Science Buddies: "Ask an Expert"

We did the bath bomb science project exactly as directed. The teacher has given the project a 65% because the two recipes have different amounts of acid/base along with the extra cornstarch. She says this makes the data and conclusions inaccurate. I understand the recipe changes were done because of proportion sizes, but is there a way to explain this so the whole project is not deemed "bad science?" Thank you for any feedback.

Hi monaleon42,

Looking at this procedure ( https://www.sciencebuddies.org/science- ... #procedure ) there may be a way to clarify this to the instructor and I'll try and break it down here:

1) the acid-baking soda ratio (the ratio of the reactants in this chemical reaction) is constant in both recipes

2) in these two recipes, the volume of reactants is reduced and the volume of the inert material, the cornstarch, is increased. the independent variable in this experiment is the ratio of the reactants to the inert binder. let's call it R/C = (acid+baking soda)/cornstarch.

3) to be precise, we don't really care about the chemical reaction-- we care about dissolution of the bath bomb. the rate of this reaction is transport limited and a function of this ratio of reactant to inert binder

4) take this independent variable to its logical extreme: (a) no cornstarch: we've got the volcano experiment. it'll react super fast and foam all over the place (b) pure cornstarch: it'll take a very,very long time to dissolve and might just be a glob of slime

I think all 4 are important points, but #2 (identification of the independent variable you're testing in your two experiments) is a strong and efficient approach. #4 may seem absurd but is helpful in visualizing the 2 'recipies' as data points along a continuous curve, where the x-axis is the reactant-cornstarch ratio.

Hope some of this helps,

-John

THANK YOU for making this understandable. I really appreciate your answer!

You're welcome!

-John

Thank you again for your explanation. We thought it was a great project, a lot of fun, and learned a lot.

I did share your answer with the instructor. Unfortunately, the instructor has sent me a very detailed explanation of why this project is still unacceptable. It is hard to believe this is all over a 6th grade project! It seems we will have to take the failing grade, but we will definitely be doing some more ScienceBuddies projects on our own over the summer. Posted below is the instructor's response, just for your reference.

Here is our analysis. We both disagree with Sciencebuddies. ( I have also discussed this with the Principal.) Science Fair is challenging. So below is our reasoning with the hope it creates further understanding; it is not meant to put down anyone (you or sciencebuddies).

The (sciencebuddies)statement:
1.the acid-baking soda ratio (the ratio of the reactants in this chemical reaction) is constant in both recipes
The student was not testing ratios, she was testing if changing the amount of cornstarch affected the reaction rate. This is what is stated in her hypothesis. This is what the science coordinator and I approved. She did not ask about testing ratios. This would not have been approved since it would be plagiarism to copy exactly the same project. For this class, we want original projects. The idea is to challenge students.

Sciencebuddies statement" 2) in these two recipes, the volume of reactants is reduced and the volume of the inert material, the cornstarch, is increased. the independent variable in this experiment is the ratio of the reactants to the inert binder. let's call it R/C = (acid+baking soda)/cornstarch."

Again, he is changing more than one variable. The ratio of his reactants are : 4tbsp inactive to 3 tbsp inactive= 4:3
and the second recipe is 4tbsp(changing the reactants in amounts) to 5 tbsp inactive ( the increase in cornstarch) 4:5

The ratios are not the same. The part of the ratio that is the same is in the numerator only (the 4 representing the active ingredients but they themselves were changed) and this was done by switching the amounts of citric acid and baking soda. Now another variable is added.

As for sciencebuddies statement:" to be precise, we don't really care about the chemical reaction-- we care about dissolution of the bath bomb" If this were the case, then one of the reagents that cause the chemical reaction should be tested: either the citric acid or the baking soda-not both. He says we don't care about the chemical reactions; we do care about the chemical reaction. The student wanted to see if it would be affected.

As for their statement:
"take this independent variable to its logical extreme: (a) no cornstarch: we've got the volcano experiment. it'll react super fast and foam all over the place (b) pure cornstarch: it'll take a very,very long time to dissolve and might just be a glob of slime" but by adding the cornstarch, which is what the student wanted to do and asked to test, she would then see this outcome. This is what she wanted to test. Just the cornstarch. Would it just sat there as he claims? would it have dissolved over time? These questions would have been answered if the other two variables were not changed.

Hi monaleon42,

I don't understand all of the details but am very sorry to hear about your experience.

On the other hand, I'm happy to hear that it has not dampened your child's (or your) enthusiasm.

-John