托福official45听力lecture3 Homogeneous And Heterogeneous原文解析+翻译音频

[00:00.00]NARRATOR: Listen to part of a lecture in a chemistry class.
[00:03.81]MALE PROFESSOR: So, just to sum up, matter is anything that has mass and volume, right? [00:09.56]Anything that takes up space—[00:11.52]and this includes solids, liquids and gases.[00:14.45]And if we combine two portions of matter, we get a mixture.
[00:19.30]Now, there are two main kinds of mixtures: homogeneous and heterogeneous. [00:24.89]Uh maybe I should put this on the board. [00:27.55]Whether a mixture is homogeneous or heterogeneous, well, this relates to the notion of “phase”.
[00:34.70]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.
[00:46.73]So, homogeneous mixtures, what are they?
[00:49.87]OK, the prefix homo means “same”, [00:53.47]so a homogeneous mixture is the same throughout; [00:57.18]it contains only one phase. [00:59.59]So if you put alcohol in water, the two liquids combine, [01:04.24]they disperse into each other, [01:06.17]and you can't perceive any boundary between the two any longer. [01:09.98]So the mixture contains only one phase—[01:13.32]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; [01:22.78]the two portions combine to form a single phase.
[01:26.18]Now, if homogeneous mixtures are ones that are the same throughout, then what do you suppose heterogeneous mixtures are? [01:35.37]Right. Mixtures that are different throughout.
[01:38.72]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. [01:49.58]They're not going to mix together like alcohol and water do. [01:53.03]You can see the boundary between them, and in fact they’re mechanically separable.
[01:58.10]The same is true for soil, which is a mixture of solid materials. [02:03.06]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. [02:11.68]Since you can see all the different components, detect distinct boundaries, we've got multiple phases; [02:19.10]and in fact you can pick out the components, [02:21.71]the various portions can be mechanically separated.
[02:24.64]Now, with some heterogeneous mixtures you can see the different phases with the naked eye.
[02:31.27]But that's not so for all of them… like smoke. [02:34.67]Actually, that's a good example, because to the naked eye it looks uniform, like it's a single phase. [02:40.26]But if you magnify it, you can see that there are tiny solid and liquid particles suspended in the air. [02:47.00]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.
[02:56.88]Another example, uh dirty water. Ok?
[03:01.63]Dirty water is water that has suspended solid matter in it. [03:06.23]That can be filtered too. [03:07.75]Pass it through a filter and the dirt and whatever else is in there will stay behind on the filter paper, [03:13.44]and the clean water will pass through it. [03:15.63]Again, depending on the size of the particles in the water, you might need magnification to see them, [03:21.32]but even so, they can be detected, [03:23.62]the boundaries are detectable, [03:25.35]so multiple phases, ok?
[03:27.79]Homogeneous mixtures, on the other hand, well, no amount of magnification could reveal a detectable boundary between the components. [03:35.74]The mixing extends all the way to the fundamental particle level. [03:40.45]And we use the term “solution” to refer to these single phase, homogeneous mixtures...
[03:46.66]When salt’s dissolved in water, no amount of magnification is going to show you separate pieces of salt, [03:53.30]there are no detectable boundaries between salt and water—[03:56.64]so it's a solution.
[03:58.52]Even so, what you can do with solutions is separate the parts by a process called distillation. [04:05.89]If you distill salt water, water gets boiled away from the solution, and only the salt remains behind. [04:13.15]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.
[04:22.82]Now, there are other ways that we can describe mixtures, [04:26.68]and one of these is by properties, uh, variable properties.
[04:30.71]A real simple example of this is the taste and color of a cup of coffee. [04:35.77]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. [04:44.29]So color and taste, these are two variable properties. [04:48.68]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. [04:59.08]A solution of salt water, for example, will have a different freezing point depending on how much salt is dissolved in the water.