut down on the volume of water also to speed up the rate of color change (remember, color change is related to concentration, so the more volume you have, the more dilute the H+ ions are, and the longer the change takes).

Thank you for the help but i just tried to do the experiment again and decrease the amount of water but the solution turned yellow immediately, rather than a bluish green like it was supposed to. why is that? what should i do?

Hi Karifula,

I'm sorry you've had some trouble with your experiments. Here are some factors to consider when working with an indicator solution.

1) Concentration of the indicator
It's important to pay attention to both the volume of water and the volume (number of drops) of indicator solution you add. Too much indicator can change too quickly, and too little can change too slowly. As Aaron pointed out, using a high volume of water would have the same effect as adding too little indicator.

2) Type of water
As the experimental instructions say, it's good to pay attention to whether your tap water may already be acidic. If it is, then the indicator will turn yellow immediately to show you that the water is already acidic. This can happen because of the salts present in tap water in some areas. Did you try using distilled water the second time? Make sure your container is very clean as well, since contaminants on the container can get into the water.

Do you have any more details that might help us to assist you further?

Heather