Q19 - Oxygen-18 is a heavier-than-normal

[armeniandude424]

I was hoping someone could explain to me what is being said in the stimulus and explain to me the answers as well.

I thought it would be best to put these two questions together because they are both based on the same stimulus.

I'd be grateful for any help I recieve

[giladedelman]

Thanks for posting! This is a difficult pair of questions.

So, let's look at what we're told in the stimulus. Oxygen-18 is a heavier isotope of oxygen (FYI, that means it has more neutrons than normal -- yay, chemistry!), and oxygen-18 molecules are rarer in rainclouds than normal oxygen. Also, a higher percentage of oxygen-18 than normal oxygen is lost when it rains.

So, why are scientists surprised that the level of oxygen-18 remains constant as the cloud moves from its Atlantic birthplace to the Amazon, where it rains constantly? Well, since more oxygen-18 descends to earth when it rains, we'd expect the amount of it to decline.

Question 19 asks us to resolve this apparent paradox. We can be pretty sure that the correct answer will have something to do with oxygen-18 being replenished in rainclouds over the Amazon -- otherwise, how would its level not decline?

Answer (B) gets us closest to this explanation. If the tropical rain forests can replenish the clouds above them, then maybe they can replenish the oxygen-18 content enough to keep it at a constant level.

(A) is out of scope. We're not interested in rain forests vs. unforested regions, we're interested in what happens when clouds move from the ocean to the rain forest.

(C) is out of scope. What does this have to do with oxygen-18?

(D) is waaay out of scope! River runoff? Huh?

(E) is ... yeah, it's out of scope. The stimulus is not at all about the effects of tropical rain forests on the atmosphere above them.

Whew! We still have another question to look at!

So, question 20 is a pretty standard inference question. It turns out that we can infer answer (A), since it's basically a restatement of the stimulus's claim that oxygen-18 is rarer in rainclouds than ordinary oxygen. Oxygen-18 is rarer = there's more ordinary oxygen.

(B) contradicts what we're told: the oxygen-18 level stays constant as a cloud passes over the Amazon.

(C) is tempting, but be careful. We're told that a higher proportion of oxygen-18 descends to earth. That doesn't mean that more oxygen-18 overall descends to earth.

(D) directly contradicts the stimulus, which tells us that a higher proportion of oxygen-18 is lost.

(E) is incorrect because of detail creep. A higher proportion of oxygen-18 is lost, but that doesn't mean that more than half of it is lost. Maybe it's 30% of regular oxygen and 35% of oxygen-18.

All right, does that clear these problems up for you? Let me know if you still have questions.

[iridium77]

We're told that a higher proportion of oxygen-18 descends to earth. That doesn't mean that more oxygen-18 overall descends to earth.

How does this work ?

Also...
what threw me off for question 19.b.) is that it says "Like the oceans, tropical rain forests can create or replenish rain clouds..."

How could this account for the greater amount of rainfall over the Amazon if the ocean also has the capacity to replenish the clouds?

[monygg85]

Can someone please elaborate on C? I'm just not understanding why its wrong...I see it has something to do with proportions.

[sumukh09]

Can someone please elaborate on C? I'm just not understanding why its wrong...I see it has something to do with proportions.

C is saying that the amount of rain inside the clouds formed over the Atlantic Ocean is the same as the amount of rainfall over the Amazon. But who cares if the amount of rainfall over the Amazon is the same as the amount of rain formed in the clouds over the Atlantic? This has nothing to do with the paradox that Oxygen 18 molecules remained fairly constant inside the clouds over the Amazon when we would expect there to be less oxygen 18 molecules since a larger proportion of them descend to earth during rainfall.

[agersh144]

I have read all of the above posts and am still utterly perplexed. Could someone maybe give an example with numbers. Maybe its because I just took the test and my head hurts but this question is just mind-numbing to me, can someone please simplify?

[gplaya123]

Killer questions but I went through it so... here we go.
Ok, here is the picture you want to imagine.
There are two clouds:
1) one above Atlantic ocean
2) one above Amazon Forest
The scientist are comparing these two clouds essentially.
The stimulus told us that when RAINS, more oxygen 18 is released.
Don't worry about the number of oxygen 18 and whatnot.
It's all about the matter of how much the cloud retains or releases oxygen 18 during and after raining.
But to make things easier, let's make some example involving number.

WHEN NOT RAINING:
there are 100 molecules, 10 is oxygen 18 while 90 is normal oxygen (this shows that oxy 18 is RARER)
However,
WHEN RAINING:
9 out of 10 oxygen 18 is released whereas only 45 out of 90 of normal oxygen is released (this shows that HIGHER Proportion of oxy 18 is released; it's 90% vs 50%).

This is where it gets crazy.
There was constant raining in Amazon forest. What does this mean? It means, the cloud should only have 1 oxy 18.
YET!
the number of oxy 18 of a cloud that is formed above the Atlantic ocean is relatively same as that of Amazon forest!
How?
The latter should have only 1 whereas the former should have like... 10, if not rained at all!
How can they be ever same?
That's what B is saying: since the cloud above Amazon forest replenishes its oxy 18, it's possible!

hope this helps.

[tommywallach]

Hey Gplaya,

I love the enthusiasm here! However, there are a few things about this explanation that give me pause. First off, we haven't discussed the wrong answers (it would be easy to think that (C) and (D) say something similar to (B)). Also, I think that bringing actual numbers in might confuses the issue here.

So the point of the passage isn't really comparing two types of clouds. There's one cloud, and it moves across the rainforest. As it does so, it rains all the time. And even though clouds lose more Oxygen-18 when it rains, the level of O-18 in the clouds never seemed to change.

(A) This isn't about one thing being poorer than another, it's about why the level didn't change with all that raining.

(B) CORRECT. If the rain forest can somehow get O-18 back up into the cloud, the O-18 level might not change.

(C) This would mean the clouds would eventually "run out" of rain altogether. It wouldn't explain why we aren't losing O-18 at a greater rate than normal oxygen.

(D) This tells us why the amount of rain in the clouds might stay the same, but not the amount of O-18 in the clouds.

(E) Effects are totally out of scope.

Hope that helps!

-t

[agersh144]

Thanks Tommy, great explanation!