#8 - Fluoride enters a region’s groundwater when rain

[astroworld]

Hello,

I was deciding between A and D but ended up choosing A.

Because in answer A, I thought of the possibility that sodium could be the source of fluoride, and therefore fluoride-bearing minerals were actually not the only/primary source of fluoride. Would this reasoning be completely false? I think if I was aware that sodium is an element that could not bear any other elements, I wouldn't have assumed this. For someone who is completely ignorant in these types of subjects, is this something that would be hard to overcome?

I also eliminated D because I thought increasing the rate of dissolution seemed a bit too broad and therefore irrelevant. In the stimulus, we are told that "relevant variables are held constant", would this include length of time from which they started the experiment to which they measured the concentration level of fluoride? Because I would assume that if groundwater with high concentration of sodium was measured way later than the other, the former would just have more time to accumulate fluoride than having the fluoride bearing minerals to dissolve at a greater speed.

[Luke Haqq]

Hi astroworld!

Please feel free to clarify if I'm not understanding your reasoning, but I think what you're suggesting about (A) is something that it seems could be true.

That, however, should move it out of contention, since this is a must be true question stem. An answer choice that is merely possible or could be true based on the stimulus will not be the correct answer for this question type. Rather, the correct answer will be a matter of logical entailment from what is contained in the stimulus.

Here, we're told that fluoride "enters a region's groundwater when rain dissolves fluoride-bearing minerals in the soil." This could be diagrammed as:

Fluoride enters groundwater Rain dissolved FBM

We're then told that concentrations of fluoride in groundwater are higher where there is a higher concentration of sodium, with other variables being kept constant (to your question, this refers to any relevant variables generally). The sodium therefore must be contributing to this process of dissolving. This is reflected in answer choice (D): "Sodium in groundwater increases the rate at which fluoride-bearing minerals dissolve."

[npant120]

Hello,

I had a question about reasons why is E is wrong. I originally chose E, but when I was reviewing my answers, I switched my choice to D. I did this because E seemed to be to be saying that all soil with sodium-bearing minerals also has fluoride-bearing minerals. However, based on the stimulus we only know that the presence of sodium somehow changes the concentration of fluoride bearing minerals, whereas this choice seems to reverse the relationship and imply that anywhere that has sodium-bearing minerals would also have fluoride-bearing minerals.

Now reading the answer choice explanations on the forum, I understand that the stimulus talks about groundwater while the answer choice talks about soil. However, is my understanding of this also being a reversed relationship and that being another reason the answer is wrong correct? Or have I misunderstood something by assuming that?

Thank you

[Adam Tyson]

Answer E is just about a correlation, npant120, and doesn't indicate which of these two things causes the other. For that reason, the problem with the answer is not that it reverses a causal relationship. The cause and effect could go either way, or they could be simply correlated with no causal relationship. The real problem with answer E is that we know nothing about sodium in the soil, but only in the water.

[Dancingbambarina]

Hi,

Where do we know for sure the relationship is causal? SUrely that is just a POSSIBLE causality?

In a any event, I selected D because if the rate is higher, that would mean more fluoride in the water becasue more of the mineral is dissolving per day (or whatever period of time). Thus, the causality is POSSIBLE due to the answer choice, butb I am unsure how the thread here is so confident there is DEFINITELY a causal relationship.

[Adam Tyson]

It's not definite, Dancingbambarina, but it is the most reasonable inference that could be made based upon these facts, and none of the other answers choices are reasonable inferences drawn from these facts. Thus, D is the answer that is MOST supported. It might not be true - something else might explain this, though it's hard to see what it could be since just about everything else is being held constant - but it doesn't have to be true. It just has to be the answer that has the most support from the stimulus.

[Dancingbambarina]

Thanks very much Adam! Brilliant response.

Thanks