Science Buddies: "Ask an Expert"

My son (5th grade) is doing the crystal experiment to figure out what temp grows the largest and purest crystals using borax. The three temps used are room temp, refrigerator, and ice bath. Experiment went well overall, but we have some confusion about the control. We assume the variable will be the three different temperatures. Would the control be the room temperature one, or would it be the solution itself? Or something different? Thanks!

If I understand your project correctly, the control or constant is the bath and the time allotted at each temperature to grow the crystals. The independent variable is the temperature and the dependent variable is the crystal size. I hope that helps.

Thank you for the reply. I'm still a little confused, though. The only "bath" used is the ice bath. To explain the experiment a little more...we made a supersaturated solution of water and borax, then poured equal amounts in 3 jars. One jar was placed on a table, one jar was placed in the refrigerator, and one jar was placed in an ice bath. We then documented the crystal growth after specific time periods. The only things that are the same in all three conditions are the solution and the time allotted for the crystals to grow. So, I thought the "control" might be the solution?? But, if the "control" is that which you compare things against, I thought it might be the room temp jar?? If someone could explain this a little more, I would really appreciate it. We were planning on calling the three different temperatures the "variable." Thanks!

Hello, and welcome to Science Buddies! Not all experiments require a control group. A control group is necessary if you want to study the effects on something when its environment has changed. However, the purpose of other experiments is to find an optimal solution. An example of this is how to make the best cookies. You would experiment with different temperatures, baking times, amounts of sugar, etc. There is no control group in this type of experiment.

Your son's experiment falls under this latter category. He is trying to optimize the temperature to grow the largest and purest crystals. You should think in terms of independent, dependent, and controlled variables.

Independent variable(s): The variable(s) that will have different levels to determine the optimal setting for your outcome (temperature)
Dependent variable(s): The variable(s) or outcome that is affected by changing the independent variable (size / purity of crystal)
Controlled variable(s): This is different from a control group. Controlled variable(s) are those that you want to hold constant during your experiment. You do this to factors in the environment you don't wish to have an effect on the outcome (borax solution)

You are correct in calling the three different temperatures the variable. More specifically, temperature is your independent variable. Distinguishing between the three variables I have described above is important.

You can find more information here:
https://www.sciencebuddies.org/science- ... evariables

I hope this helps. Be sure to write back if you have further questions.
Good luck to your son!
Cheers,
Deana