These are the methods that keep coming back — titration, filtering, distilling, refluxing, calorimetry, chromatography. Paper 3 loves asking why a step is done, not just what it is. So for each technique here, keep asking yourself: what would go wrong if I skipped this bit?
📘 What you need to know
Titration finds an unknown concentration by adding one solution to another until they exactly react.
Filtration separates an insoluble solid from a liquid. Recrystallisation purifies a solid that is already there.
Distillation collects the substance that boils off. Reflux heats a mixture for a long time without losing any of it.
Calorimetry measures heat using q = mcΔT, where m is the mass of the solution, not the solid.
Chromatography separates a mixture by how strongly each component sticks to the paper compared with how much it likes the solvent. Rf identifies it.
Gas volumes are collected in a gas syringe or over water; only insoluble gases work over water.
For every technique, be ready to say why each step is there. That is where the Paper 3 marks are.
Titration
A titration answers one question: how much of solution B is needed to exactly react with a known amount of solution A? Once you know that, the concentration falls straight out of the mole ratio.
Water left in the burette or pipette dilutes the solution you carefully measured. Water left in the conical flask does not, because the number of moles inside it is unchanged.
🧩 Running a titration
Rinse. Burette with the solution it will hold, pipette with the solution it will hold, conical flask with distilled water only.
Pipette exactly 25.00 cm3 into the conical flask and add 2–3 drops of indicator. No more — indicator is a weak acid itself.
Fill the burette above zero, run some through the tip to remove the air bubble, then take the initial reading.
Rough titre first. Run it in fast to find roughly where the endpoint is.
Accurate titres next, adding dropwise near the endpoint and swirling constantly.
Repeat until two or more titres are within 0.10 cm3 of each other. Average only those.
Only concordant titres go into the mean. If you have 23.45, 23.50 and 24.80, you average the first two and leave the third out — but you still write it in your table, and you still say why you excluded it.
WORKED EXAMPLE
Finding a concentration from titres
25.00 cm3 of 0.100 mol dm–3 NaOH is titrated with HCl. The titres are 24.80, 23.45 and 23.50 cm3. Find the concentration of the HCl. The equation is NaOH + HCl → NaCl + H2O.
Step 1: Pick the concordant titres
23.45 and 23.50 agree within 0.10. Drop the 24.80 (rough run).
Mean titre = (23.45 + 23.50) ÷ 2 = 23.475 cm³Step 2: Moles of the known solutionn(NaOH) = 0.100 × 25.00 ÷ 1000 = 2.50 × 10⁻³ molStep 3: Use the ratio (1:1 here)n(HCl) = 2.50 × 10⁻³ molStep 4: Divide by the volume in dm³c = 2.50 × 10⁻³ ÷ 0.023475 = 0.10650…0.107 mol dm⁻³ (3 s.f.)averaging all three titres would have given 0.0993 — a whole mark thrown away
Separating and purifying
Four techniques that students blur together. The difference is what you are trying to keep and what state it is in.
Technique
Separates
You keep
Key detail
Filtration
Insoluble solid from a liquid
Either the residue or the filtrate
Wash the residue with cold distilled water to remove trapped solution
Recrystallisation
A solid from its soluble impurities
Purified crystals
Dissolve in the minimum hot solvent, cool slowly, then filter
Simple distillation
A liquid from a much higher boiling mixture
The distillate that comes over
Thermometer bulb level with the side arm, not down in the liquid
Reflux
Nothing — it keeps everything in
The reaction mixture
Vertical condenser returns the vapour, so nothing escapes over hours of heating
Reflux is what you use to make a product; distillation is what you use afterwards to get it out clean. Many organic preparations use both, in that order.
Why cold water goes in at the bottom: filling against gravity keeps the jacket completely full and gives the steepest temperature difference where the vapour is hottest. Feed it in at the top and you get a half-empty jacket and poor cooling.
Calorimetry: measuring the heat
You cannot measure energy directly. What you measure is a temperature change in a known mass of solution, and then convert.
Heat transferredq = m × c × ΔT
m is the mass of the solution being heated in grams — not the mass of the fuel or the solid you dissolved.
c is the specific heat capacity, 4.18 J g–1 K–1 for dilute aqueous solutions.
ΔT is the temperature change. A rise in temperature means the reaction gave out heat, so ΔH is negative.
WORKED EXAMPLE
Enthalpy change from a calorimetry experiment
0.0100 mol of a solid is dissolved in 50.0 cm3 of water in a polystyrene cup. The temperature rises by 8.4 °C. Take the density of the solution as 1.00 g cm–3 and c = 4.18 J g–1 K–1. Find ΔH in kJ mol–1.
Step 1: Mass of solution being heated50.0 cm³ × 1.00 = 50.0 gStep 2: Heat releasedq = 50.0 × 4.18 × 8.4 = 1755.6 J = 1.7556 kJStep 3: Divide by moles1.7556 ÷ 0.0100 = 175.56 kJ mol⁻¹Step 4: Temperature went up, so it is exothermicΔH = −176 kJ mol⁻¹ (3 s.f.)the mass is 50.0 g of solution, not the mass of the solid — this is the classic slip
Real cups leak heat, so your value always comes out too small. The fix that examiners want to hear is: insulate the cup, add a lid, and extrapolate the cooling curve back to the moment of mixing to find the temperature rise you would have got with no loss at all.
Chromatography
Chromatography splits a mixture using a tug-of-war. Every component is pulled along by the moving solvent and held back by the paper. Whichever one clings to the paper least travels furthest.
The value is always between 0 and 1, because a spot can never travel further than the solvent that carries it.
Retardation factor
Rf = distance moved by the spot ÷ distance moved by the solvent
Rf is fixed for a given substance in a given solvent, so it is only useful for identification if you compare it with a known sample run on the same plate, at the same time, in the same solvent. Change the solvent and the number changes.
If a spot is colourless — amino acids, for example — you develop the plate afterwards with a locating agent such as ninhydrin, or view it under UV light. You cannot measure a spot you cannot see.
Collecting a gas
If the reaction gives off a gas, the volume of it tells you how far the reaction has gone. There are two ways to catch it.
Method
Good for
Watch out for
Gas syringe
Any gas, and it reads volume directly
The plunger sticks if it is wet; it has a maximum of about 100 cm3
Collection over water
Cheap, and fine for hydrogen or oxygen
Useless for soluble gases — CO2, HCl and NH3 dissolve and you lose them
Mass loss on a balance
Heavy gases such as CO2
Light gases like hydrogen barely change the mass at all
Whichever you use, gas escapes in the first second or two while you push the bung in. That is a systematic error, and it always makes your initial rate look too slow. Mention it and you have a real evaluation point rather than a vague one.
💡 Exam tip
Learn the rinsing rules for titration. They come up again and again, and the reasoning is what earns the mark.
In q = mcΔT, always ask “the mass of what is being heated?” It is the solution, every time.
If asked to improve a method, name a specific change and its effect: “add a lid to reduce heat loss to the air”, not “be more accurate”.
For chromatography, always say pencil baseline and above the solvent level. Both are standard mark points.
“Why reflux rather than just heat it?” → because the reactants and product would boil away otherwise.
Watch the units: cm3 to dm3 is a factor of 1000, and J to kJ is another. Two of the easiest marks to drop.
⚠ Common mix-up
Rinsing the conical flask with the solution instead of water. That adds extra moles and ruins the titre.
Averaging every titre including the rough one. Only concordant results go into the mean.
Using the mass of the solid in q = mcΔT. Use the mass of the solution that actually warmed up.
