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英['lɪs(ə)n] 美['lɪsn]
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草莓小菇凉:说的非常好,十分有道理,棒棒棒!

06-08 15:44:55

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TPO 45-L3 .homogeneous and heterogeneous 点击收藏

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    Listen to part of the lecture in a chemistry class.

    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.

    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?

    Okay. 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 combined 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 the oil's lower density.

    They are not going to mix together like alcohol and water do.

    You can see the boundary between them and in fact, they are 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 are going to 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 got in smoke are three phases: solid, liquid and gas, which you can separate by a process of filtration.

    Another example, uh, dirty water. Okay? 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 in 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, okay?

    Homogeneous mixtures, on the other hand, well, no amount of magnification could reveal a detectable boundary between the components.

    Their mixing extends all the way to the fundamental particle level.

    And we use the term ,solution, to refer to the single phase, homogeneous mixtures.

    When salts dissolve 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.

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