Official 45 Set 5

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Homogeneous and Heterogeneous

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What does the professor mainly discuss?
  • A. Characteristics of different types of mixtures

  • B. Differences between mixtures and solutions

  • C. Ways of separating components of mixtures

  • D. Identifying variable properties of solutions

显示答案 正确答案: A

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    NARRATOR:Listen to part of a lecture in a chemistry class.

    MALE PROFESSOR:So, just to sum up, matter is anything that has mass and volume, right?Anything that takes up space—and this includes solids, liquids and gases.

    And if we combine two portions of matter, we get a mixture.

    Now, there are two main kinds of mixtures: homogeneous and heterogeneous.Uh maybe I should put this on the board.

    Whether a mixture is homogeneous or heterogeneous, well, this relates to the notion of "phase".

    Remember, we defined the word "phase" as being one physical state, whether solid, liquid or gas, that, well, that has distinct boundaries and uniform properties.

    So, homogeneous mixtures, what are they?OK, the prefix homo means "same", so a homogeneous mixture is the same throughout; it contains only one phase.So if you put alcohol in water, the two liquids combine, they disperse into each other, and you can't perceive any boundary between the two any longer.So the mixture contains only one phase—even though two phases went into it—it now contains one phase and we can't detect any boundary between the water and the alcohol once they're mixed together; the two portions combine to form a single phase.

    Now, if homogeneous mixtures are ones that are the same throughout, then what do you suppose heterogeneous mixtures are?Right. Mixtures that are different throughout.If you mix oil and water together, the mixture contains two liquid phases because the oil will float on top of the water because of oil's lower density.They're not going to mix together like alcohol and water do.You can see the boundary between them, and in fact they're mechanically separable.

    The same is true for soil, which is a mixture of solid materials.So if you look closely at a sample of soil, you're gonna see bits of sand, some black matter, maybe even pieces of vegetation.Since you can see all the different components, detect distinct boundaries, we've got multiple phases; and in fact you can pick out the components, the various portions can be mechanically separated.

    Now, with some heterogeneous mixtures you can see the different phases with the naked eye.But that's not so for all of them... like smoke.Actually, that's a good example, because to the naked eye it looks uniform, like it's a single phase.But if you magnify it, you can see that there are tiny solid and liquid particles suspended in the air.So actually, what you've got in smoke are three, three phases-solid, liquid and gas, which you can separate by the process of filtration.

    Another example, uh dirty water. Ok?Dirty water is water that has suspended solid matter in it.That can be filtered too.Pass it through a filter and the dirt and whatever else is in there will stay behind on the filter paper, and the clean water will pass through it.Again, depending on the size of the particles in the water, you might need magnification to see them, but even so, they can be detected, the boundaries are detectable, so multiple phases, ok?

    Homogeneous mixtures, on the other hand, well, no amount of magnification could reveal a detectable boundary between the components.The mixing extends all the way to the fundamental particle level.And we use the term "solution" to refer to these single phase, homogeneous mixtures...When salt's dissolved in water, no amount of magnification is going to show you separate pieces of salt, there are no detectable boundaries between salt and water-so it's a solution.

    Even so, what you can do with solutions is separate the parts by a process called distillation.If you distill salt water, water gets boiled away from the solution, and only the salt remains behind.And in your next lab, actually, we'll be using these processes, distillation and filtration, to show how we can separate the different parts of some mixtures.

    Now, there are other ways that we can describe mixtures, and one of these is by properties, uh, variable properties.A real simple example of this is the taste and color of a cup of coffee.The more coffee that's dissolved in the water, the stronger the taste of the coffee, and the darker the color—the darker the solution.So color and taste, these are two variable properties.And these variable properties, they vary of course, because of the relative amounts of the components.

    And the melting or freezing points of liquids, too.A solution of salt water, for example, will have a different freezing point depending on how much salt is dissolved in the water.

  • 旁白:请听化学课上的部分内容。

    教授:总结一下,物质就是一切有着质量和体积的东西,对吗?一切占据了空间的事物,包括固体、液体和气体。

    如果我们把两种物质结合在一起,我们会得到一个混合物。

    有两种主要的混合物:同质混合物和异种混合物。也许我应该把这个写到黑板上。

    一种混合物究竟是同质的还是异种的......这和状态的概念有关。

    还记得我们把“状态”定义为一种物理状态,无论固体、液体还是气体,它有着明显的界限和一致的性质。

    那么,同质混合物......它们是什么呢?“homo-”这个前缀指的是“相同的”,所以同质混合物自始至终是相同的;它只包含了一种状态。如果你把酒精放进水中,这两种液体会结合,它们会消散进彼此之中,你在这两种物质之间再也察觉不到任何界限了。所以这种混合物只包含了一种状态,虽然有两种状态进入了其中,它现在只包含一种状态,一旦水和酒精混合到了一起,我们就发现不了它们之间的界限了;这两部分结合起来形成了一个单独的状态。

    如果同质混合物自始至终都是相同的混合物,那你们认为异种混合物是什么呢?没错,自始至终都不相同的混合物。如果你把油和水混合在一起,这种混合物就会包含两种液体状态,因为油会浮在水上面,因为油的密度更低。它们不会像水和酒精那样混合在一起。你能看到它们之间的界限,而且事实上,它们在物理上是可以分离的。

    这同样也适用于泥土,泥土是固体材料的混合物。如果你仔细看一个泥土样品的话,会看到一些沙子、一些黑色的物质,也许甚至还能看到几片植物。既然你能看到所有不同的成分,发现明显的界限,那我们就有了多种状态;事实上,你可以把里面的成分挑出来,这些不同的部分在物理上是可以分离的。

    有一些异种混合物,你用肉眼就能看到不同的状态。但不是所有的都是这样的,比如烟。实际上这是个很好的例子,因为在肉眼看来,它是一致的,就像只有单一的状态一样。但是如果你把它放大,会看到空气中悬浮着微小的固体和液体颗粒。所以事实上烟里面有三种状态:固体、液体和气体,而通过过滤的步骤你可以把它们分离开来。

    另一个例子是脏水,好吗?脏水是里面悬浮着固体物质的水。那也能被过滤。让它穿过一个过滤器,然后其中的灰尘和其他的物质会留在过滤纸上,而干净的水会穿过它。还是一样,根据水中颗粒的大小,你也许需要放大才能看见它们,但是即使如此,它们也能被发现,它们之间的界限是可检测到的,所以它们有多种状态。

    但是同质混合物,换句话说,即使放大到再大也揭示不了这些成分之间可检测的界限。它们的混合一直延展到了最根本的颗粒层面。我们用“溶解”这个术语指代这种单一的状态,这种同质混合物。当盐溶在水中时,放大再多倍也看不到单独的盐粒,盐和水之间没有可检测到的界限,所以这是溶液。

    即便如此,你对溶解物能做的就是通过一个叫做蒸馏的过程把各部分分离。如果你蒸馏盐水,水分会从溶解物中汽化,只有盐分留了下来。实际上,你们下次做实验的时候,我们会使用这些处理方法,蒸馏和过滤,来给你们展示我们能分离一些混合物的不同部分。

    我们还有其他方式描述这些混合物,其中之一就是通过性质,可变性质。一个很简单的例子就是一杯咖啡的味道和颜色。溶解在水中的咖啡越多,咖啡的味道就越强烈,颜色也就越深。所以,颜色和味道,这是两种可变性质。而这些可变性质,它们使过程多样化,因为这些成分的相对含量不同。

    还有液体的融点和冰点也不一样。比如一个盐水的溶解物会有不同的冰点,这取决于水中溶解了多少盐。

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    题型分类:主旨题

    原文定位:Now, there are two main kinds of mixtures: homogeneous and heterogeneous. Uh maybe I should put this on the board. [pause] Whether a mixture is homogeneous or heterogeneous, well, this relates to the notion of “phase”.

    选项分析:教授开篇就讲到混合物的问题,混合物分两种:同质混合物和异质混合物,接下来展开讲这两种混合物的特征,对应选项A。

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