Most of these procedures exist to protect one number. A standard solution is only worth making if its concentration is exactly what you say it is, and every fiddly step — the rinsing, the meniscus, the inverting — is there to keep that promise.
📚 What you need to know
A standard solution has a precisely known concentration, made with a 3 decimal place balance and a volumetric flask.
Diluting changes the volume but not the number of moles, so the concentration falls in proportion.
The equivalence point is when stoichiometrically equal amounts have been mixed; the endpoint is the colour change you can see.
Concordant titres agree to within 0.10 cm3. Average only those, and discard the rough titration.
Separation methods exploit a physical difference: solubility, boiling point, or affinity for a stationary phase.
Recrystallisation purifies a solid; a melting point checks whether it worked. Pure means sharp, impure means broad and lowered.
Reflux heats a mixture without losing anything; distillation heats it in order to remove something.
Making a standard solution
Two pieces of apparatus carry the accuracy: a balance reading to three decimal places for the mass, and a volumetric flask for the final volume. Everything else is technique.
Dissolve the solid before it goes near the flask. A volumetric flask has a narrow neck and cannot be swirled properly, so undissolved solid tends to stay undissolved.
Two details that carry marks. Make up to the mark so the bottom of the meniscus sits on the graduation, at eye level — overshoot and you cannot undo it. Then stopper and invert the flask several times, because the last water added sits on top and the solution is not uniform until you mix it.
Diluting a solution
Adding water changes the volume but does not change the number of moles of solute already there. So if the volume goes up by a factor of ten, the concentration goes down by a factor of ten.
Dilution
n(before) = n(after) c1V1 = c2V2
Serial dilution is the same move repeated. Take 100 cm3 of stock, make it up to 1000 cm3, and you have a 1:10 dilution; use that as the new stock and repeat to reach very low concentrations that would be impossible to weigh out directly.
Running a titration
Two words that get used loosely and should not be. The equivalence point is a fact about the chemistry: the moment stoichiometrically equivalent amounts of the two reactants have been mixed. The endpoint is what you actually observe — a colour change — and a well-chosen indicator makes the two coincide.
🧩 The titration itself
Pipette a fixed volume (usually 20.0 or 25.0 cm3) of one solution into a conical flask.
Fill the burette with the other solution and record the initial reading to 2 decimal places.
Add a few drops of indicator if one is needed, and stand the flask on a white tile.
Run the titrant in, swirling after each addition, and slow to dropwise near the endpoint.
Stop when one drop produces a permanent colour change, and record the final reading.
Repeat until you have concordant titres, agreeing to within 0.10 cm3.
Only a few drops of indicator, and only if you need one. Indicators are usually weak acids themselves, so a generous squirt takes part in the reaction you are trying to measure. Some redox titrations need none at all — manganate(VII) is its own indicator, going from purple to colourless as it is used up.
The first titration is deliberately rough: you overshoot to find out roughly where the endpoint is. Discard it. Average the concordant titres only, because including a result you know is wrong drags the mean towards it.
Separating a mixture
Every separation technique exploits one physical difference between the components. Naming that difference is usually where the mark is.
Technique
What it separates
The difference it uses
Filtration
an insoluble solid from a liquid
particle size against the filter pores
Crystallisation
a dissolved solid from its solution
solubility falls as solvent evaporates
Simple distillation
a liquid from a solution
a large difference in boiling point
Fractional distillation
two or more miscible liquids
small differences in boiling point
Paper or thin-layer chromatography
components of a mixture in solution
solubility in the mobile phase
Recrystallisation
a pure solid from its impurities
solubility changes with temperature
Chromatography deserves a note because the vocabulary is examined. The stationary phase is the paper, or a thin silica or alumina layer on a plate. The mobile phase is the solvent that travels through it. Components that are more soluble in the mobile phase travel further. If the spots are colourless, they are revealed with UV light or a locating agent such as ninhydrin.
Purifying a solid and checking it worked
Recrystallisation works on a single idea: the compound you want is much less soluble cold than hot, while the impurities — present in small amounts — stay dissolved. Dissolve the impure solid in the minimum of hot solvent, let it cool slowly, and the pure compound crystallises out while the impurities remain in solution.
The melting point then tells you whether it worked, because impurities do two things to it at once.
Melting points also identify. Comparing the measured value with a data book figure is a quick way to confirm which compound you have actually made.
Technique matters here too. The sample must be totally dry and finely powdered, crushed with the back of a spatula onto filter paper. Heat slowly — use the first tube to find the approximate range, then repeat much more slowly to pin it down, and take about three measurements.
Reflux, and the rest of the toolkit
Many organic reactions are slow at room temperature and need heating, but heating a volatile organic mixture in an open flask evaporates the reactants away. Reflux solves it: a vertical condenser sits on top of the flask, so vapour condenses and runs straight back down.
The water jacket runs bottom to top so the condenser stays full of water. Fed the other way, it would drain and cool far less efficiently.
Practical points for reflux: use a pear-shaped or round-bottomed flask, add anti-bumping granules for smooth boiling, and heat with a water bath or heating mantle rather than a naked flame when the contents are flammable. Typical uses are oxidising a primary alcohol all the way to a carboxylic acid, and esterification with a concentrated acid catalyst.
Three more you should recognise
Drying to constant mass. Weigh, heat, cool in a desiccator, reweigh, and repeat until the mass stops changing — the sign that all the water has gone. It is how the water of crystallisation in a hydrated salt is found.
Colorimetry and spectrophotometry. Both measure how much light a solution absorbs, and absorbance is related to concentration. A colorimeter is quick but less precise; a spectrophotometer scans wavelengths and is sensitive enough for low concentrations. Either way, the reading is converted using a calibration curve made from known standards.
Molecular modelling. Physical models (balls and sticks) show geometry and bond angles; digital models handle larger molecules, use real bond lengths and angles, and can simulate properties and interactions.
WORKED EXAMPLE
Calculate the mass of anhydrous sodium carbonate, Na2CO3, needed to prepare 250 cm3 of a 0.100 mol dm–3 standard solution.
Step 1 — convert the volume250 cm³ = 0.250 dm³Step 2 — find the moles neededn = cV = 0.100 × 0.250 = 0.0250 molStep 3 — find the molar massM = (2 × 22.99) + 12.01 + (3 × 16.00) = 105.99 g mol⁻¹Step 4 — find the massm = nM = 0.0250 × 105.99 = 2.6498 g2.65 gWeigh it on a 3 decimal place balance, so 2.650 g. The extra digit is the whole reason that balance is specified.
WORKED EXAMPLE
A student records four titres: 24.35, 23.90, 23.95 and 23.85 cm3. State which results should be used and calculate the mean titre.
Step 1 — find the rough result24.35 cm³ is the first titration and sits well away from the others, so it is discarded.Step 2 — check concordancy23.95 − 23.85 = 0.10 cm³The remaining three agree to within 0.10 cm³, so all three are concordant.Step 3 — average them(23.90 + 23.95 + 23.85) ÷ 3 = 71.70 ÷ 3mean titre = 23.90 cm³Never average in the rough titre. Say explicitly that it was discarded — that statement is usually worth a mark on its own.
WORKED EXAMPLE
A sample of benzoic acid melts between 114 °C and 119 °C. The data book value is 122 °C. Comment on the purity of the sample and suggest what the student should do next.
What the numbers sayThe melting point is 3 to 8 degrees below the literature value, and it is spread over a 5 degree range rather than melting sharply.lowered by up to 8 °C, range = 5 °Cthe sample is impureWhat to do nextPurify it by recrystallisation, dry the crystals fully, then measure the melting point again.a sharper range nearer 122 °C would show it workedUse both pieces of evidence. A lowered value alone is weaker than a lowered value plus a broad range.
💡 Exam tip
For a standard solution, mention the 3 decimal place balance, the rinsings, the meniscus on the mark and inverting to mix.
Distinguish equivalence point (stoichiometry) from endpoint (what you see).
Define concordant as agreeing to within 0.10 cm3, and say the rough titre is discarded.
Name the physical difference a separation technique relies on, not just the technique.
For purity, quote both the value and the range, and say impurities lower and broaden the melting point.
Explain reflux as preventing loss of volatile substances, and contrast it with distillation.
⚠️ Common mix-up
Averaging in the rough titre. It is deliberately an overshoot and it is discarded.
Dissolving the solid in the volumetric flask. Dissolve it in a beaker first, then transfer and rinse.
Adding a large volume of indicator. A few drops only; indicators are weak acids and take part in the reaction.
Saying impurities raise the melting point. They lower it, and they broaden the range.
Describing reflux as a way of separating a product. Nothing is removed — that is distillation.
That completes Tool 1. Between them, these three pages cover the planning (is it safe and responsible?), the measuring (is the number any good?) and the doing (is the procedure right?) — which is exactly the order an examiner reads a practical answer in.
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