Reading Comprehension

PT143 · S2 · P4 · Q27 The Myth of Liquid Glass

A free, expert breakdown of this official LSAT Reading Comprehension question.

Save & drill this skill — build targeted practice sets from questions like this one
Video walkthroughs — watch every question solved step by step
81 official LSATs — as questions, timed sections & full-length tests

Start practicing free

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.

27.

The passage suggests that which one of the following statements accurately characterizes the transition temperature of glass?

It is higher for medieval
Unsupported Comparison: medieval v. modern
There are two implied comparisons between medieval glass and modern glass: 1. medieval glass is thicker at the bottom than modern glass is 2. medieval glass contains more impurities than modern glass does Could either of those comparisons translate into a higher glass transition temperature? A higher glass transition temperature would mean that if you cooled liquid glass (medieval and modern), the medieval glass would start taking on the physical properties of a solid sooner, because it transitions at a higher temperature. We know that the author doesn't think that #1 is related to glass transition temperature, because she explains in the final paragraph that it's more related to manufacturing methods. When it comes to impurities, all we know is that impurities in medieval glass would make the warmer, more liquid-y version more prone to flowing downward (the impurities "lower the viscosity and speed the flow"). The medieval glass is more prone to being liquid-y, whereas higher glass transition temperature would mean that it becomes a solid sooner than modern glass does (which makes the modern glass sound more liquid-y). So, if anything, this answer seems to go against the passage. But rather than getting too lost in that deep technical rationale, we can feel fine eliminating this based on not having any good way to compare glass transition between medieval and modern.
2% picked this
It has only recently been
Out of Scope: calculating transition temp
The recent calculations in this passage all come from Zanotto, and he never calculates a glass transition temperature. He is calculating how long it would take for the viscous flow of glass to result in any noticeable lump at the bottom of the glass. He calculated the "flow rate" of different types of glass, but didn't come up with some precise calculation of the glass transition temperature (the range through which a glass goes from being liquid to being pretty much solid).
6% picked this
Correct
Its upper extreme is well
Why this is right
This answer is combining two facts: - glass takes a few hundred degrees to go from solid to liquid (it doesn't flow like a liquid until it's closer to the upper end of those few hundred degrees) - glass doesn't start acting like a liquid at all until you're at at least 350º Thus, we can tell that at 350º, with only a negligible ability to flow, glass has NOT yet completed the journey through its glass transition temperature. It still has a ways to go, and thus the upper end of the glass transition is well above 350º. This is a brutal correct answer, and it requires that we do some searching in the passage, because 350º isn't anywhere near our Support Window at the end of the 1st paragraph. We have to ask ourselves agnostically, "Hmm ... did we say anything about 350 degrees?" The end of the 3rd paragraph says: for glass to have more than a negligible ability to flow, it would have to be heated to at least 350 degrees Celsius. Remember that glass transition temperature is defined as the range of a few hundred degrees over which glass goes from solid to liquid, or liquid to solid (depending on whether you're heating a solid into a liquid or cooling a liquid into a solid). Let's say for Glass X, its glass transition range is from 25º - 325º Celsius. That would mean that at any temperature below 25º, glass is acting like a solid. At any temperature above 325º, it's acting like a liquid. Any temperature in between, like 150º, would be a hybrid. It would be a "very viscous liquid" or a "somewhat spongy solid". Could there be a glass like Glass X, which is totally liquid by the time it hits the upper end of its glass transition temperature at 325º? No. That would contradict the line we just read that, "glass doesn't really flow at all until you get above 350º". By the time Glass X gets to 325º, it has fully transitioned into a liquid form, so it would flow like water or like maple syrup. It sounds more like a typical transition temperature would be that Glass X goes from solid to liquid as we go from 300 - 600º Celsius. That would be more in line with the idea that as you get to around 350º, the glass starts to be able to flow downward somewhat (because it's loosening from solid form to liquid form).
Skill tested: Inference · how this choice captures the passage's function is the move to repeat next time.
54% picked this
It does not affect the
Contradicted
This answer is saying, "Some kinds of glass flow downward, and it has nothing to do with their glass transition temperature". But since glass transition temperature refers to the temperature range through which glass goes from being a solid that doesn't flow to being something more liquid-y that does flow, the glass transition temperature is definitely related to whether or not a given type of glass (at a given temperature) would or wouldn't have any tendency to flow downward.
24% picked this
For some types of glass,
Contradicted: specific
The transition temperature is never a specific singular temperature; glass doesn't have a precise freezing / melting point. It's a range, usually, of a few hundred degrees Celsius.
13% picked this

Continue the review in LSAT Lab

Save this question, watch the video walkthrough, and drill similar questions in your LSAT Lab account.

Save and practice this skill

LSAT Lab

Turn this review into a targeted study plan.

Save this question, drill more like it, watch the video walkthrough, and track your progress in your LSAT Lab account.

Start practicing free