Acids, Bases & Salts
IGCSE Chemistry Topic 7 13:06 English narration · English + 中文 subtitles burned in
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Lemon juice, vinegar, and the acid in your stomach — acids are everywhere.
柠檬汁、醋，还有你胃里的酸——酸无处不在。
So are their opposites: alkalis, like soap and oven cleaner.
它们的对立面也一样：碱，比如肥皂和烤箱清洁剂。
Mix an acid and an alkali, and they cancel each other out.
把酸和碱混合，它们会相互抵消。
Add a special dye, and the liquid changes colour to reveal which is which.
加一种特殊的染料，液体就会变色，显示出哪个是哪个。
Today we meet acids, bases, and the colourful salts they make together.
今天我们来认识酸、碱，以及它们一起生成的多彩的盐。
Welcome to acids, bases and salts.
欢迎来到酸、碱和盐。
Their reactions, the pH scale, and how to make a salt.
它们的反应、pH标度，以及怎样制备盐。
Let's begin.
让我们开始吧。
An acid is a substance that makes hydrogen ions when it dissolves in water.
酸是溶解在水中时生成氢离子的物质。
Acids have three key reactions to learn.
酸有三个关键反应要记。
Acid plus a metal gives a salt, plus hydrogen gas.
酸加金属，生成盐，加氢气。
Acid plus a base gives a salt, plus water — this one is called neutralisation.
酸加碱，生成盐，加水——这个叫中和。
And acid plus a carbonate gives a salt, plus water, plus carbon dioxide.
酸加碳酸盐，生成盐，加水，加二氧化碳。
In the exam, watch for observation marks: you see bubbles, called effervescence, and the solid dissolving — never write that hydrogen is given off, because you cannot see a gas.
考试时，注意现象分：你看到气泡，叫做泡腾，以及固体溶解——绝不要写放出氢气， 因为你看不见气体。
Look at the three branches carefully.
仔细看这三条分支。
Acid plus a metal makes a salt and hydrogen gas — for example, magnesium with hydrochloric acid.
酸加金属生成盐和氢气——例如镁和盐酸。
Acid plus a base makes a salt and water; that is neutralisation.
酸加碱生成盐和水； 那就是中和。
Acid plus a carbonate makes a salt, water, and carbon dioxide.
酸加碳酸盐生成盐、水和二氧化碳。
Naming the products is only half of what the exam wants.
说出产物只是考试要求的一半。
The other half is describing what you actually see with your eyes — the observations that earn their own marks.
另一半是描述你用眼睛实际看到的现象——这些现象分单独计分。
Learn the observation words.
记住现象用语。
Bubbles of gas coming off — say effervescence, or fizzing, or bubbling.
冒出气泡——说泡腾，或冒泡。
The solid getting smaller and vanishing — the solid dissolves or disappears.
固体变小并消失——固体溶解或消失。
If the solution changes colour, name the colour.
如果溶液变色，说出颜色。
When bubbling stops, say no more effervescence — the acid is used up.
当冒泡停止时，说不再泡腾——酸已用完。
For magnesium in hydrochloric acid: effervescence, and the solid dissolves, and the tube gets warm.
镁放入盐酸：泡腾，固体溶解，试管变热。
For black copper oxide in warm sulfuric acid: the solid dissolves and the solution turns blue.
黑色氧化铜放入温热硫酸： 固体溶解，溶液变蓝。
Never write hydrogen is given off as an observation — you cannot see hydrogen.
绝不要把放出氢气当作现象——你看不见氢气。
And never write a salt forms — that is a conclusion, not something you see.
也绝不要写生成了盐——那是结论，不是你看到的东西。
Here is neutralisation up close.
这是中和的近距离观察。
An acid contains hydrogen ions; an alkali contains hydroxide ions.
酸含有氢离子；碱含有氢氧根离子。
When you mix them, the hydrogen ion and the hydroxide ion join together to make a simple molecule of water.
当你把它们混合， 氢离子和氢氧根离子结合在一起，生成一个简单的水分子。
As the ions pair off, the solution becomes neutral — no longer acidic, no longer alkaline.
随着离子配对， 溶液变成中性——不再是酸性，也不再是碱性。
The metal and the acid's other part are left behind, dissolved, as the salt.
金属和酸的另一部分则以溶解的盐的形式留下来。
Write this ion picture into your notes.
把这幅离子图写进笔记。
Acids in water contain hydrogen ions.
酸的水溶液含有氢离子。
Alkalis in water contain hydroxide ions.
碱的水溶液含有氢氧根离子。
That is why indicators change colour: they respond to which of these ions dominates.
这就是指示剂变色的原因：它们响应哪种离子占主导。
And that is why neutralisation works: the hydrogen ions and the hydroxide ions join to make water.
这也是中和起作用的原因： 氢离子和氢氧根离子结合生成水。
Leave the symbols on the screen; say the names out loud.
符号留在屏幕上；口头说出名称。
How do we tell acids from alkalis?
我们怎样区分酸和碱？
With indicators — dyes that change colour.
用指示剂——会变色的染料。
Litmus turns red in acid and blue in alkali.
石蕊在酸中变红，在碱中变蓝。
But to measure how acidic something is, we use the pH scale, from zero to fourteen.
但要测量某物有多酸，我们用pH标度，从零到十四。
Universal indicator gives a whole rainbow: red for a strong acid, green at neutral pH seven, and blue or purple for an alkali.
通用指示剂给出一整条彩虹： 强酸是红色，中性pH七是绿色，碱是蓝色或紫色。
The rule to remember: the lower the pH, the more hydrogen ions, and the more acidic.
要记住的规则：pH越低， 氢离子越多，就越酸。
Here is litmus paper in the lab.
这是实验室里的石蕊试纸。
Blue litmus turns red in an acid.
蓝石蕊在酸中变红。
Red litmus turns blue in an alkali.
红石蕊在碱中变蓝。
Litmus is a yes-or-no test — acid or alkali — not a measure of how strong.
石蕊是是否判断——酸还是碱——不是强度测量。
For a full colour scale you need universal indicator, which we just met.
完整的颜色标度需要通用指示剂， 我们刚刚见过。
Keep both tools straight: litmus for type, universal for the number on the scale.
把两种工具分清：石蕊看类型，通用指示剂看标度上的数值。
The exam can name other indicators too.
考试也可能考其他指示剂。
Litmus is red in acid and blue in alkali.
石蕊在酸中红、在碱中蓝。
Thymolphthalein is colourless in acid and blue in alkali.
百里酚酞在酸中无色、在碱中蓝。
Methyl orange is red in acid and yellow in alkali.
甲基橙在酸中红、在碱中黄。
Learn the three pairs of colours.
记住这三对颜色。
Any of them can appear in a table question, so do not rely on litmus alone.
任何一种都可能出现在表格题中， 所以不要只依赖石蕊。
The pH scale runs from zero to fourteen.
pH标度从零到十四。
Below seven is acidic — universal indicator shows red, orange, or yellow.
低于七是酸性——通用指示剂显示红、橙或黄。
Exactly seven is neutral — green.
正好七是中性——绿色。
Above seven is alkaline — blue or purple.
高于七是碱性——蓝色或紫色。
A higher hydrogen ion concentration means a lower pH, so the solution is more acidic.
氢离子浓度越高，pH越低，溶液就越酸。
Strong acids sit at the low end; strong alkalis sit at the high end.
强酸在低端；强碱在高端。
Read the colour, then place the number.
先读颜色，再对应数字。
A word of warning: strong and weak do not mean concentrated and dilute.
一句提醒：强和弱不是指浓和稀。
They describe how much the acid splits into ions in water.
它们描述酸在水中分裂成离子的程度。
A strong acid, like hydrochloric acid, is completely dissociated — every molecule breaks into ions.
强酸，比如盐酸，完全分裂——每个分子都裂成离子。
A weak acid, like the ethanoic acid in vinegar, only splits a little — most of it stays as whole molecules, so we use a reversible arrow.
弱酸，比如醋里的乙酸， 只分裂一点点——大部分仍是完整的分子，所以我们用可逆箭头。
Strong and weak are about splitting, not about how much water you add.
强和弱是关于分裂， 而不是关于你加了多少水。
An acid is also a proton donor — a hydrogen ion is just a proton, so the acid gives away hydrogen ions.
酸也是质子供体——氢离子就是一个质子，所以酸给出氢离子。
A base is a proton acceptor.
碱是质子受体。
Strength is about dissociation: how fully the acid splits in water.
强度关乎电离：酸在水中分裂得有多完全。
Hydrochloric acid is strong, so its equation uses a one-way arrow to hydrogen ions and chloride ions.
盐酸是强酸，所以方程用单向箭头指向氢离子和氯离子。
Ethanoic acid is weak, so its equation uses a reversible arrow — only partly split.
乙酸是弱酸，所以方程用可逆箭头——只部分分裂。
Same concentration of two acids can still give different pH values if one is strong and one is weak.
两种酸浓度相同， 若一个强一个弱，仍可能给出不同的pH值。
Now the opposites of acids: bases.
现在来看酸的对立面：碱。
A base is a metal oxide or hydroxide.
碱是金属氧化物或氢氧化物。
A base that dissolves in water is called an alkali.
溶于水的碱叫做可溶性碱。
This links to oxides.
这和氧化物有关。
Metal oxides, like copper oxide, are basic.
金属氧化物，比如氧化铜，是碱性的。
Non-metal oxides, like carbon dioxide, are acidic.
非金属氧化物，比如二氧化碳， 是酸性的。
And a few special oxides, like aluminium oxide and zinc oxide, are amphoteric — they react with both acids and bases.
还有少数特殊的氧化物，比如氧化铝和氧化锌，是两性的——它们既与酸反应， 也与碱反应。
So remember: metals make basic oxides, non-metals make acidic ones.
所以记住：金属生成碱性氧化物，非金属生成酸性氧化物。
Bases have two typical reactions to learn.
碱有两个典型反应要记。
First, with an acid: base plus acid gives a salt plus water — neutralisation again.
第一，与酸：碱加酸生成盐加水——又是中和。
Second, with an ammonium salt: heating a base with an ammonium salt releases ammonia gas.
第二，与铵盐：把碱与铵盐一起加热，会放出氨气。
That second reaction is a classic test for ammonium compounds and a classic way to make ammonia in the lab.
第二个反应是检验铵盐的经典方法， 也是实验室制氨的经典途径。
Do not skip it when you revise bases.
复习碱时不要跳过它。
Dip universal indicator paper into an alkali and you get blue or purple — pH greater than seven.
把通用指示剂试纸浸入碱液，你会得到蓝色或紫色——pH大于七。
Soap, oven cleaner, and sodium hydroxide solution all sit on that alkaline side.
肥皂、烤箱清洁剂和氢氧化钠溶液都在碱性这一侧。
Remember: every alkali is a base, but not every base is an alkali.
记住：每个可溶性碱都是碱， 但不是每个碱都是可溶性碱。
Copper oxide is a base, yet it does not dissolve in water, so it is not an alkali.
氧化铜是碱，却不溶于水，所以它不是可溶性碱。
Limestone is mostly calcium carbonate.
石灰石主要是碳酸钙。
A metal carbonate acts as a base: it neutralises acids, making a salt, water, and carbon dioxide.
金属碳酸盐充当碱：它中和酸，生成盐、水和二氧化碳。
That is why acid rain eats into limestone buildings, and why farmers add limestone to acidic soil.
这就是为什么酸雨腐蚀石灰石建筑，以及为什么农民把石灰石加到酸性土壤中。
Oxides of metals such as calcium oxide are basic; carbon dioxide and sulfur dioxide are acidic oxides of non-metals.
钙等金属的氧化物是碱性的；二氧化碳和二氧化硫是非金属的酸性氧化物。
Amphoteric oxides, aluminium oxide and zinc oxide, sit in the middle and react with both.
两性氧化物氧化铝和氧化锌居中，既与酸也与碱反应。
To make a salt, you first need to know if it dissolves.
要制备盐，你首先需要知道它是否溶解。
Here are the rules.
规则如下。
All sodium, potassium, and ammonium salts are soluble.
所有钠盐、钾盐、铵盐都可溶。
All nitrates are soluble.
所有硝酸盐都可溶。
Most chlorides are soluble, except silver and lead.
大多数氯化物可溶，除了银和铅。
Most sulfates are soluble, except barium, calcium, and lead.
大多数硫酸盐可溶，除了钡、钙和铅。
But most carbonates are insoluble, and so are most hydroxides.
但大多数碳酸盐不溶，大多数氢氧化物也不溶。
Learn these — they decide how you make the salt.
记住这些——它们决定你怎样制备盐。
Drill the exceptions until they stick.
把例外练到记牢。
Sodium, potassium, ammonium, and all nitrates are always soluble — no exceptions.
钠、钾、铵以及所有硝酸盐永远可溶——没有例外。
Chlorides are soluble except lead and silver.
氯化物可溶，除了铅和银。
Sulfates are soluble except barium, calcium, and lead.
硫酸盐可溶，除了钡、钙和铅。
Carbonates and hydroxides are insoluble, except the sodium, potassium, and ammonium ones — and calcium hydroxide is partly soluble.
碳酸盐和氢氧化物不溶，除了钠、钾、铵的——氢氧化钙部分可溶。
Those exceptions decide precipitation products in the next steps.
这些例外决定了后面沉淀产物是什么。
The method depends on whether the salt is soluble or insoluble.
方法取决于盐是可溶还是不溶。
For a soluble salt, react an acid with an excess of a solid — a metal, a base, or a carbonate.
对可溶的盐，让酸与过量的固体反应——金属、碱或碳酸盐。
Then filter off the leftover solid, evaporate some water, and let the solution crystallise.
然后过滤掉剩余的固体，蒸发掉一些水，让溶液结晶。
For an insoluble salt, use precipitation: mix two solutions that each carry one of the ions you need, and the salt drops out as a solid.
对不溶的盐，用沉淀法： 把两种溶液混合，每种带有你需要的一种离子，盐就以固体形式析出。
Then filter, wash, and dry it.
然后过滤、洗涤、干燥。
Follow the four steps for a soluble salt from an insoluble solid.
用不溶固体制备可溶盐，按四步走。
One: add excess solid to the acid — a metal, an insoluble base, or an insoluble carbonate — so all the acid is used up.
一：向酸中加过量固体——金属、不溶碱或不溶碳酸盐—— 让酸全部用完。
Two: filter to remove the leftover solid.
二：过滤除去剩余固体。
Three: evaporate some of the water to concentrate the solution.
三：蒸发掉一些水以浓缩溶液。
Four: leave the solution to crystallise.
四：让溶液结晶。
Excess solid is deliberate: if acid is left over, it contaminates the crystals.
过量固体是故意的：如果留下酸，会污染晶体。
There is another route to a soluble salt: titration.
制备可溶盐还有另一条路：滴定。
Use it when both reactants are solutions — an acid and an alkali.
当两种反应物都是溶液——酸和碱——时使用。
You cannot add excess alkali the way you add excess solid, so you must measure the exact volumes that neutralise each other, using an indicator.
你不能像加过量固体那样加过量碱，所以必须用量准确的体积使它们恰好中和，并用指示剂。
Then take a pure sample without the indicator colour, evaporate and crystallise.
然后取一份不含指示剂颜色的纯样品，蒸发并结晶。
Titration for two solutions; excess solid for a solid reactant.
两种溶液用滴定； 有固体反应物用过量固体法。
These bright blue crystals are copper sulfate.
这些亮蓝色晶体是硫酸铜。
Slow crystallisation gives well-shaped crystals.
缓慢结晶能得到形状良好的晶体。
If you boil the solution dry too fast, you get a powder of tiny crystals instead.
如果把溶液煮干太快，只会得到细小晶体粉末。
In the method: after filtering off excess solid, evaporate only some of the water, then leave the dish to cool so crystals can grow.
在方法中：过滤掉过量固体后， 只蒸发掉一部分水，然后让器皿冷却，晶体才能长大。
That is the salt you set out to make.
那就是你要制备的盐。
An insoluble salt is made by precipitation.
不溶盐用沉淀法制备。
Mix two solutions that each contain one of the needed ions.
把两种溶液混合，每种含有所需离子中的一种。
The insoluble salt forms as a solid and sinks or clouds the mixture.
不溶盐以固体形式生成，下沉或使混合物变浑浊。
Then filter off the solid, wash it with distilled water to remove leftover soluble ions, and dry it.
然后过滤出固体， 用蒸馏水洗涤除去剩余可溶离子，再干燥。
You choose the two solutions so that the other salt formed stays dissolved and washes away.
选择两种溶液时，要让另一种生成的盐保持溶解并被洗走。
Worked example.
例题。
Which method would you use to prepare copper sulfate, and which for barium sulfate?
你会用哪种方法制备硫酸铜，哪种制备硫酸钡？
Check the solubility rules first.
先查溶解度规则。
Copper sulfate is soluble — only barium, calcium and lead sulfates are not — so make it from an insoluble solid and an acid: add excess copper oxide to warm dilute sulfuric acid, filter off the leftover solid, then evaporate some water and crystallise.
硫酸铜可溶——只有钡、钙、铅的硫酸盐不溶——所以用不溶固体和酸制备： 向温热的稀硫酸中加过量氧化铜，过滤剩余固体，再蒸发一些水并结晶。
Barium sulfate is insoluble, so make it by precipitation: mix two solutions that each carry one of the needed ions, then filter, wash and dry the solid.
硫酸钡不溶，所以用沉淀法：混合两种各带一种所需离子的溶液，然后过滤、洗涤并干燥固体。
The solubility of the salt you want decides the method.
你要的盐的溶解度决定方法。
One last salt idea: water inside the crystal.
最后一个盐的概念：晶体里的水。
A hydrated substance is chemically joined with water.
水合物质与水化学结合。
An anhydrous substance contains no water.
无水物质不含水分。
The water molecules locked inside hydrated crystals are called water of crystallisation.
锁在水合晶体里的水分子叫做结晶水。
Blue copper sulfate is a classic hydrated salt — five water molecules for each formula unit.
蓝色硫酸铜是经典的水合盐—— 每个式量单位有五个水分子。
Gently heat it and the water leaves, and the solid turns white and anhydrous.
轻轻加热，水离开，固体变白成为无水物。
Add water back and the blue colour returns.
再加水，蓝色又回来。
Three marks to lock in.
锁住三个分。
First: learn the three acid reactions — with a metal gives hydrogen, with a base gives water, with a carbonate gives carbon dioxide.
第一：记住三个酸的反应——加金属生成氢气，加碱生成水， 加碳酸盐生成二氧化碳。
Second: strong and weak are about how much the acid splits into ions, not how concentrated it is.
第二：强和弱是关于酸分裂成离子的程度，而不是它有多浓。
Third: pick the salt method by solubility — soluble means acid plus excess solid, then filter and crystallise; insoluble means precipitation.
第三：按溶解度选择制盐方法——可溶就用酸加过量固体，然后过滤、结晶； 不溶就用沉淀法。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。