Logical ReasoningDifficulty: Hard

PT1 S3 Q11 Explanation

Water vapor evaporated from the

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TopicsMost Supported

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Full official LSAT questions are available through LawHub. This page provides LSAT Lab's explanation, strategy, and review tools without republishing the full official question.

Stimulus

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 falls on ice caps, where it is trapped as ice.

What this question is testing

Most Supported

Premise

Think about it this way. The water that evaporates off the ocean is lighter in composition — more O-16 (light), less O-18 (heavy). In normal times, that's a wash because the vapor falls back as rain into the ocean.

Evaluate

But during an ice age, that vapor doesn't come back. It piles up on ice caps and stays there. So the ocean is steadily losing the lighter oxygen-16 with no replacement. What's left behind in the ocean? More of the heavier stuff: oxygen-18.

Imagine pouring a salt-water solution through a coffee filter that catches one of the components. The water that drips through is missing one ingredient, and what stays in the filter has a higher concentration of the other. Same idea here — the ocean is the "filter," and what remains is enriched in O-18.

Goal

Find the answer that says O-18 in seawater is more concentrated during ice ages.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question
11.

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

Answer choices

  1. Contradicted7% picked this

    The proportions of oxygen-16 and oxygen-18 are the same in vapor from seawater as in

    The stimulus explicitly says vapor has more oxygen-16 and less oxygen-18 than seawater — that is, the proportions in vapor and seawater are different. This answer says the proportions are the same. That directly contradicts the premise.

  2. Correct56% picked this

    The concentration of oxygen-18 in seawater

    Why this is right

    This is the inference the stimulus directly supports. Evaporation pulls oxygen-16-rich water out of the ocean. Normally that's a wash because precipitation returns it, but during an ice age the precipitation gets locked up in ice caps. So the ocean is losing oxygen-16 with no replacement — meaning the oxygen-18 share of what remains in the ocean rises. The concentration of oxygen-18 in seawater is increased.

    Skill tested: Most Supported · how this choice captures the argument's function is the move to repeat next time.

  3. Contradicted12% picked this

    Rain and snow contain relatively more oxygen-16 than they do in

    This compares ice-age precipitation with interglacial precipitation, claiming ice-age rain/snow has relatively more oxygen-16. The stimulus says vapor is consistently richer in oxygen-16 regardless of period — and gives no reason to think ice-age vapor differs from normal vapor in composition. The mechanism the stimulus describes affects the ocean over time, not the composition of any single precipitation event compared to interglacial events.

  4. Unsupported11% picked this

    During the ice age, more of the Earth’s precipitation falls over land than falls

    The stimulus says a large amount of precipitation falls on ice caps. It does not say more ice-age precipitation falls on land than on the ocean. We're told only that some falls on ice caps and gets trapped — nothing about a land-vs-ocean breakdown. This goes beyond what's supported.

  5. Unsupported15% picked this

    The composition of seawater changes more slowly than it does in

    The stimulus actually suggests the opposite: in interglacial periods composition is stable (vapor returns as precipitation), but in ice ages composition shifts because vapor gets trapped. So if anything, ice-age composition changes faster than interglacial composition. This answer reverses that.

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