Water vapor evaporated from the ocean contains a greater proportion of

Question

Water vapor evaporated from the ocean contains a greater proportion of oxygen-16 and a smaller proportion of the heavier oxygen-18 than does seawater. Normally, this phenomenon has no effect on the overall composition of the ocean, because evaporated seawater returns to the ocean through precipitation. During an ice age, however, a large amount of precipitation falls on ice caps, where it is trapped as ice.

Which one of the following conclusions about a typical ice age is most strongly supported by the statements above?

Which one of the following conclusions about a typical ice age is most strongly supported by the statements above?

(A) The proportions of oxygen-16 and oxygen-18 are the same in vapor from seawater as in the seawater itself.

(B) The concentration of oxygen-18 in seawater is increased.

(C) Rain and snow contain relatively more oxygen-16 than they do in interglacial periods.

(D) During the ice age, more of the Earth’s precipitation falls over land than falls over the ocean.

(E) The composition of seawater changes more slowly than it does in interglacial periods.

dimitri92 · Accepted Answer

Premise:  no effect on the overall composition of the ocean, because evaporated seawater returns to the ocean through precipitation 
Premise:  During an ice age, however, a large amount of precipitation falls on ice caps, where it is trapped as ice.

the premises tell us that although the overall composition of ocean does not change BUT during an ice age it does because a large portion of precipitation (more O-16) is trapped as ice on ice caps.

this must mean that the concentration of O-18 is now increased.

B is correct

nusmavrik · Answer

water vapor from ocean O16 > O18
water vapor from sea water O16 > O18---------->TRAP.
Water vapor evaporated from the ocean contains a greater proportion of oxygen-16 and a smaller proportion of the heavier oxygen-18 than does   seawater

(A) The proportions of oxygen-16 and oxygen-18 are the  same   in vapor from seawater as in the seawater itself. >> Out of scope / Cannot be inferred.
(B) The concentration of oxygen-18 in seawater is  increased . >> O16 evaporates causing O18 to increase in seawater. O16 is not returned. Answer
(C)  Rain and snow  contain relatively more oxygen-16 than they do in interglacial periods. >> Rain is alright.But snow is wrong. OUT
(D) During the ice age,  more of the Earth’s precipitation falls   over land than falls over the ocean. >> Out of scope
(E) The composition of seawater  changes more slowly  than it does in interglacial periods. >> Out of scope

nusmavrik · Answer

Hey All Now a question - " Water vapor evaporated from the ocean contains a greater proportion of oxygen-16 and a smaller proportion of the heavier oxygen-18 than does seawater ." It means water vapor from ocean has O16 > O18 obviously But the second half. Is it comparing water vapor 's concentration from ocean with sea water ? or is it comparing water vapor from ocean w/ water vapor from sea water :roll:

? Can some one answer this pls?

dimitri92 · Answer

critical thinking :wink:

actually this is a comparison of composition ... between water vapor (of the ocean) and seawater e.g. let's assume water vapor (from the ocean): 75% O-16, 25% O-18 seawater: 65% O-16, 35% O-18 now the argument is comparing the oxygen concentration between the two. i.e. water vapor (from the ocean) has greater composition of O-16 than does saltwater -----> equal amount of seawater btw i hope i made some sense!

feruz77 · Answer

Your explanation is acceptable with regard to the OA.

But I think, the text is a bit confusing to find the right answer. A, C, D, E are clearly out.

And let's consider the OA in the light of the main text.

The first premise is "Water vapor evaporated from the ocean contains a greater proportion of oxygen-16 and a smaller proportion of the heavier oxygen-18 than does seawater."
This premise separates the seawater from the ocean water. On this basis the second premise - "Normally, this phenomenon has no effect on the overall composition of the ocean, because evaporated seawater returns to the ocean through precipitation." - seems clearly ambiguous because of the use of the word "return".

Moreover, one may, because of the words " precipitation falls on ice caps", assume two cases, i.e. either water circulation from the ocean to the sea or vise versa. Herewith the reader can also be trapped in so far as the ocean water does usually not become icy (here the ice age is not an absolute evidence to disprove this fact and otherwise as in the GMAT-format the test-taker is not assumed to posses the knowledge of the special subject, as here the case relates to the geography), the same is true under conditions regarding the seawater.

Take my evaluations into consideration then even B is also not a proper ans. choice.

I would say it a bad question because of such internal contradictions in its construction and ambuqity of its content.

law258 · Answer

B is correct.

Logic goes like this: Water evaporates with fewer O 18 molecules than O 16. In ice age, O 16 molecules are trapped, leaving relatively more heavy O 18 molecules in seawater than before.

Hi gatreya14 - In response to your question, LSAT reasoning questions are very good prep for GMAT CR section. LSAT as a whole is heavily focused on logic and would therefore be a great resource to utilize in preparation for this exam. I'm not saying to go out and start purchasing LSAT books, but utilize these questions when you are feeling shaky on this topic, especially if you're working in the 700-level questions. Have faith though. Keep practicing the 700 level and you will get better

mvictor · Answer

so O-16 is being evaporated, but not going back in the seawater, therefore, there will be more O-18 left in seawater during the ice age than during normal times.

(A) The proportions of oxygen-16 and oxygen-18 are the same in vapor from seawater as in the seawater itself.
doesn't explain anything. since same amount is evaporated vs. what is left - then nothing should be changed though...

(B) The concentration of oxygen-18 in seawater is increased.
aha - so more O-18 is in the seawater, as O-16 is evaporated and not coming back...looks like a good answer.

(C) Rain and snow contain relatively more oxygen-16 than they do in interglacial periods.
seems tempting...but interglacial periods - what is that? it speaks about different period than we are looking for..out

(D) During the ice age, more of the Earth’s precipitation falls over land than falls over the ocean.
sure? 2/3 surface of earth is ocean - so i don't think it is true...

(E) The composition of seawater changes more slowly than it does in interglacial periods.
again - interglacial periods...a period that we are not interested in...

eliaslatour · Answer

This is an inference question, in which we must determine which of the following answers can be concluded from the above.

What do we know? We know that the ocean contains both oxygen-16 and oxygen-18. However, water made from oxygen-16 evaporates more readily. During normal times, this precipitation falls on the land or in snow packs and then melts in the spring to return to the oceans.

In an ice age, however, more and more ice accumulates on land. This ice is made up mostly of water containing oxygen-16. Meanwhile the concentration of oxygen-18 increases in the ocean because oxygen-16 is reduced and never replaced.

This information best supports answer choice (B).

Aim4gmat2024 · Answer

Present day : more oxygen 16 is evaporated, but it comes back to sea . Composition of sea water wrt 0xygen 16 and 18 remains unchanged

Glacial age : More oxygen 16 is evaporated , but it does not back to ocean , rather its trapped in ice. Thus composition of Oxygen 18 increases in sea water . Thus B is the derived inference

Hoehenheim · Answer

Bunuel  I've been having trouble understanding this, could you post an OA? Or maybe help out with an expert reply?  KarishmaB   AjiteshArun   GMATNinja   mysterymanrog   MartyTargetTestPrep   AjiteshArun   egmat   chetan2u

To derive the desired conclusion, ocean and seawater are once used interchangeably. "Seawater returns to ocean" warrants an understanding of the water-cycle. Most importantly, the way the last line is phrased, it is impossible for me to say with certainty which precipitate (ocean or seawater) is being talked about..... Since it is an official LSAT Q, it must mean I am missing something.

AjiteshArun · Answer

Hi Hoehenheim,

Is it that you've understood the question (and answer) but are concerned that the language doesn't seem too (ahem) watertight, or is there something else in the question that you're not sure about?

egmat · Answer

Hoehenheim ,

You've pointed out two important points about this question:
 
 The interchangeable use of "seawater" and "ocean water": You're right about this. The question does use these terms interchangeably. While this is scientifically accurate, it's a good observation on your part. GMAT writers usually aim for more precise language. This interchangeability, though not wrong, is slightly less careful than what you might typically see on that exam. 
 The assumption that test-takers know about the water cycle: Your observation is correct again and in this case, it's a fair expectation. Understanding the basic water cycle (evaporation, condensation, precipitation) is considered general knowledge for graduate-level exams. Test-makers usually assume candidates will know this kind of basic scientific concept.  
Keep up this careful analysis - it's a valuable skill for these exams.

Rajat

Hoehenheim · Answer

Hey there Ajitesh!

Apologies for not providing a watertight (ahem back) explanation of what it is that I couldn't figure out.
Allow me to justify the war I've waged against this piece of text.
 
We know that Ocean Water Vapour is +16 -18. (where '+' stands for greater porportion '-' vice versa) I will draw no conclusions that Ocean Water itself has the exact same or lesser proportion.
As Rajat  egmat  pointed out, only about a basic understanding of the Water Cycle could be tested here, and not Chemistry where it is a given that isotopes have different physical properties, so technically Liquid and Gaseous forms would have different composition. I will proceed with the question using basic science and logic.

Here is my first issue. Taking the author for their word, the intentional omission of 'vapour' after the word seawater would lead one to believe that Ocean Water Vapour proportions are being compared with Sea Water proportions. Given no change in overall Ocean Water composition would mean that even if more O16 than O18 escapes as Ocean Vapour, the equlibrium in the composition is restored when Sea Water Vapour precipitates onto Ocean Water. Clearly Sea Water Vapour is filling the gap left by +O16 -O18, so it'd have to be ++16 and +O18 as compared to Ocean water. But the first line says Ocean Water Vapour has a greater 16/18 ratio than Seawater. Comparing both lines, to avoid a contradiction, Seawater and Seawater Vapour would need to have different compositions.

Second: I am surely no grammar expert, but it is my understanding that using "than" leads to a 1 to 1 comparison, so even if seawater vapour is not mentioned, I would think they are comparing ocean water vapour to sea water vapour. And, if Sea water vapour may have lower +16 -18 as compared to ocean wv, the second line falls apart.
 
Third: Now now, I have no way of knowing which vapour, ocean or water is expected to precipitate onto ice caps. My rough understanding of Water Cycle is that Ocean water vaporizes and precpitates as rain and water in all other forms return back to the Ocean. So, does Ocean Water or Sea Water get trapped, how do the lines make it clear?
If Sea Water gets trapped, we know ++O16 and +O18 gets trapped, so loss of greater O16 would increase concentration of O18 in SeaWater and Voila! We have arrived at the correct answer.

I seem to have understood the question. I have arrived at the answer as well. The journey however, was hurdled by too many Ifs for my taste.

My concern remains, this took me quite a while and I would like to replicate a thought process to arrive at the correct answer in the given time on the test day. But, the amount of analysis and reasoning required to arrive at the solution was time consuming and confusing honestly. So did I miss out on the comprehension aspect? Possibly overthink things? I tried logic to circumvent my grasp of the information as understood by me in the passage. Is this how to go about it? I'm just trying to find the logic-comprehension sweet spot. And how to understand when authors try to trick you with words, as indicated by my line of reasoning.

Hoehenheim · Answer

Thanks for your kind words Rajat, it is really motivating to get affirmation from experts when you move up from scratching the mere surface to peeling of the layers.

AjiteshArun · Answer

Hi Hoehenheim,

You've clearly understood the question (and the answer). Your broader question seems to be about similar situations where the GMAT may trick us.

1.  Our mistakes: We do need to accept that it's okay to say that the ocean is made up of seawater (that is, the LSAT is not at fault here). The question does help ("evaporated seawater  returns  to the ocean"), but more generally, GMAT (and LSAT, GRE...) questions are in English, and communication in English (as in other languages) needs the audience to bring in a  lot  of background knowledge.

2.  Give GMAT questions the respect they deserve, no more: There's a tendency on GMAT club to treat official questions as truly amazing. Unfortunately, GMAT questions are generally pretty good, but that's about it. I'm not qualified to comment on the quant and DI sections, but when I take the GMAT, I never expect the verbal questions to be perfect.

3.  Outside information: GMAT questions may need us to use some "outside knowledge". There's no clear definition, except that we don't need specialised knowledge to solve GMAT questions. Here's  a discussion on this issue .

I'm not sure whether my reply helps or not, but I'd be happy to answer any follow-up questions.

Hoehenheim · Answer

Thanks for the follow-up!

Broadly, I'd love it if you can point me towards developing utmost precision and not missing out on questions in Verbal. I considered it to be my strength, however I scored a paltry 80 when I first gave the exam. RC was 100%ile and I have no idea how CR could have gone so wrong.

I've practiced since. I am at a stage now where when I take adaptive tests on Forum Quiz using LSAT, I reach a 50-50 in ~705-805 level questions. But it still feels like things could go wrong on D-Day.

How can I make my Verbal airtight?

AjiteshArun · Answer

I don't think V80 is a bad score. It may indicate underperformance though, especially if you were getting higher scores in your official practice tests.

There's no easy way to increase accuracy, but it's good to see that you've managed to do that and are now feeling more confident. Keep working through official questions and analysing your approach.

All the best.

egmat · Answer

@ Hoehenheim - improving is CR is fairly simple. First, identify why you are faltering, then fix that habit, and finally, track that you have actually improved. Here is a framework below to identify why/where you may be faltering: Incomplete passage understanding: Missing key details or inferences Failing to identify the author's main point or conclusion Weak pre-thinking: Skipping the pre-thinking step entirely Unable to generate potential weakeners/strengtheners Not considering multiple angles or perspectives Answer choice traps: Selecting choices that are out of scope Falling for choices that use familiar language from the passage Choosing answers that sound good but don't address the core issue This video may help as well: https://www.youtube.com/watch?v=rg3mBi428Fk -Rajat

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