Confused with 'control' in biology experiment?

[emchun]

I did an experiment for biology where we had two flasks, both had 100ml bromothymol blue (indicator), and elodea plant in each of them. We blew into the flasks so that the colour would turn yellow because we need carbon dioxide to be present in each of the flasks. We then left one to be exposed by lamps, and the other wrapped in foil and hidden in a cupboard.
This practical is about seeing if light and carbon dioxide is necessary for photosynthesis to occur.

What I don't understand is, which flask is the control and how do I know it is the control.

I hope somebody can help me! Thank you.

[Yacoubb]

I did an experiment for biology where we had two flasks, both had 100ml bromothymol blue (indicator), and elodea plant in each of them. We blew into the flasks so that the colour would turn yellow because we need carbon dioxide to be present in each of the flasks. We then left one to be exposed by lamps, and the other wrapped in foil and hidden in a cupboard.
This practical is about seeing if light and carbon dioxide is necessary for photosynthesis to occur.

What I don't understand is, which flask is the control and how do I know it is the control.

I hope somebody can help me! Thank you.

I'd say the control flask is the one that is not exposed to light, and wrapped in foil & hidden in the cupboard. By blowing into both flasks, you are controlling CO₂ concentration, which is a very important factor in photosynthesis. Therefore, by exposing the second flask to light, and observing photosynthetic rate there, you can say that both light & CO2 are involved in photosynthesis, and that the rate of photosynthesis in the flask wrapped in foil would probably cease due to the fact it is not exposed to light.

How do you know its the control - firstly, what is the control variable? A control variable is ultimately a point of comparison. A control is used so that you are forming this point to compare your DV to when exposed to the IV. So, the control is not exposed to the IV, and the experimental variable is when the DV is exposed to the IV. Furthermore, the control variable involves controlling all extraneous variables and basically disregarding any outlying factors that could have an effect on the DV, increase the reliability & credibility of your experimental findings.

Hope this helped

[ealam2]

Controlled variables and the control group are two different things. Controlled variables are variables that are kept the same all throughout the experiment in both the experimental and the control group.

The control group is the group not exposed to the independent variable. In your case, the flask wrapped in foil and hidden in a cupboard as the independent variable for this experiment is availability of light. This is used as a standard of comparison and for the experimenter to be sure that the results are due to the independent variable and nothing else.

[emchun]

Thank you for the help!   The answers are great!

[M-D]

talking about controls in biology pracs, what's the difference between positive and negative controls?

[psyxwar]

talking about controls in biology pracs, what's the difference between positive and negative controls?

Positive control = where a change in DV is expected to happen. I suppose an example would be testing the ability of a new indicator to measure the presence of glucose. Your positive control would be a different type of indicator that has been proven to work with glucose -- obviously, if this shows up negative, then your solution probably doesn't have glucose and therefore your experiment is invalid.

Negative control = where no change in DV is expected to happen -> if there is a change in the DV, then it's due to an extraneous variable. eg. a starch solution without amylase would be your negative control, while your starch solution with amylase would be your experimental group. If your DV is say, the amount of starch lost, then in your control no starch is expected to be lost,  because you have no amylase to break it down. If there is a loss of starch in your control, then obviously something is wrong as it is not your IV (presence of amylase) that is causing the change in the DV.