Experimental Techniques
IGCSE Chemistry Topic 12 13:26 English narration · English + 中文 subtitles burned in
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You are handed an unknown white powder and a colourless solution. No labels.
有人递给你一种未知的白色粉末和一瓶无色溶液，没有标签。
How do you find out what they are?
你怎么弄清它们是什么？
Chemistry gives you a detective's toolkit: add a reagent, then watch for a colour, a precipitate, or a gas.
化学给了你一套侦探工具：加入试剂，然后观察颜色、沉淀或气体。
Each clue narrows it down.
每一条线索都把范围缩小。
In this topic you learn to measure carefully, to separate mixtures, and to run the tests that identify exactly what you have.
在这个专题里，你将学会精确测量、分离混合物，以及做出准确鉴定所需的各种检验。
Welcome to experimental techniques.
欢迎来到实验技术。
Apparatus, titrations, chromatography, separation, and the tests for ions and gases.
仪器、滴定、色谱法、分离，以及离子和气体的检验。
Let's begin.
让我们开始吧。
First, choose the right tool for the job.
首先，为每项工作选对工具。
For time, a stop-watch.
测时间用秒表。
For temperature, a thermometer.
测温度用温度计。
For mass, a balance.
测质量用天平。
For an accurate volume, a burette or a volumetric pipette — but for a rough volume, a measuring cylinder is enough.
测准确的体积用滴定管或移液管——但测粗略的体积，量筒就够了。
For the volume of a gas, a gas syringe.
测气体体积用注射器。
And learn these words: the solvent does the dissolving, the solute is what dissolves, and together they make a solution.
还要记住这些词：溶剂负责溶解，溶质是被溶解的东西，两者合起来构成溶液。
After filtering, the solid left behind is the residue, and the liquid that passes through is the filtrate.
过滤之后，留下的固体是残渣，通过滤纸的液体是滤液。
One more key word is easy to miss.
还有一个关键词容易漏掉。
A saturated solution holds the maximum amount of solute that will dissolve at a given temperature.
饱和溶液在给定温度下溶解了能溶解的最大量溶质。
Warm the mixture and more solid can often dissolve; cool it again and crystals may form.
加热混合物，往往能溶解更多固体；再冷却，就可能析出晶体。
That idea underpins crystallisation later in this topic.
这个想法支撑着后面要讲的结晶法。
A titration finds the exact volume of one solution that reacts with another.
滴定用来求出一种溶液与另一种恰好反应所需的准确体积。
Use a volumetric pipette to put a fixed volume into a conical flask.
用移液管把固定体积移入锥形瓶。
Add a few drops of indicator.
滴几滴指示剂。
Then run the second solution in from a burette, slowly, swirling as you go.
然后从滴定管慢慢滴入第二种溶液，边滴边摇。
The moment the indicator just changes colour is the end-point — the reaction is exactly complete.
指示剂刚刚变色的那一刻就是终点——反应恰好完成。
Read the burette, and you have your volume.
读取滴定管，你就得到了体积。
Look at the set-up.
看这套装置。
The burette is clamped upright above a conical flask.
滴定管竖直夹在锥形瓶上方。
The flask holds a known volume of solution plus a few drops of indicator.
瓶里是已知体积的溶液和几滴指示剂。
Often the flask stands on a white tile so the colour change is easy to see.
瓶下常垫白色瓷砖，方便看清颜色变化。
You run solution down from the burette until the indicator just changes colour — that is the end-point.
从滴定管往下滴加溶液，直到指示剂刚刚变色—— 那就是终点。
Then you read the burette scale carefully to find the volume that was added.
然后仔细读取滴定管刻度，得到加入的体积。
Here is a real titration in the lab.
这是实验室里真实的滴定。
Several burettes hang above conical flasks.
几支滴定管挂在锥形瓶上方。
In this flask the indicator has already turned orange — you swirl gently as you add drops, and stop the moment the colour stays changed.
这个瓶里指示剂已经变成橙色—— 边滴边轻轻摇，颜色一稳定变掉就停。
Never race through the last few drops; the end-point is a single drop late or early.
最后几滴不要抢；终点就差一滴。
Record the initial and final burette readings, then subtract to get the titre volume.
记录滴定管的初读数和终读数，相减就得到滴定体积。
Paper chromatography separates a mixture of soluble substances.
纸色谱法分离可溶物质的混合物。
Put a spot of the mixture on a pencil baseline near the bottom of the paper, then stand the paper in solvent.
在纸靠近底部的铅笔基线上点一滴混合物， 然后把纸立在溶剂中。
As the solvent rises, it carries the substances with it — but each one travels a different distance, so they separate into spots.
溶剂上升时会带着这些物质一起走——但每一种走的距离不同， 所以它们分离成一个个斑点。
Coloured substances show up by themselves; colourless ones need a locating agent sprayed on.
有色物质自己就能显现；无色的则需要喷显色剂。
The finished paper is a chromatogram, and a pure substance gives just one spot.
做好的纸叫色谱图，纯净物只给出一个斑点。
On this diagram the solvent climbs from the bottom.
在这张图上，溶剂从底部向上爬。
The mixture starts as a spot on the pencil baseline.
混合物先以斑点落在铅笔基线上。
Different dyes travel different heights and stop as separate spots below the solvent front.
不同染料走的高度不同， 在溶剂前沿下方停成一个个斑点。
The distance from the baseline to the centre of a spot is letter a.
从基线到斑点中心的距离记为甲。
The distance from the baseline to the solvent front is letter b.
从基线到溶剂前沿的距离记为乙。
You will use a over b to find the R-f value next.
下一步用甲除以乙求Rf值。
Real chromatograms look like this.
真实的色谱图就像这样。
Each strip started with a mixture near the bottom.
每条纸条底部先点了混合物。
As the solvent rose, the dyes separated into coloured spots at different heights.
溶剂上升时，染料分离成不同高度的彩色斑点。
You can identify an unknown by running known pure samples beside it on the same paper — matching height and colour means the same substance.
你可以把已知纯样品和未知物点在同一张纸上对照——高度和颜色一致就说明是同一种物质。
A pure sample leaves only one spot; a mixture leaves more than one.
纯样品只留下一个斑点；混合物会留下多个。
Two exam uses of a chromatogram.
色谱图在考试中有两个用途。
First: identify an unknown by comparing its spots with known substances run under the same conditions.
第一：在相同条件下与已知物质对照，鉴定未知物。
Second: check purity — a pure substance gives a single spot; two or more spots mean a mixture.
第二：检查纯度——纯净物只给出一个斑点；两个或更多斑点说明是混合物。
Remember the baseline must be pencil, never ink, or the ink itself will rise with the solvent and spoil the result.
记住基线必须用铅笔，不能用墨水，否则墨水自己也会随溶剂上升，毁掉结果。
And keep the baseline above the solvent level so the spots do not dissolve into the liquid below.
还要把基线放在溶剂液面之上，免得斑点溶进下面的液体。
Each spot has an R-f value: the distance moved by the substance, divided by the distance moved by the solvent.
每个斑点都有一个Rf值：物质移动的距离除以溶剂移动的距离。
Both are measured from the pencil baseline, and to the centre of the spot.
两者都从铅笔基线量起， 并量到斑点的中心。
Here the solvent front rose eight centimetres and the spot moved six, so R-f is six over eight, which is nought point seven five.
这里溶剂前沿上升了八厘米，斑点移动了六厘米，所以Rf是六除以八， 等于零点七五。
R-f has no units, and it is always less than one — because a spot can never travel further than the solvent carrying it.
Rf没有单位，而且总是小于一——因为斑点不可能走得比带着它的溶剂更远。
If you get more than one, you divided the wrong way round.
如果你算出大于一，就是除反了。
Choosing a separation method depends on the mixture.
选择分离方法取决于混合物。
To get an insoluble solid out of a liquid, use filtration — the solid stays on the paper as the residue.
要把不溶的固体从液体中取出，用过滤—— 固体作为残渣留在滤纸上。
To get a soluble solid back from its solution, use crystallisation.
要从溶液中取回可溶的固体，用结晶。
To get the solvent itself back from a solution, use simple distillation.
要从溶液中取回溶剂本身，用蒸馏。
And to separate two liquids with different boiling points, use fractional distillation.
要分离两种沸点不同的液体，用分馏。
You can also check purity: a pure substance melts and boils at sharp fixed temperatures, while impurities lower the melting point and raise the boiling point.
你还可以检验纯度：纯净物在尖锐、固定的温度熔化和沸腾，而杂质会降低熔点、升高沸点。
Filtration separates an insoluble solid from a liquid.
过滤把不溶的固体从液体中分开。
Fold filter paper into a cone, place it in a funnel, and pour the mixture through.
把滤纸折成圆锥，放进漏斗，把混合物倒进去。
The solid stays on the paper — that is the residue.
固体留在滤纸上——那就是残渣。
The clear liquid that drips into the beaker below is the filtrate.
滴入下方烧杯的清液是滤液。
Use filtration when the solid will not dissolve in the liquid.
固体不溶于液体时用过滤。
If the solid is soluble, filtration cannot recover it from the solution.
如果固体可溶，过滤无法从溶液中把它取回。
To recover a soluble solid from its solution, use crystallisation instead.
要从溶液中取回可溶固体，改用结晶。
Warm the solution gently so solvent evaporates and the solution becomes saturated — it holds as much dissolved solid as it can at that temperature.
轻轻加热溶液，让溶剂蒸发，溶液变成饱和—— 在该温度下已溶解了尽可能多的固体。
Then cool it.
然后冷却。
Crystals form as the solution can no longer hold all the solute.
溶液装不下全部溶质时，晶体就会析出。
You can filter off the crystals, wash them with a little cold solvent, and dry them.
可以滤出晶体，用少量冷溶剂洗涤，再干燥。
Filtration alone would not work here, because the solid is dissolved, not suspended.
单靠过滤在这里不行，因为固体是溶解的，不是悬浮的。
Simple distillation recovers the solvent from a solution.
简单蒸馏从溶液中取回溶剂。
Heat the flask so the liquid boils.
加热烧瓶使液体沸腾。
Vapour rises into a water-cooled condenser, cools back to liquid, and drips into a collecting beaker as the distillate.
蒸汽升入水冷冷凝管，冷却变回液体， 作为馏出液滴入收集烧杯。
The dissolved solid stays behind in the flask.
溶解的固体留在烧瓶里。
A thermometer checks that you collect at the correct boiling point.
温度计用来确认在正确沸点收集。
Use this method when you want the pure liquid, not the solid.
当你要的是纯液体而不是固体时，用这个方法。
Fractional distillation separates two or more liquids with different boiling points.
分馏用来分离两种或更多沸点不同的液体。
The extra piece is a fractionating column packed with glass beads, fitted between the flask and the condenser.
多出来的部件是装有玻璃珠的分馏柱， 装在烧瓶和冷凝管之间。
Vapour rises and condenses repeatedly on the beads.
蒸汽在珠子上反复上升和冷凝。
The liquid with the lower boiling point reaches the top first and is collected.
沸点较低的液体先到顶部并被收集。
Watch the thermometer at the top of the column — when the temperature jumps, switch receivers and collect the next fraction.
注意柱顶的温度计——温度一跳，就换接收器，收集下一段馏分。
After you separate a product, check how pure it is.
分离出产物后，检查它有多纯。
A pure substance melts and boils at sharp fixed temperatures — the whole sample changes at one clear value.
纯净物在尖锐、固定的温度熔化和沸腾——整份样品在一个明确的值上变化。
Impurities lower the melting point and raise the boiling point, and they make those ranges wider and less sharp.
杂质会降低熔点、升高沸点，还会让这些范围变宽、变钝。
So a melting point that is too low, or a boiling point that is too high, is a warning that the sample is not pure yet.
所以熔点偏低或沸点偏高，都在警告样品还不够纯。
Now the tests.
现在说检验。
For anions — negative ions.
先看阴离子——带负电的离子。
Add dilute acid to a carbonate and it fizzes, and the gas turns limewater milky.
往碳酸盐里加稀酸会冒泡， 气体使石灰水变浑浊。
For the halides, add dilute nitric acid then silver nitrate: chloride gives a white precipitate, bromide cream, and iodide yellow.
对于卤化物，先加稀硝酸再加硝酸银：氯化物生成白色沉淀， 溴化物是乳白色，碘化物是黄色。
For a sulfate, add nitric acid then barium nitrate for a white precipitate.
对于硫酸盐，加硝酸再加硝酸钡，生成白色沉淀。
For cations — positive ions — add sodium hydroxide and read the precipitate colour: copper is light blue, iron two is green, and iron three is red-brown.
再看阳离子——带正电的离子——加氢氧化钠，看沉淀的颜色：铜是浅蓝色， 二价铁是绿色，三价铁是红棕色。
Aluminium and zinc both give a white precipitate that dissolves in excess.
铝和锌都生成白色沉淀，且在过量时溶解。
Two more anion tests complete the set.
再补两个阴离子检验就齐了。
For a nitrate, add aluminium foil and sodium hydroxide, then warm.
对于硝酸盐，加铝箔和氢氧化钠，然后加热。
Ammonia gas is given off — check it with damp red litmus, which turns blue.
会放出氨气——用湿润的红色石蕊试纸检验，试纸变蓝。
For a sulfite, add acidified potassium manganate seven: the purple colour fades.
对于亚硫酸盐，加酸化的高锰酸钾： 紫色褪去。
Do not confuse sulfate with sulfite.
不要把硫酸盐和亚硫酸盐搞混。
Sulfate gives a white precipitate with barium nitrate; sulfite decolourises the purple manganate.
硫酸盐与硝酸钡生成白色沉淀； 亚硫酸盐使紫色高锰酸盐褪色。
For cations, add aqueous sodium hydroxide, or aqueous ammonia, and watch the precipitate.
对于阳离子，加氢氧化钠水溶液或氨水，观察沉淀。
Copper two gives a light blue precipitate with sodium hydroxide; with ammonia the light blue dissolves to deep blue.
二价铜与氢氧化钠生成浅蓝色沉淀； 与氨水时浅蓝色溶解成深蓝色。
Iron two is green and stays; iron three is red-brown and stays.
二价铁是绿色且不溶；三价铁是红棕色且不溶。
Chromium three is green and dissolves in excess sodium hydroxide, but stays with ammonia.
三价铬是绿色，过量氢氧化钠中溶解，但在氨水中不溶。
Calcium gives a white precipitate with sodium hydroxide that stays, but no precipitate with ammonia.
钙与氢氧化钠生成不溶的白色沉淀， 但与氨水无沉淀。
Ammonium gives no coloured precipitate — warm with sodium hydroxide and ammonia gas comes off.
铵没有彩色沉淀——与氢氧化钠加热会放出氨气。
Aluminium and zinc both give a white precipitate that dissolves in excess sodium hydroxide.
铝和锌都生成白色沉淀，且在过量氢氧化钠中溶解。
So how do you tell aluminium from zinc?
那怎么区分铝和锌？
Switch to aqueous ammonia.
改用氨水。
Zinc's white precipitate also dissolves in excess ammonia.
锌的白色沉淀在过量氨水中也会溶解。
Aluminium's white precipitate stays — it does not dissolve in excess ammonia.
铝的白色沉淀留下——它不溶于过量氨水。
Same first step with sodium hydroxide; the ammonia column is what separates them.
氢氧化钠这一步相同；真正把它们分开的是氨水那一列。
Five gas tests you must know.
有五个气体检验你必须知道。
Hydrogen: a lighted splint gives a squeaky pop.
氢气：点燃的木条会发出尖锐的爆鸣声。
Oxygen: a glowing splint relights.
氧气：带火星的木条会复燃。
Carbon dioxide: bubble it through limewater and it turns milky.
二氧化碳：通入石灰水会变浑浊。
Chlorine: it bleaches damp litmus paper white.
氯气：使湿润的石蕊试纸漂白成白色。
Ammonia: it turns damp red litmus blue.
氨气：使湿润的红色石蕊试纸变蓝。
And finally, flame tests for metal ions: lithium burns red, sodium yellow, potassium lilac, calcium orange-red, barium light green, and copper blue-green.
最后是金属离子的焰色试验：锂烧成红色，钠黄色，钾淡紫色，钙橙红色， 钡浅绿色，铜蓝绿色。
Hold the tests side by side.
把检验并排放在一起。
A lighted splint pops in hydrogen.
点燃的木条在氢气中爆鸣。
A glowing splint relights in oxygen.
带火星的木条在氧气中复燃。
Those two are easy to mix up — remember: lit means hydrogen, glowing means oxygen.
这两个容易混——记住：点燃对应氢气，带火星对应氧气。
Carbon dioxide turns limewater milky.
二氧化碳使石灰水变浑浊。
Chlorine bleaches damp litmus white.
氯气使湿润石蕊漂白成白。
Ammonia turns damp red litmus blue.
氨气使湿润红石蕊变蓝。
One more gas: sulfur dioxide decolourises acidified potassium manganate seven — the purple fades, just as with a sulfite.
还有一种气体：二氧化硫使酸化的高锰酸钾褪色——紫色褪去，和亚硫酸盐一样。
A flame test identifies some metal cations by the colour they give a flame.
焰色试验靠金属阳离子给火焰上色来鉴定它们。
Dip a clean wire in the sample, then hold it in a Bunsen flame.
把干净的金属丝蘸上样品，伸进本生灯火焰。
Lithium is red.
锂是红色。
Sodium is yellow — a strong yellow that can hide other colours if the wire is dirty.
钠是黄色——黄色很强，如果金属丝不干净会盖住别的颜色。
Potassium is lilac, also called purple.
钾是淡紫色，也叫紫色。
Calcium is orange-red.
钙是橙红色。
Barium is light green.
钡是浅绿色。
Copper two is blue-green.
二价铜是蓝绿色。
Always clean the wire in acid and rinse before you start, or last sample's colour will fool you.
开始前一定要用酸洗净金属丝并冲洗，否则上一个样品的颜色会骗你。
Three marks to lock in.
锁住三个分。
First: pick apparatus by the measurement — a burette or pipette for accurate volumes, a measuring cylinder only for rough ones.
第一：按测量内容选仪器——准确体积用滴定管或移液管，量筒只用于粗略测量。
Second: R-f is the substance distance over the solvent distance, so it is always less than one, and a pure substance gives a single spot.
第二：Rf等于物质距离除以溶剂距离，所以总是小于一，纯净物只给出一个斑点。
Third: learn the halide precipitates with silver nitrate — chloride white, bromide cream, iodide yellow — and the gas tests: hydrogen pops, oxygen relights, carbon dioxide turns limewater milky.
第三：记住卤化物与硝酸银的沉淀——氯化物白色，溴化物乳白色，碘化物黄色—— 以及气体检验：氢气爆鸣，氧气复燃，二氧化碳使石灰水变浑浊。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。
Two more lists worth reciting.
还有两份值得背的清单。
Anions: a carbonate fizzes with acid and the gas turns limewater milky.
阴离子：碳酸盐遇酸冒泡，气体使石灰水变浑浊。
A sulfate gives a white precipitate with barium nitrate after acid.
硫酸盐在酸化后与硝酸钡生成白色沉淀。
Nitrate needs aluminium foil, sodium hydroxide and heat to release ammonia.
硝酸盐需要铝箔、氢氧化钠和加热才能放出氨气。
Sulfite fades purple manganate.
亚硫酸盐使紫色高锰酸盐褪色。
For cations, add sodium hydroxide and read the precipitate colour: copper two light blue, iron two green, iron three red-brown.
对于阳离子，加氢氧化钠，读沉淀颜色： 二价铜浅蓝、二价铁绿、三价铁红棕。
Use ammonia when white precipitates look the same — that is how zinc and aluminium part company.
白色沉淀看起来一样时用氨水——锌和铝就靠这一步分开。