Experimental Techniques and Chemical Analysis: Measure, Separate, Identify
Lab Detective: Spot the Ion 🔬
Introduction
1. Introduction
Okay, this is the practical chapter, and here's the good news: almost everything in it is a fixed fact or a fixed method you can learn, so these are basically free marks if you know your stuff. The whole game is being a lab detective: pick the right tool, separate the mess, and run the right test to figure out what an unknown actually is.
Here's the cheat code for the entire chapter: every test is the same three beats. You add something → you see something → you say what it means. Nail that triple and the questions answer themselves. This is your fast refresh, not the full textbook: one key idea per topic, one quick worked example, and the exact word the marker wants. Let's go full lab detective! 🔬
Here's the cheat code for the entire chapter: every test is the same three beats. You add something → you see something → you say what it means. Nail that triple and the questions answer themselves. This is your fast refresh, not the full textbook: one key idea per topic, one quick worked example, and the exact word the marker wants. Let's go full lab detective! 🔬
2. Picking the Right Apparatus
Different tools measure volume with wildly different accuracy, and the exam loves testing it. A measuring cylinder is for rough volumes. A pipette gives one fixed accurate volume (like 25.0 cm³). A burette gives any accurate volume to 0.1 cm³ through a tap, which is why it runs titrations. Ask yourself two things: does it need to be accurate, and is the volume fixed or different each time? Fixed + accurate = pipette; different + accurate = burette; rough = measuring cylinder. Oh, and read the scale with your eye level with the bottom of the meniscus, or you'll get a parallax error.
Key idea🔑 Key idea: Pipette = one fixed accurate volume. Burette = any accurate volume (0.1 cm³). Measuring cylinder = rough. Finer scale = smaller uncertainty = more accurate. 🎯
Worked example
Worked Example: Which Tool for 25.0 cm³?
Worked Example: Fixed and Accurate 🧪
A student needs exactly 25.0 cm³ of a solution, measured accurately. Measuring cylinder, pipette, beaker, or burette?
- 1It's a fixed volume (always 25.0 cm³) and must be accurate. A beaker and measuring cylinder are too rough.
- 2A pipette is built to deliver one fixed volume accurately.
So the answer is the pipette. ✅
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3. The Lab Vocabulary You Keep Losing Marks On
These six words sound obvious but cost real marks. Solute = the thing that dissolves. Solvent = the liquid doing the dissolving. Solution = the two combined. A saturated solution is holding the maximum it can, so no more dissolves. When you filter, the insoluble solid stuck on the paper is the residue, and the liquid that drips through is the filtrate. Big trap: the filtrate is not pure water, because anything already dissolved stays dissolved and rides straight through. That's also why you can't filter sugar out of water: it's dissolved, so it's way too small to be trapped.
Key idea🔑 Key idea: Residue = solid trapped on the paper. Filtrate = liquid through the paper (still has dissolved stuff in it). You can't filter out anything that's dissolved. 💧
Worked example
Worked Example: Residue, Filtrate and Sugar
Worked Example: Solid Stays, Liquid Goes 🧴
Sand and salt solution are filtered; the sand stays on the paper. (a) Name the sand and the liquid below. (b) Why can't you filter sugar out of water?
- 1The insoluble sand on the paper = residue. The liquid through the paper = filtrate (it's salt solution, not pure water).
- 2Dissolved sugar is far too small to be trapped, so it passes straight through.
So sand = residue, liquid = filtrate, and sugar can't be filtered. ✅
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4. Titrations and the End-Point
A titration measures how much acid exactly reacts with an alkali. A pipette puts a fixed volume of alkali in the conical flask, you add a few drops of indicator, stand it on a white tile (so you can see the colour clearly), and run acid in from the burette, swirling, until the colour just flips. That flip is the end-point. The classic indicator is methyl orange: yellow in alkali, red in acid, and orange at the end-point. The volume of acid you used is the titre, and you always get it by subtracting: final reading − first reading.
Key idea🔑 Key idea: End-point = one sharp colour flip (methyl orange yellow → orange). Titre = final reading − first reading. White tile to see it clearly. ⚗️
Worked example
Worked Example: The Methyl Orange Flip
Worked Example: Yellow to Orange 🟠
Acid is added to an alkali using methyl orange. What colour change is seen at the end-point?
- 1Methyl orange is yellow in alkali, so it starts yellow.
- 2When the acid finishes neutralising the alkali, methyl orange turns orange.
So the change is yellow → orange. ✅
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5. Working Out the Titre
Never just read the last number off the burette. You take the reading before you start (first reading) and the reading at the end-point (final reading), and subtract. Both are read to 0.1 cm³. To get a reliable answer you repeat the titration and average only the concordant titres (the ones within about 0.1 cm³ of each other); you bin the rough first run. And don't use universal indicator for a titration: it slides through loads of colours, so there's no single sharp end-point.
Key idea🔑 Key idea: Titre = final − first. Average only concordant titres. Universal indicator is useless here (too many colours, no sharp flip). 🧮
Worked example
Worked Example: Finding the Titre
Worked Example: Just Subtract 💧
The final burette reading is 18.8 cm³ and the first reading is 4.2 cm³. What volume of acid was added?
- 1titre = final − first.
- 2titre = 18.8 − 4.2 = 14.6 cm³.
So 14.6 cm³ of acid was added. ✅
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6. Reading a Chromatogram
Paper chromatography separates soluble coloured substances (like the dyes in an ink) using a suitable solvent. Draw the baseline in pencil (ink would run), keep the solvent below the baseline so the spots don't wash off, and let the solvent rise. To identify an unknown, run it next to known (reference) substances on the same paper with the same solvent. A spot at the same height as a reference means the unknown contains that substance. One spot means one substance, so it's pure; two or more spots means a mixture.
Key idea🔑 Key idea: Pencil baseline, solvent below the spots. Same height = same substance. One spot = pure; more than one spot = mixture. 🎨
Worked example
Worked Example: A Dye That Is a Mixture
Worked Example: Count the Spots 🔎
Dyes P, Q, R and S are run on one chromatogram. P, Q and R each give one spot. S gives two spots: one level with Q, and one that matches none of P, Q or R. What can you say about each dye?
- 1P, Q and R each give one spot, so each is pure.
- 2S gives two spots, so it's a mixture. One spot is level with Q, so S contains Q; the other matches no reference, so it's a substance that isn't P, Q or R.
So P, Q and R are pure, and S is a mixture of Q and one other substance. ✅
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7. Separating Mixtures
Pick the method by what's mixed with what. Filtration pulls an insoluble solid out of a liquid. Crystallisation gets a dissolved solid back (evaporate some water, then cool slowly so crystals grow). Simple distillation gives you a pure liquid (like pure water) from a solution. Fractional distillation splits two mixed liquids with different boiling points, and the one with the lower boiling point comes off first. Always wash the residue with distilled water before you dry it.
Key idea🔑 Key idea: Filter (insoluble solid), crystallise (dissolved solid, cool slow), simple distil (pure liquid), fractional distil (lower boiling point first). Wash the residue! 🧰
Worked example
Worked Example: Splitting Ethanol from Water
Worked Example: Low Boiler Wins 🌡️
Ethanol (boils 78 °C) and water (boils 100 °C) are fully mixed. Which method separates them, and which comes off first?
- 1Two mixed liquids, different boiling points → fractional distillation.
- 2Ethanol has the lower boiling point, so it comes off first.
So: fractional distillation, ethanol first. ✅
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8. Checking Purity
This one's a freebie if you remember the rule. A pure substance melts and boils at one sharp, fixed temperature. An impurity messes that up: it makes the substance melt (or boil) over a range of temperatures instead of at one point, and it lowers the melting point. So if something melts over a spread of temperatures, it's impure; if it melts or boils bang on one fixed value, it's pure. Always give the reason, not just the verdict.
Key idea🔑 Key idea: Pure = one sharp fixed temperature. Impure = melts/boils over a range. Say the reason! 📉
Worked example
Worked Example: Pure or Impure?
Worked Example: Sharp vs Spread 🔍
A wax melts from 45 °C to 49 °C. A liquid boils and stays at 141 °C. Which is pure?
- 1The wax melts over a range (45–49 °C), not sharply → impure.
- 2The liquid boils at one fixed temperature (141 °C) → pure.
So the wax is impure and the liquid is pure. ✅
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9. Naming Ions
The grand finale: identifying ions. A flame test gives each metal a signature colour: lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green, copper(II) blue-green. For more cations, add aqueous sodium hydroxide, a few drops then excess, and read the precipitate: copper(II) light blue, iron(III) red-brown, and green for iron(II) or chromium(III). Excess sodium hydroxide splits the greens: chromium(III) dissolves, iron(II) stays. Three give a white precipitate: calcium stays in excess, aluminium and zinc dissolve. Then excess aqueous ammonia is the tiebreaker: zinc dissolves, aluminium stays, and calcium gives no precipitate (or a very slight white one). Ammonium gives no precipitate but gives off ammonia when warmed with sodium hydroxide. Learn every test as add → see → means.
Key idea🔑 Key idea: Flame: Li red, Na yellow, K lilac, Ca orange-red, Ba light green, Cu blue-green. Green precipitate? Excess NaOH: Cr³⁺ dissolves, Fe²⁺ stays. White precipitate? Excess NaOH: Ca²⁺ stays, Al³⁺ and Zn²⁺ dissolve; then excess ammonia: Zn²⁺ dissolves, Al³⁺ stays. Ammonium: ammonia gas on warming. 🔥
Worked example
Worked Example: Aluminium or Zinc?
Worked Example: Ammonia Decides 🟰
Two solutions both give a white precipitate with sodium hydroxide that dissolves in excess. Adding ammonia instead, what tells aluminium from zinc?
- 1Both behave the same with sodium hydroxide, so that test can't separate them.
- 2With excess ammonia, the zinc precipitate dissolves but the aluminium one doesn't.
So zinc dissolves in excess ammonia, aluminium doesn't. ✅
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10. Naming a Whole Salt
Exam questions love to make you name a whole salt from its reactions. Do it in three moves: find the metal (flame test + the sodium hydroxide precipitate), find the negative ion (its own test), then join them and balance the charges. The anion tests: carbonate fizzes with dilute acid and the gas turns limewater milky; nitric acid then silver nitrate gives chloride white, bromide cream, iodide yellow; nitric acid then barium nitrate gives sulfate a white precipitate; nitrate gives ammonia when warmed with sodium hydroxide and aluminium foil (check for ammonia before adding the foil, because ammonium salts give it too); sulfite turns acidified potassium manganate(VII) from purple to colourless. Acidify with nitric acid first to remove carbonate, which would otherwise give a false positive. The gas tests: ammonia turns damp red litmus blue, carbon dioxide turns limewater milky, chlorine bleaches damp litmus, hydrogen pops with a lighted splint, oxygen relights a glowing splint, sulfur dioxide turns acidified manganate(VII) purple to colourless.
Key idea🔑 Key idea: Salt = cation test + anion test, then balance charges. Acidify with nitric acid before halide/sulfate tests. Sulfite and sulfur dioxide both turn manganate(VII) colourless. Hydrogen: lighted splint pops. Oxygen: glowing splint relights. 🧂
Worked example
Worked Example: What Salt Is It?
Worked Example: Green + Yellow 🟢
Solution J gives a green precipitate with sodium hydroxide (stays in excess), and a yellow precipitate with acidified silver nitrate. Name the salt.
- 1Green precipitate, stays in excess = iron(II) (chromium(III) would dissolve).
- 2Yellow precipitate with silver nitrate = iodide.
So J is iron(II) iodide, FeI₂. ✅
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