Group 2
A-Level Chemistry Topic 10 8:01 English narration · English + 中文 subtitles burned in
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Before a stomach X-ray, the patient is asked to drink a thick white liquid.
做胃部X光检查之前，医生会让病人喝下一种浓稠的白色液体。
It is full of barium — and barium ions are poisonous.
它含有大量的钡——而钡离子是有毒的。
Swallow a soluble barium salt and it can kill you.
吞下可溶性的钡盐，可能会致命。
Yet this drink is completely safe, and on the X-ray the gut glows bright white.
可是这杯饮料却完全安全，而且在X光片上，肠道会显出明亮的白色。
One single idea explains the whole trick, and it lives here in Group 2.
只用一个概念就能解释这个把戏，而它就藏在第二族里。
Welcome to Group 2, the alkaline earth metals.
欢迎来到第二族——碱土金属。
Today: how these metals react, how their compounds fall apart when you heat them, and the two solubility trends the exam asks for every year.
今天我们要讲：这些金属如何反应， 它们的化合物受热时如何分解，以及考试年年都考的那两条溶解度规律。
Let's begin.
让我们开始吧。
Start with the atom.
先从原子说起。
Every metal in Group 2 has exactly two electrons in its outer shell.
第二族的每一种金属，最外层都正好有两个电子。
Magnesium, calcium, strontium, barium — all the same.
镁、钙、锶、钡——全都一样。
In a reaction the metal loses both of them, and what is left is an ion with two positive charges.
反应时金属把这两个电子都失去， 剩下的就是一个带两个正电荷的离子。
That never changes down the group.
沿着族向下，这一点从不改变。
So every Group 2 formula you write must use that ion.
所以你写的每一个第二族化学式，都必须用这个离子。
Get the charge wrong and the whole equation is wrong.
电荷写错，整个方程式就全错了。
Now go down the group.
现在沿着族往下走。
One thing changes: size.
有一件事在变：原子的大小。
From magnesium to barium the atom gets bigger, so the two outer electrons sit further from the nucleus, with more full shells shielding them.
从镁到钡，原子越来越大， 最外层的两个电子离原子核更远，中间还有更多充满的电子层起屏蔽作用。
The pull holding them is weaker, so the ionisation energy falls.
对它们的吸引更弱，所以电离能下降。
A weaker hold means the electrons come off more easily — and that is why the metals get more reactive as you go down.
吸引得越弱，电子就越容易失去—— 这正是沿族向下金属越来越活泼的原因。
That one sentence explains almost every trend in this topic.
这一句话几乎能解释本专题里所有的趋势。
So how do they react?
那么它们怎么反应呢？
With oxygen they burn to form an oxide.
与氧气反应时，它们燃烧生成氧化物。
Magnesium gives that famous dazzling white flame, and white magnesium oxide powder is left behind.
镁会发出著名的耀眼白光，留下白色的氧化镁粉末。
With water they give a hydroxide and hydrogen gas, more vigorous as you go down.
与水反应时，生成氢氧化物和氢气，沿族向下更剧烈。
Magnesium is the exception you must know: it is very slow in cold water, but in steam it reacts fast — and there the product is the oxide, not the hydroxide.
镁是你必须记住的例外：它在冷水中非常慢，但在水蒸气中反应很快—— 而且这时的产物是氧化物，不是氢氧化物。
With dilute hydrochloric acid, or with dilute sulfuric acid, you get a salt and hydrogen.
与稀盐酸或稀硫酸反应，都会生成盐和氢气。
State the observations, because they carry marks: the mixture fizzes and the metal slowly disappears.
现象一定要写出来，因为那是有分的：混合物冒泡，金属慢慢消失。
But sulfuric acid hides a trap.
但硫酸藏着一个陷阱。
Down the group the sulfates become insoluble, so a solid layer of sulfate coats the metal surface and blocks the acid.
沿族向下，硫酸盐变得难溶， 于是金属表面会积起一层固体硫酸盐，把酸挡住。
With sulfuric acid the reaction therefore gets slower down the group, not faster.
所以用硫酸时，沿族向下反应反而变慢，而不是变快。
Now the compounds.
现在讲化合物。
The oxides and the hydroxides of Group 2 are bases.
第二族的氧化物和氢氧化物都是碱。
Watch what a base does to an acid.
看看碱对酸做了什么。
Hydroxide ions meet the hydrogen ions from the acid, and each pair joins to make one water molecule.
氢氧根离子遇上酸里的氢离子，每一对结合成一个水分子。
As the hydrogen ions disappear, the pH climbs towards seven.
随着氢离子消失，pH 向 7 上升。
That is neutralisation, and it is why every Group 2 oxide and hydroxide reacts with dilute acid to give a salt and water.
这就是中和反应—— 也正因如此，每一种第二族的氧化物和氢氧化物都能与稀酸反应，生成盐和水。
Three equations to learn.
有三个方程式必须记住。
An oxide plus water gives the hydroxide.
氧化物加水，生成氢氧化物。
A hydroxide plus acid gives a salt and water — the neutralisation you just watched.
氢氧化物加酸，生成盐和水——就是你刚才看到的中和反应。
A carbonate plus acid gives a salt, water and carbon dioxide, and that one has a give-away observation: it fizzes, and the gas turns limewater milky.
碳酸盐加酸，生成盐、水和二氧化碳，而这一个有一个很明显的现象： 冒泡，而且气体能使石灰水变浑浊。
Balance them carefully. The metal ion carries two charges, so it needs two chlorides, and that means two moles of acid.
配平要小心： 金属离子带两个正电荷，所以要配两个氯，也就是要用两摩尔的酸。
Now heat them.
现在给它们加热。
Thermal decomposition means using heat to break one compound into simpler ones.
热分解的意思是用加热把一种化合物拆成更简单的物质。
Heat a Group 2 carbonate and it splits into the metal oxide and carbon dioxide.
加热第二族的碳酸盐，它会分解成金属氧化物和二氧化碳。
Heat a Group 2 nitrate and you get three products: the metal oxide, oxygen, and brown nitrogen dioxide gas.
加热第二族的硝酸盐，会得到三种产物：金属氧化物、氧气， 以及棕色的二氧化氮气体。
That brown gas is a classic exam answer — if the question asks what you see, say a brown gas is given off.
那种棕色气体是经典的考试答案—— 如果题目问你看到什么，就答：放出棕色气体。
But not every carbonate needs the same heat.
但并不是每种碳酸盐都需要同样的热量。
Here is why.
原因在这里。
A small metal ion packs its two positive charges into a tiny space, so its charge density is high.
小的金属离子把两个正电荷挤在很小的空间里，所以电荷密度很高。
It pulls hard on the electrons of the carbonate ion and distorts it — we say it polarises the anion.
它强烈地拉扯碳酸根离子的电子，把它扭曲——我们说它极化了这个阴离子。
A distorted carbonate has a weakened bond, so it breaks apart easily.
被扭曲的碳酸根离子，键被削弱了，所以很容易断裂。
A big ion like barium spreads the same charge over far more space, pulls gently, and barely disturbs the carbonate at all.
而像钡这样的大离子，把同样的电荷分散在大得多的空间里， 拉扯很温和，几乎不会扰动碳酸根。
Let's use that.
我们来用一用。
Magnesium carbonate decomposes at about five hundred and forty degrees. Barium carbonate needs about one thousand three hundred and sixty.
碳酸镁大约在五百四十摄氏度分解， 碳酸钡则需要大约一千三百六十摄氏度。
Explain the trend, and predict calcium carbonate.
请解释这个趋势，并预测碳酸钙。
Pause here and try it yourself.
先暂停，自己试一试。 好了吗？
Ready? Argue the whole chain — cation size, then polarising power, then distortion of the anion.
要把整条推理链写全——阳离子尺寸，然后极化能力， 然后对阴离子的扭曲。
One: going down the group the cation gets larger.
第一，沿族向下，阳离子变大。
Two: the same charge is spread out, so the charge density and polarising power fall.
第二，同样的电荷被分散开， 所以电荷密度和极化能力都下降。
Three: less distortion of the anion, so more heat is needed.
第三，对阴离子的扭曲变小，因此需要更多热量。
Thermal stability increases down the group, and calcium sits between magnesium and barium — so about nine hundred degrees.
热稳定性沿族向下增大；而钙位于镁和钡之间——所以大约九百摄氏度。
The chain earns the marks, not the number.
得分靠的是这条链，不是那个数字。
And that prediction is not only an exam answer.
这个预测不只是一个考试答案。
Limestone is calcium carbonate, and a lime kiln holds it at roughly nine hundred degrees — exactly the temperature we just predicted.
石灰石就是碳酸钙， 而石灰窑把它保持在大约九百摄氏度——正好是我们刚才预测的温度。
The carbon dioxide escapes and calcium oxide is left: quicklime, one of the most used industrial chemicals in the world.
二氧化碳跑掉，留下氧化钙：也就是生石灰， 世界上用量最大的工业化学品之一。
It goes into cement, into steel making, and into treating acid soil.
它用于水泥、炼钢，还用来改良酸性土壤。
Two more trends, and they run in opposite directions.
还有两条趋势，而且方向相反。
Going down the group, the hydroxides increase — they get more soluble.
沿族向下，氢氧化物越来越易溶。
Magnesium hydroxide is almost insoluble — that is the chalky white medicine for indigestion — while barium hydroxide dissolves well.
氢氧化镁几乎不溶——那正是治胃酸的白色药糊——而氢氧化钡溶解得很好。
The sulfates do the opposite: solubility decreases down the group, so they get less soluble.
硫酸盐正好相反：溶解度沿族向下减小，所以越来越难溶。
Magnesium sulfate dissolves easily, but barium sulfate hardly dissolves at all.
硫酸镁很容易溶解， 而硫酸钡几乎完全不溶。
Hydroxides up, sulfates down.
氢氧化物向上，硫酸盐向下。
Learn it as one line, and half the solubility questions in this topic are already answered.
就记这一句，本专题里一半的溶解度题就已经答完了。
So back to that drink.
回到那杯饮料。
Barium sulfate is the least soluble sulfate in the group, so almost no barium ion ever reaches the blood.
硫酸钡是这一族里最难溶的硫酸盐， 所以几乎没有钡离子会进入血液。
The patient is safe, and because barium is a heavy atom the drink blocks X-rays and shows up bright white.
病人是安全的； 而且因为钡原子很重，这杯饮料能挡住X射线，在片子上显出明亮的白色。
The same insolubility gives you a test. Add barium ions to an unknown solution: if a white precipitate appears, sulfate ions are there.
同样的难溶性还给了你一个检验方法：向未知溶液中加入钡离子， 如果出现白色沉淀，就说明其中有硫酸根离子。
One trend, two completely different uses.
同一条规律，两种完全不同的用途。
One more exam skill: using a trend to predict.
还有一项考试技能：用趋势去做预测。
Radium sits below barium, so just continue every line.
镭位于钡的下面， 所以只要把每一条规律继续延伸就行。
It will be the most reactive of them all, and fastest with water.
它会是其中最活泼的，与水反应最快。
Its hydroxide will be the most soluble of the group.
它的氢氧化物会是全族最易溶的。
Its sulfate will be the least soluble.
它的硫酸盐会是最难溶的。
And its carbonate will need the highest temperature to decompose.
而它的碳酸盐需要最高的温度才能分解。
You never learned radium — you worked it out.
你从没学过镭——你是自己推出来的。
Three marks students throw away.
三个学生常丢的分。
First, when you explain reactivity, name the reason: the atoms get bigger, so the ionisation energy falls.
第一，解释活泼性时要写出原因： 原子变大，所以电离能下降。
Second, for thermal stability never write only that it is more stable.
第二，讲热稳定性时，绝不要只写"更稳定"。
Give the chain: bigger ion, lower charge density, less distortion of the carbonate.
要把链条写全：离子更大、电荷密度更低、对碳酸根的扭曲更小。
Third, keep the two solubility trends apart — hydroxides more soluble down the group, sulfates less.
第三，两条溶解度趋势不要弄混——沿族向下，氢氧化物更易溶，硫酸盐更难溶。