Question about variables in the Bath Bomb project

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lbeerkir
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Question about variables in the Bath Bomb project

Post by lbeerkir »

So, My daughter wanted to do the Bath Bomb project for her submission that will be part of her evaluation into a charter school. I looked at the project and there are two "recipes" where is seems the independent variable is the amount of cornstarch in each recipe. The problem is some of the other ingredients (the acid and base) are also specified for different amounts in each of the two recipes (although they are proportionally the same). I have a slightly advanced background in science and am used to in kid's experiments usually all constant variables have the same value. What gives? She's a 5th grader and the proportionally equal amount of some ingredients between recipes seems like it will be hard to explain to her....or rather to have her explain it in her submission!

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AmyCowen
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Re: Question about variables in the Bath Bomb project

Post by AmyCowen »

Hi Ibeerkir - Great questions.

The reply below is from our science team.

It is a slightly more advanced concept, but we're confident your daughter can both understand and explain it! As you've noted, it is more common for kids' experiments to have constant variables, but for many recipes (like this one) that ends up in clear failure. So instead we've introduced the concept of keeping the proportion the same.

Since 5th graders have learned about fractions, it is probably best to sit down with your student and help her think through the recipe as a series of fractions. As the introduction explains, the reactants that cause the fizzing are the citric acid and the baking soda. They are kept at a constant 1:2 ratio, or for whatever amount of baking soda there is 1/2 (half) as much citric acid. This is the same in both recipes.

As the introduction explains, the cornstarch is the filler. So what changes in the two recipes is the ratio of reactants to filler. If you add up the total volume of reactants (citric acid+baking soda) and filler, they are the same in both recipes. If we add in the Epsom salt, too, (which is held constant) you can see that the fraction of cornstarch to other dry goods is what is changing in the two recipes -- 4.5/15 tablespoons in the first recipe and 7.5/15 tablespoons in the second recipe.

That brings us to the wet ingredients. They are constant except for water. This is because cornstarch is hygroscopic. It absorbs water better than the other ingredients. To get the same moldable bath bomb, you need to increase water as you increase cornstarch. So, rather than keeping the water volume constant, we've kept the consistency of the final product constant. This is explained in step 6d. Your daughter can think of it as "add water until both recipes have the same consistency."

I hope this helps!

Amy
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lbeerkir
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Re: Question about variables in the Bath Bomb project

Post by lbeerkir »

In my many years of posting questions to internet forums, I have rarely received such a complete, clear, and easily understandable answer than yours, Amy! Thanks very much for taking the time to reply!

Larry
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Re: Question about variables in the Bath Bomb project

Post by AmyCowen »

Thanks for the reply, Larry! Full credit goes to our dedicated science team, but I am happy to hear that the information helped clarify the questions you had. I hope your daughter has a fantastic science project/fair experience. We would love to hear how it goes.

Amy
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