TPO 52
标题：Snowflakes and Ozone
Listen to part of a lecture in a chemistry class.
Professor:
Ok, so, today we’re going to talk about the Arctic, ozone depletion and snowflakes. And it’s all related. Let’s start with snowflakes.
Now, I find snowflakes fascinating. To even begin to understand them, you need to understand physics, chemistry, and mathematics. Even though there’s been a lot of research, there’re still a lot about snowflakes that we don’t understand yet. Hard to believe, I know.
Anyway, snowflakes have a particular form, there’s a six-sided center with six branches andor arms that radiate out from it. But how did they get that way? Well, you start with water vapor. You need a pretty humid atmosphere. And that water vapor condenses directly into ice, into an ice crystal. At this point it looks kind of like a thin dinner plate that rather than being circular, is hexagonal with six flag flat edges.
It’s at this point in the process were we begin to see why each snowflake is unique. Imagine this dinner plate is floating around in the wind, right? And when it encounters water vapor, molecules from that vapor attached to each of the six sides. You begin this development with six arms or branches radiating out from the center plate. Each time the snowflake encounters water vapor, more molecules attached to it, leading to more and more complex structures. And of course, each snowflake takes unique route through the clouds on its way down. And so the quantity of water vapor that it goes through is going to be unique for each one.
Now one important characteristic of snowflakes is that they had something called a quasi-liquid layer, the QLL. Our snowflake is an ice crystal, right? Well, we find a quasi-liquid layer on the surface of ice is basically a thin layer of water that’s not completely frozen. And anthe existed temperature is well below frizzingfreezing. And the thickness varysvaries at different temperatures. Now this quasi-liquid layer, it plays an important role on what we are going to talk about next.
Ah, yes, Mary?
Mary:
How can liquid exist below frizzingfreezing? Why doesn’t it frieeze?
Professor:
Well, when water becomes ice, the molecules bond together and it get sort of…locked in the place. They can’t move around as much anymore. So each molecule is surrounded by other molecules. And they are all locked together. But what about the exterior of the ice? There is a layer of water molecules on the surface, they attached molecules only on one side. So, they are a bit freer. They can move around a bit more. Think of a… think of a brick wall. The bricks in the wall, they have other bricks above and below them, and they are all locked against each other. But that top layer, it only has a layer below it. Now this can only be taken so far because of course bricks don’t move at all. They are not liquid. But the bricks of water molecules, well, this top layer would be the quasi-liquid layer. And it wouldn’t be completely frozen. Does that make sense?
So, finally we get to the connection between snowflakes and ozone. Ozone is a gas found in the atmosphere of Earth. Now there is the ozone found in the stratosphere which is the layer the atmosphere from 6 to 30 miles above the Earth. This is considered good ozone, which occurs naturally and helps block harmful radiation from the Sun.
But there is also ground-level ozone. It’s exactly the same gas but it’s found closer to the surface of the Earth. This ground level ozone results from human activities, at a high concentrations it can be a pollutant. Now snowflake’s quasi-liquid layer plays an important role in some complex chemical reactions. We’re going to be looking at these in detail later today. But basically, these reactions cause certain chemicals to be released. And these chemicals reduce the amount of ground-level ozone. So the more branches you have in an ice crystal, the more quasi-liquid layer there is. The more quasi-liquid layer, the more reactions and the more chemicals that reduce ground-level ozone. So you can see why this is such an important system to study and understand.
TPO 52 托福听力Lecture 3题目
6. What aspects of snowflakes does the professor mainly discuss?
Click on 2 answers
How they develop into complex structures
How they are affected by the presence of ozone
The challenges researchers face in studying them
The function of their quasi-liquid layer
7. What does the professor say about the role of water vapor in snowflake formation?
Too much water vapor prevents the initial "dinner plate" from forming.
Water vapor's role in snowflake formation is not completely understood.
Water vapor molecules in snowflakes attract ice particles from the air.
Water vapor is necessary for snowflakes to be able to form branches.
8. What factor helps explain why no two snowflakes are alike?
They all freeze at different rates.
They all form in slightly different air temperatures.
They all begin with a different number of water molecules.
They all follow different paths through clouds.
9. How do molecules in the quasi-liquid layer differ from those in other parts of the snowflake?
They are not held in place as tightly as other molecules.
They react with ozone to keep the layer from completely freezing.
They prevent ice crystals from forming additional branches.
They are thinner than other molecules.
10. What does the professor imply about ice crystals with a large number of branches?
They help block harmful radiation from the Sun.
They form as a result of complex reactions with ozone.
They contribute to a reduction in ground-level ozone.
They have a thinner quasi-liquid layer than ice crystals with fewer branches."
重听题：11. What can be inferred about the professor when he says this:
He doubts that the students have understood his explanation.
He does not think that bricks are an ideal illustration of his point.
He is not sure that the information he has just given is accurate.
He thinks that the similarities between liquid and bricks are surprising.
TPO 52 托福听力Lecture 3答案解析
6.答案：AD
解析：此题出处是：
Anyway, snowflakes have a particular form, there’s a six-sided center with six branches andor arms that radiate out from it. But how did they get that way?
……
So, finally we get to the connection between snowflakes and ozone.
……
此题需要结合整篇讲座进行归纳。讲座前半段是在讲雪花是怎么形成的，后半段是在讲雪花和臭氧的关系，选项A和选项D合适。选项B是说雪花是如何被臭氧的存在而影响的，选项C是说研究雪花的过程中研究者所面对的挑战，都不合适。双选AD。
7.答案：D
解析：此题出处是：
Imagine this dinner plate is floating around in the wind, right? And when it encounters water vapor, molecules from that vapor attached to each of the six sides. You begin this development with six arms or branches radiating out from the center plate. Each time the snowflake encounters water vapor, more molecules attached to it, leading to more and more complex structures.
这里教授讲到，雪花是空气中的水蒸气在碰到雪花六边形主体的时候被冻在上面，一步步形成了向外辐射的分支，所以水蒸气在雪花形成过程中不可或缺，选项D合适。选项A是说太多的水蒸气会阻碍雪花主体薄片的形成，选项B是说我们并不完全明白水蒸气在雪花形成中的作用，选项C是说雪花里的水蒸气会帮助吸引空气中的冰粒子，都不合适。选择D。
8.答案：D
解析：此题出处是：
And of course, each snowflake takes unique route through the clouds on its way down. And so the quantity of water vapor that it goes through is going to be unique for each one.
这里教授提到，因为每一片雪花下落的路线不同，会附着在它上面的水蒸气的量也不一样，所以每一片雪花都是独一无二的，选项D合适。选项A说它们结冰的速率不同，选项B说它们形成时候的温度有细小的差别，选项C说它们一开始所含有的水分子的量不同，都不对。选择D。
9.答案：A
解析：此题出处是：
But the bricks of water molecules, well, this top layer would be the quasi-liquid layer. And it wouldn’t be completely frozen.
这里教授提到，最外层的水分子因为只受到一面的附着力，相对内部的水分子而言就更容易移动一下，更加自由，选项A正确。选项B是说它们和臭氧反应，以不让这一层彻底冻上，选项C说它们组织冰晶形成多余的分支，选项D说它们比别的分子更薄，都不合适。选择A。
10.答案：C
解析：此题出处是：
So the more branches you have in an ice crystal, the more quasi-liquid layer there is. The more quasi-liquid layer, the more reactions and the more chemicals that reduce ground-level ozone.
这里教授讲到，冰晶的分支越多，其类液态薄层越多，类液态薄层越多，会导致地面臭氧含量降低的化学反应就越多，选项C合适。选项A说它们帮助我们抵挡有害太阳辐射，选项B说它们由和臭氧的复杂化学反应形成，选项D说它们比分支少的冰晶具有更薄的类液态薄层，都不合适。选择C。
11.答案：B
解析：此题出处是：
The bricks in the wall, they have other bricks above and below them, and they are all locked against each other. But that top layer, it only has a layer below it. Now this can only be taken so far because of course bricks don’t move at all. They are not liquid.
这里教授先是想以砖块墙类比雪花，但是他形容完砖块之间的结合方式后，又表示砖块毕竟不是液态，砖块本身就不可以移动，这说明其实他自己觉得用砖块来类比类液态薄层其实并不是很合适，选项B正确。选项A是说他怀疑学生没有理解他的解释，选项C是说他并不确定他说的信息是正确的，选项D是说他认为液体和砖块之间的相似性令人惊奇，都不合适。选择B。
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