Introduction
Okay, this is the practical chapter, and here's the plot twist: it's the most-tested area in the whole Core course. 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! 🔬
Picking the Right ApparatusC12.1
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: Pipette = one fixed accurate volume. Burette = any accurate volume (0.1 cm³). Measuring cylinder = rough. Finer scale = smaller uncertainty = more accurate. 🎯
Worked Example: Which Tool for 25.0 cm³?C12.1
Problem A student needs exactly 25.0 cm³ of a solution, measured accurately. Measuring cylinder, pipette, beaker, or burette?
Step 1 It's a fixed volume (always 25.0 cm³) and must be accurate. A beaker and measuring cylinder are too rough.
Step 2 A pipette is built to deliver one fixed volume accurately.
So the answer is the pipette. ✅
The Lab Vocabulary You Keep Losing Marks OnC12.1
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: 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: Residue, Filtrate and SugarC12.1
Problem 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?
Step 1 The insoluble sand on the paper = residue. The liquid through the paper = filtrate (it's salt solution, not pure water).
Step 2 Dissolved sugar is far too small to be trapped, so it passes straight through.
So sand = residue, liquid = filtrate, and sugar can't be filtered. ✅
Titrations and the End-PointC12.2
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 → orange in acid. The volume of acid you used is the titre, and you always get it by subtracting: final reading − first reading.
> 🔑 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: The Methyl Orange FlipC12.2
Problem Acid is added to an alkali using methyl orange. What colour change is seen at the end-point?
Step 1 Methyl orange is yellow in alkali, so it starts yellow.
Step 2 When the acid finishes neutralising the alkali, methyl orange turns orange.
So the change is yellow → orange. ✅
Working Out the TitreC12.2
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: Titre = final − first. Average only concordant titres. Universal indicator is useless here (too many colours, no sharp flip). 🧮
Worked Example: Finding the TitreC12.2
Problem The final burette reading is 18.8 cm³ and the first reading is 4.2 cm³. What volume of acid was added?
Step 1 titre = final − first.
Step 2 titre = 18.8 − 4.2 = 14.6 cm³.
So 14.6 cm³ of acid was added. ✅
The R_f ValueC12.3
Chromatography separates dissolved substances (like the dyes in an ink). 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. The $R_f$ value is just how far a substance moved compared with the solvent: $R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$. Measure both from the baseline. It's always between 0 and 1 and has no units, and the same substance always gives the same $R_f$.
> 🔑 Key idea: Pencil baseline, solvent below the spots. $R_f$ = substance ÷ solvent (0 to 1, no units). 🎨
Worked Example: Calculating R_fC12.3
Problem The solvent moved 8.0 cm; a dye's spot moved 6.0 cm. Calculate its $R_f$.
Step 1 $R_f = \frac{\text{dye}}{\text{solvent}} = \frac{6.0}{8.0}$.
Step 2 $\frac{6.0}{8.0} = 0.75$.
So $R_f = 0.75$ (no units). ✅
Separating MixturesC12.4
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: Filter (insoluble solid), crystallise (dissolved solid, cool slow), simple distil (pure liquid), fractional distil (lower boiling point first). Wash the residue! 🧰
Worked Example: Splitting Ethanol from WaterC12.4
Problem Ethanol (boils 78 °C) and water (boils 100 °C) are fully mixed. Which method separates them, and which comes off first?
Step 1 Two mixed liquids, different boiling points → fractional distillation.
Step 2 Ethanol has the lower boiling point, so it comes off first.
So: fractional distillation, ethanol first. ✅
Checking PurityC12.4
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: Pure = one sharp fixed temperature. Impure = melts/boils over a range. Say the reason! 📉
Worked Example: Pure or Impure?C12.4
Problem A wax melts from 45 °C to 49 °C. A liquid boils and stays at 141 °C. Which is pure?
Step 1 The wax melts over a range (45–49 °C), not sharply → impure.
Step 2 The liquid boils at one fixed temperature (141 °C) → pure.
So the wax is impure and the liquid is pure. ✅
Naming IonsC12.5
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 sodium hydroxide and read the precipitate colour: copper(II) light blue, iron(II) green, iron(III) red-brown. Three give a white precipitate (aluminium, zinc, calcium), so you add excess reagent to tell them apart — and ammonia is the tiebreaker: zinc dissolves in excess ammonia, aluminium doesn't. Learn every test as add → see → means.
> 🔑 Key idea: Flame: Li red, Na yellow, K lilac, Ca orange-red, Ba light green, Cu blue-green. White precipitate? Use excess: Al and Zn dissolve in excess NaOH; only Zn dissolves in excess ammonia. 🔥
Worked Example: Aluminium or Zinc?C12.5
Problem Two solutions both give a white precipitate with sodium hydroxide that dissolves in excess. Adding ammonia instead, what tells aluminium from zinc?
Step 1 Both behave the same with sodium hydroxide, so that test can't separate them.
Step 2 With excess ammonia, the zinc precipitate dissolves but the aluminium one doesn't.
So zinc dissolves in excess ammonia, aluminium doesn't. ✅
Naming a Whole SaltC12.5
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 — silver nitrate for halides, barium nitrate for sulfate, acid for carbonate), then join them and balance the charges. For the anion tests, always acidify with nitric acid first to remove carbonate, which would otherwise give a false positive.
> 🔑 Key idea: Salt = cation test + anion test, then balance charges. Acidify with nitric acid before halide/sulfate tests. 🧂
Worked Example: What Salt Is It?C12.5
Problem Solution J gives a green precipitate with sodium hydroxide (stays in excess), and a yellow precipitate with acidified silver nitrate. Name the salt.
Step 1 Green precipitate, stays in excess = iron(II).
Step 2 Yellow precipitate with silver nitrate = iodide.
So J is iron(II) iodide, FeI₂. ✅
Key Points to Remember
- Pick apparatus by accuracy: pipette (one fixed accurate volume), burette (any accurate volume, 0.1 cm³), measuring cylinder (rough). Smaller uncertainty = more accurate; avoid parallax (eye level with the meniscus).
- Residue = solid on the paper; filtrate = liquid through it (still holds dissolved stuff); you can't filter out anything dissolved.
- Collect a gas by its density and solubility: over water (insoluble), downward delivery (denser than air), upward delivery (less dense than air).
- Titration end-point = one sharp colour change (methyl orange yellow → orange); titre = final − first reading; average only concordant titres; universal indicator is no good.
- $R_f = \frac{\text{distance by substance}}{\text{distance by solvent}}$, 0 to 1, no units; pencil baseline, solvent below the spots; same height = same substance, more than one spot = mixture.
- Separation: filter (insoluble solid), crystallise (dissolved solid, cool slow), simple distil (pure liquid), fractional distil (lower boiling point first). Wash the residue.
- Purity: pure melts/boils sharply at one fixed temperature; impurity melts over a range (and lowers the melting point).
- Identify ions: flame colours (Li red, Na yellow, K lilac, Ca orange-red, Ba light green, Cu blue-green); NaOH precipitates name the cation (Al and Zn dissolve in excess NaOH — use ammonia to separate); acidify with nitric acid before anion tests; learn each test as add → see → means.