States of Matter
A-Level Chemistry Topic 4 7:31 English narration · English + 中文 subtitles burned in
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Diamond melts at three thousand five hundred and fifty degrees Celsius. Iodine melts at one hundred and fourteen.
金刚石在三千五百五十摄氏度熔化，碘在一百一十四度就熔化了。
Both are made only of non-metal atoms, and in both, those atoms are joined by strong covalent bonds.
两者都只由非金属原子组成，而且在两者之中，原子都是由很强的共价键连接的。
So why is one of them the hardest material in nature, while the other is a soft solid you can melt on a hotplate? The answer is not the bond.
那么，为什么一个是自然界最硬的材料，另一个却是在电热板上就能熔化的软固体？
It is the structure — and by the end of this lesson you will be able to read a substance's structure straight off its properties.
答案不在化学键，而在结构——等这节课结束， 你就能直接从物质的性质读出它的结构。
Welcome to states of matter.
欢迎来到物质的状态。
First gases, and the one equation that describes them. Then the four structures that decide how every solid behaves.
先讲气体，以及描述气体的那一个方程； 再讲决定每一种固体行为的四种结构。
Let's begin.
让我们开始吧。
Start with a gas.
先看气体。
Its molecules move fast, in every direction, and they keep colliding with the walls of their container.
它的分子高速运动，方向四面八方，不断与容器壁碰撞。
Each hit gives the wall a tiny push.
每一次撞击都给器壁一个微小的推力。
Add up the countless pushes landing on every part of the wall, and that total is what we call pressure.
把落在器壁各处的无数次推力加起来， 这个总和就是我们所说的压强。
Two things make it rise. Heat the gas, and the molecules move faster, so each hit is harder and the hits come more often. Or squeeze the gas into a smaller box, so the same molecules strike the walls more frequently.
有两件事会让压强升高：加热气体， 分子运动更快，于是每次撞击更用力、撞击也更频繁； 或者把气体压进更小的盒子，同样多的分子撞击器壁就更频繁。
To calculate with a gas, we use a model called an ideal gas.
要对气体做计算，我们使用一个模型，叫做理想气体。
It makes two assumptions, and the exam asks for both.
它有两条假设，考试两条都要写。
First, the particles themselves take up no volume at all. Second, there are no forces of attraction between them.
第一，粒子本身不占任何体积；第二，粒子之间没有相互吸引力。
No real gas is truly like that, but many come very close — at low pressure and high temperature, where the particles are far apart and moving fast.
没有哪种真实气体完全如此，但许多气体非常接近——在低压和高温下， 粒子相距很远、运动很快。
A real gas behaves least like the model at high pressure and low temperature, when the particles are crowded together and both their real size and their attractions start to matter.
而在高压和低温下，实际气体与该模型偏离最大， 因为此时粒子被挤在一起，它们真实的大小和相互吸引都开始起作用。
The model gives us one equation that ties everything together: pressure times volume equals the number of moles, times the gas constant, times the temperature.
这个模型给了我们一个把一切联系起来的方程：压强乘以体积， 等于物质的量乘以气体常量再乘以温度。
It is worth more marks than anything else in this topic — and almost every mark lost on it is lost to a unit.
它在这一章里比任何东西都值分—— 而在它上面丢掉的分，几乎全都丢在单位上。
Pressure must be in pascals, so a kilopascal value is multiplied by one thousand.
压强必须用帕， 所以千帕的数值要乘以一千。
Volume must be in cubic metres, not cubic centimetres and not cubic decimetres.
体积必须用立方米，不是立方厘米，也不是立方分米。
Temperature must be in kelvin, so add two hundred and seventy-three to a Celsius value.
温度必须用开尔文，所以摄氏度的数值要加二百七十三。
And the gas constant is eight point three one.
气体常量是八点三一。
Let's use it.
我们来用一用。
Find the volume of zero point five zero moles of an ideal gas at twenty-seven degrees Celsius and one hundred kilopascals.
求零点五零摩尔理想气体在二十七摄氏度、一百千帕下的体积。
Convert first, every single time. Twenty-seven degrees Celsius becomes three hundred kelvin. One hundred kilopascals becomes one times ten to the fifth pascals.
每一次都要先换算单位：二十七摄氏度变成三百开尔文， 一百千帕变成一乘以十的五次方帕。
Now rearrange the equation: volume equals moles times the gas constant times temperature, all divided by pressure.
然后把方程变形： 体积等于物质的量乘以气体常量乘以温度，再除以压强。
Put the numbers in and you get zero point zero one two five cubic metres — which is twelve point five cubic decimetres.
代入数字，得到零点零一二五立方米——也就是十二点五立方分米。
The same equation can weigh a gas for you.
同一个方程还能替你"称"出气体的质量。
Since the number of moles equals mass divided by molar mass, you can substitute that in and rearrange, to get molar mass equals mass times the gas constant times temperature, divided by pressure times volume. Try it.
因为物质的量等于质量除以摩尔质量， 你可以把它代入并变形，得到：摩尔质量等于质量乘以气体常量乘以温度， 再除以压强与体积的乘积。
A flask holds zero point nine six grams of a gas in six hundred cubic centimetres, at one hundred kilopascals and twenty-seven degrees Celsius.
试一试：一个烧瓶在一百千帕、二十七摄氏度下， 装有零点九六克气体，体积六百立方厘米。
Convert the volume: six hundred cubic centimetres is six times ten to the minus four cubic metres.
先换算体积： 六百立方厘米等于六乘以十的负四次方立方米。
Put everything in, and the molar mass comes to about forty grams per mole.
全部代入， 摩尔质量约为四十克每摩尔。
Now to solids.
现在讲固体。
Every crystalline solid is one of four structures — and the structure, not the bond, is what decides how it behaves.
每一种晶体都属于四种结构之一——而决定它性质的是结构，不是化学键。
Giant ionic: a huge regular lattice of positive and negative ions, pulled together in every direction.
离子晶体：由正、负离子组成的巨大规则晶格，各个方向上都彼此吸引。
Simple molecular: small separate molecules — iodine, fullerene, ice; the bonds inside each molecule are strong, but only weak forces hold one molecule to the next.
分子晶体：由小而分开的分子组成——碘、富勒烯、冰；每个分子内部的键很强， 但分子与分子之间只有弱作用力。
Giant molecular, also called giant covalent: one unbroken network of atoms — silicon I V oxide, graphite, diamond — every single one covalently bonded to its neighbours.
原子晶体，也叫巨型共价结构： 一整片没有断口的原子网络——二氧化硅、石墨、金刚石，每一个原子都与邻居以共价键相连。
And giant metallic: positive metal ions sitting in a sea of delocalised electrons.
金属晶体：金属正离子浸在离域电子的海洋中。
Which answers our opening question.
这就回答了开头的问题。
Iodine is simple molecular.
碘是分子晶体。
The covalent bond inside each iodine molecule is strong — but it is not the bond you break when you melt it.
每个碘分子内部的共价键很强—— 但熔化时你破坏的并不是这个键。
You only pull the molecules apart, and the forces between them are weak, so iodine melts at one hundred and fourteen degrees.
你只是把分子彼此拉开， 而分子之间的作用力很弱，所以碘在一百一十四度就熔化了。
Diamond is giant molecular. Every carbon is bonded to four others in a rigid three-dimensional network, so melting it means breaking countless strong covalent bonds.
金刚石是原子晶体：每个碳都与另外四个碳在刚性的三维网络中成键， 所以熔化它意味着要断开无数个很强的共价键。
Graphite is carbon too, but each atom bonds to only three neighbours, forming flat layers that slide over each other — and its spare electron is delocalised, which is why graphite conducts and diamond does not.
石墨也是碳， 但每个原子只与三个邻居成键，形成可以相互滑动的平面层—— 而且它多余的那个电子是离域的，这就是石墨导电而金刚石不导电的原因。
So here is the table you must know — melting point, boiling point, electrical conductivity and solubility.
这就是你必须记住的表——熔点、沸点、导电性和溶解度。
Giant ionic: a high melting point, and it conducts only when molten or dissolved, because only then can the ions move. It usually dissolves in water.
离子晶体：熔点高； 只有熔化或溶解时才导电，因为只有那时离子才能移动；通常能溶于水。
Simple molecular: a low melting point, and it never conducts, because there are no free charges at all.
分子晶体：熔点低；从不导电，因为根本没有可自由移动的电荷。
Giant molecular: a very high melting point, and no conduction — with graphite as the one exception.
原子晶体：熔点非常高； 不导电——石墨是唯一的例外。
Giant metallic: a high melting point, and it conducts both as a solid and as a liquid, because those delocalised electrons are always free to move.
金属晶体：熔点高； 固态和液态都能导电，因为那些离域电子始终可以自由移动。
And you can read that table backwards, which is what the exam usually asks for.
而你也可以反过来读这张表，考试通常正是这样考的。
An unknown white solid melts at eight hundred degrees. It does not conduct when solid, but it does conduct once it is molten, and it dissolves in water. Take the clues one at a time.
一种未知的白色固体在八百度熔化；固态时不导电，但熔化后能导电，而且能溶于水。
A high melting point rules out simple molecular. Conducting only when molten rules out metallic. And dissolving in water rules out giant covalent.
把线索一条一条地看：熔点高，排除分子晶体；只有熔化后才导电，排除金属晶体； 能溶于水，排除原子晶体。
Only one structure fits all three: giant ionic.
三条线索都符合的只有一种结构：离子晶体。
Always quote the evidence for each step — not just the final answer.
每一步都要写出依据——而不只是给出最后的答案。
Three marks students throw away.
三个学生常丢的分。
First, in the ideal gas equation, use SI units every time: pascals, cubic metres and kelvin.
第一，用理想气体方程时，每次都要用国际单位： 帕、立方米、开尔文。
Forgetting to convert the volume is the single most common slip in this whole topic.
忘记换算体积，是这一章里最常见的失误。
Second, if you are asked about ideal behaviour, state both assumptions — no molecular volume, and no intermolecular forces — and say that real gases deviate at high pressure and low temperature.
第二，如果题目问理想行为，两条假设都要写出来——粒子不占体积、 粒子间没有作用力——并说明实际气体在高压低温下偏离最大。
Third, when you explain a melting point, name the force that breaks.
第三，解释熔点时，要说清楚断开的是哪一种作用力。
Iodine melts easily because weak intermolecular forces break, not because its covalent bonds are weak.
碘容易熔化，是因为断开的是弱的分子间作用力，而不是因为它的共价键弱。