Q20 - Oxygen-18 is a heavier-than-normal

[weiyichen1986]

Hey there,

I have no idea about this question, can anyone explain this to me? Thank you in advance. I usually have problem approaching science-based question...

[giladedelman]

Thanks for posting!

This is a good example of a "silver bullet" type inference question: the right answer only depends on one line in the stimulus.

In this case, answer (A) is correct. The stimulus tells us, "In
a rain cloud, water molecules containing oxygen-18 are
rarer than water molecules containing normal oxygen." That means that a rain cloud has to contain more ordinary oxygen than oxygen-18. The "formed over the Amazon" part doesn't really matter here; we know rainclouds have more regular oxygen than oxygen-18 from that one line.

(B) is tempting, but actually all we know is that the cloud has more oxygen-18 than we would expect. We don't know that it has a greater than normal amount. We don't even know what a normal amount would be.

(C) is the opposite of what we're told about oxygen-18 in rain clouds.

(D) is also the opposite of what we're told: the cloud actually surrenders a greater percentage of its oxygen-18.

(E) is unsupported; we know the cloud loses a higher percentage of oxygen-18 than normal oxygen, but we don't know if it loses less than half or more than half of its oxygen-18.

Does that answer your question?

[rshapiro14]

I'm still confused about this, I knew that (A) was stated in the stimulus but I thought that (C) was too. The stimulus says that clouds have more normal oxygen then Oxygen-18 which is rare (answer A), but it also says that in RAINFALL more Oxygen-18 molecules drop from the cloud. Doesn't (C) say that there is more Oxygen-18 in the RAINFALL of the cloud? I couldn't decide between these two because I thought they were both stated....

Thanks for your help!

[geverett]

Not sure about this one for the same reasons the above poster cited. Would love to hear some more explanation on this.

[giladedelman]

Hi guys, thanks for your questions!

Looking back at my original post, my explanation of (C) was off the mark. It's not the opposite of what we're told. Here's what's happening:

The stimulus tells us that in rainfall, a higher proportion of the oxygen-18 molecules leaves the cloud compared to regular oxygen. But that doesn't mean more oxygen-18 leaves the cloud; it just means that a bigger percentage of the oxygen-18 than of the regular oxygen falls out.

For example, maybe 60% of oxygen-18 molecules leave the cloud, but only 30% of regular oxygen does. But this doesn't tell us which one is more present in rainfall, because we don't know exactly what the ratio of 18 to regular is, and we don't know exactly what proportion of each falls from the cloud.

Indeed, maybe ALL the oxygen-18 leaves the cloud -- but maybe oxygen-18 is only 10% of the water in the cloud! So even if only half the regular oxygen falls out of the cloud, the rainfall is going to be mostly regular oxygen.

Does that answer your questions, or are you still confused here?

[austindyoung]

Giladedelman, I thanked you for your reply because it is spot on.

However, I do think that the question still hinges upon some unstated assumptions that if cleared up will lend to understanding of what this problem is getting at.

There is less O18 than normal O inside of a rain cloud.

Yet rainfall contains a higher concentration of O18 than other O (I guess we could call it O16 per the very similar PT1-3-11).

So, since it says "all water molecules," I can deduce that not only in proportion but also in number, is O18 greater than any other type of oxygen in the rainfall. This would mean that more O18 than normal O had come from the cloud.

But, then (C) would be correct- and it is not. That is because in this Q, rainfall is not talking about to rain-water.

So, this thinking above leads us to sucker-choice (C) Which, IMO, is not so much a logical misstep at first, then a common sense assumption that rainfall=rain. Not so here.

It seems the test-writers mean rainfall to mean, as gilad explained, to be what has *fallen* out from the cloud. NOT what the rain (something different) is composed of. This leads to a completely different interpretation and is actually tipped off by the same phrase "of all water molecules."

The key is that this phrase is repeated twice, making two groups.

In this case, the relative proportions of O18 to normal O would not be known.

What would be known, is how gilad put it. If there are 100 normal O and 10 18O, then even though 20 normal O leave and 5 18O leave, a greater proportion of 18O, relative to itself, will leave than normal O.

We know that the stimulus is speaking of relative proportions because the two group are separated. If we were speaking of an overall proportion- then obviously 5 18O out of 110 molecules (per my numbering above) would be a lower percentage than the 20 normal O, relative to ALL oxygen.

I hope this makes sense. # and % problems can really trip me up.

This problem is super sneaky about discerning whether you are comparing two- subgroups to the group from which they are a part of, or the sub-groups, relative unto themselves as they appear in a larger group they constitute. The second-kind is going on here.

The use of "all" makes it tricky- but it's saying ALL of subgroup 1 and ALL of subgroup 2

Any differences or if I perhaps messed this up- let me know

[Mab6q]

I hate this question. The stimulus seems to begin by giving us a general rule about the concentration of Oxygen 18 and normal oxygen in rain clouds and in rainfall, but then goes talking about rain clouds over the Atlantic, where it appears to show that what we would expect to see, the Oxygen 18 content decrease, actually does not happen.

So it is was very hard for me to focus in on A because it doesn't seem to reconcile the part in the stimulus about the Oxygen 18 levels being constant. How is it that they were constant? The only way is if more of the normal was released while over the Atlantic, which seems contradictory to the first part. That's why I saw C as the only answer choice that made sense.

Could we get some more discussion here from the experts. Maybe a better breakdown of the stimulus.

[tommywallach]

I wouldn't overthink it. Gilad's explanation really is the whole reason (A) is right!

-t

[asafezrati]

I answered this one correctly, but as I re-examined it something came up:

The stimulus speaks of the proportion of different water molecules, and not of the variations of oxygen directly. Answer choice A relates to the oxygen atoms directly, and not via the water molecules.

Does the LSAT expects us to remember that each water molecule contains exactly 1 oxygen (H2O)?
What if I didn't have that well known fact in my head at the moment I answered the question, and I was considering a possibility of a cloud that contains 2 water molecules with 1 normal oxygen atom each, and 1 water molecule which has 6 oxygen-18???

Moreover, I think that all of us know that rain clouds contain mainly water, but I'm no expert in the subject. What if the clouds contain other materials besides water, like some sort of acid, dust of some rare exotic element - how can I be certain that the cloud doesn't include oxygen-18 in a different form, meaning not as a part of a water molecule?
Example: 2 water molecules with 1 normal oxygen each, 1 water molecule with 1 oxygen-18, and 1 dust molecule with 20,000 oxygen-18 atoms.

Your thoughts?

Thanks

[bortnik22]

[size=150][/size]

so it might be some other element in the water molecule of rain water that makes plants grow like crazy . i asumed it was the extra osigen ?

[thobyking]

Strictly speaking, I think this question requires us to make an assumption, which is to say it does not give us all the necessary information:

The stimulus compares the proportions of "water molecules containing [ either] oxygen 18 [or] normal oxygen." However, the answer options (including A, the correct one) deal in broader categories: "Rain clouds above tropical forests are poorer in oxygen 18 than rain clouds above unforested regions."

According to the information we've been given, we have no idea what proportions of oxygen and oxygen 18 the rain clouds contain overall. All we've been told is the proportions of the types of oxygen contained within water molecules. Clouds could very well (I believe they do) contain large amounts of oxygen not bound up in molecules of H2O.

Doesn't this constitute a problem?

[YiS366]

We know oxygen-18 is rarer than normal oxygen. Say there are 100 oxygen-18 and 1000 normal oxygen.

During rainfall:
90% of oxygen-18 gets out for a total of 90
50% of normal oxygen gets out for a total of 500
Even though a higher proportion of oxygen 18 went out, the absolute number of oxygen 18 is still less than that of normal oxygen.

Answer C says during rainfalls (as in for all rainfalls), the number of oxygen 18 is greater than normal oxygen. This is not true because the example I gave is at least one way of disproving that.

[obobob]

This question is really confusing.

Can someone please explain how the second sentence let us to infer that there are more normal oxygen than oxygen-18?

I don’t know anything about chemical components of rain clouds, though I don’t think that’s the necessary part of the test, anyways, and when I just read the second sentence, I was thinking that there may be possible instances where there are more oxygen-18 than normal oxygen despite of rain clouds containing more water molecules with ordinary oxygen than those with oxygen-18.
Like, may be, there are random oxygen-18 composing the clouds outside water molecules, or something like that.

So this gives me trouble understanding why (A) is the correct answer.

(A) is saying that there are more ordinary oxygens than oxygen-18.
Do we have to just think that there are more normal oxygen by line of thought that clouds are evaporated & condensed version of water, so, commonsensically, clouds are mostly, if not all, composed of water molecules?

[smiller]

Like, may be, there are random oxygen-18 composing the clouds outside water molecules, or something like that.

That's possible.

Generally speaking, there are two types of Inference questions on the LSAT. One type asks us to choose the answer that must be true. The other type asks for the answer that is most strongly supported. This question is the latter type. The second sentence doesn't guarantee that answer choice (A) is correct, but it definitely supports answer choice (A). Choice (A) doesn't include statements that are contradicted by the stimulus or new concepts that aren't discussed in the stimulus, as other answers do.