Reading Comprehension

PT143 · S2 · P4 · Q24 The Myth of Liquid Glass

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To glass researchers it seems somewhat strange that many people throughout the world share the persistent belief that window glass flows slowly downward like a very viscous liquid.

Topic

The myth that old window glass flows downward over time, and the science debunking it.

Framework

Challenge Position

Main Point

The commonly held belief that window glass flows downward over time (causing old windows to be thicker at the bottom) is a myth; scientific calculations and historical glassmaking techniques explain the thickness differences instead. (The main point is most directly addressed at the beginning of paragraph 2 and fully explained in the last paragraph.)

P1: Introduction to the Glass Flow Myth

A lot of people (even in science books) mistakenly believe that window glass is a super slow-moving liquid and gradually flows downward, explaining the thicker bottoms in old windows. This myth likely started because glass's atoms are not arranged like in crystals, but the structure alone doesn't mean glass is liquid—it's solid at room temperature, just with a unique atomic arrangement.

P2: Scientific Debunking

A recent study shows that, although glass *can* flow under gravity, it would take longer than the age of the universe for old windows to change thickness from flow. So, that's not the reason old windows are thicker at the bottom!

P3: Why Flow Still Isn’t the Answer

Even the easiest-flowing glass (like germanium oxide glass) would still take trillions of years to change shape noticeably, and medieval glass—even with impurities—wouldn't flow in just a few centuries. The science is clear: for glass to flow, it actually needs to be heated to at least 350°C.

24.

The passage suggests that the atomic structure of glass is such that glass will

  1. behave as a liquid even

    Contradicted

    The last sentence of the 1st paragraph is saying that solid glass "takes on the physical properties of a solid", which means "how the object behaves". I can knock on solid glass, just like I'm knocking on the wall. It behaves like a solid. My knuckles have no idea that the glass's atomic structure is actually very similar to that of liquid glass.

    31% picked this

  2. be noticeably deformed by the

    Contradicted

    This language about being deformed by its own weight isn't coming from where we looked at the end of the 1st paragraph, but if we consider it, it seems to deal more with the 2nd paragraph's concerns. The author thinks that Zanotto has debunked the idea that gravity will cause glass to sag downward until the bottom is thicker than the top (noticeably deformed). Zanotto has calculated that "the time needed for flow of glass to change by a noticeable amount is well more than the age of the universe", let alone a period of a few millenia.

    4% picked this

  3. behave as a solid even

    Too Strong

    This is saying that glass always behaves as a solid. If say, "My dog begs for food even when there's food in his bowl", we mean that he always begs for food, whether there's food or not food. So this answer is saying that whether glass has gone full liquid or full solid, it will behave as a solid. The end of the first paragraph was making it sound like molten (liquid, flowy) glass switches into having the properties of a solid, once you've dipped below its glass transition temperature. But we're not hearing that liquid glass behaves as a solid. p.s. something that makes this answer so frustrating to interpret is the notion of "reaching" your glass transition temperature. Again, that's a self-contradictory notion, because the glass transition isn't a specific value you can reach. It's a range of a few hundred degrees.

    34% picked this

  4. Correct

    flow downward under its own

    Why this is right

    This is an unreasonably hard correct answer to get. It goes counter to the author's main Challenge Misconception purpose, so it's not at all tempting. And it has nothing to do with the end of the first paragraph, where the keywords take us. But our ultimate measuring stick for "suggests / most likely to agree / most reasonably inferred" is just which answer is most supportable. If you heat solid glass until it becomes liquid glass, do you think it would flow downward under its own weight? Of course! All liquids would flow downward under their own weight. They're liquids. The force of gravity pulls them crashing to the floor. The concept of weight is basically (mass + gravity). What this answer is trying to do is a common trait of correct answers on Most Supported tasks: they take something that was said one way and present the inverse version of that same thing. If we learned that, studying multiple languages reduced people's risk of Alzheimer's, a correct answer might give that back as being monolingual has at least some drawbacks to one's health. Since we learned that, cooled below its transition temperature, molten glass takes on the properties of a solid this answer is testing us on, heated to its transition temperature, solid glass takes on the properties of a liquid We could also support this answer from the last sentence of the 3rd paragraph, where it implies that there is a way for glass to have a non-negligible rate of downward flow: it would have to be heated to at least 350 degree Celsius. One property of a liquid is that you would flow downward, like a waterfall, or a mountain river. The game on this question for LSAC seemed to be to draw our eyes to the confusing end of the 1st paragraph, present some wrong ideas about what that was saying, and then bury this simple idea of "liquid flows downward" in weird language that kind of sounds like the opposite of the main point.

    Skill tested: Inference · how this choice captures the passage's function is the move to repeat next time.

    28% picked this

  5. stop flowing only if the

    Unsupported Relationship

    The author never presents a harsh, conditional relationship that says "if the atoms aren't fixed crystalline structure, then glass will never stop flowing". Also, the language of this answer is somewhat nonsensical. By saying, "Glass will stop flowing only if the atoms are arranged in a fixed crystal structure", we'd seemingly be implying that there is some possibility that glass's atoms will be in fixed crystal structure. But we don't think glass will ever be arranged in a fixed structure, so it would be weird to say the passage suggests this meaning.

    2% picked this

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