Pearson Edexcel International GCSE in Chemistry · 4CH1

Titrations

Finding the exact volume that neutralises — and then making the salt again without the indicator in it.

Topic 2 · Inorganic chemistry — one of 19 lessons in this topic, and one of 61 in Chemistry.

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

  • 2.33 — How to carry out an acid–alkali titration (bold C statement — Paper 2 only)
  • 2.40 — Preparing a pure dry soluble salt from an acid and an alkali (bold C statement — Paper 2 only)

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

A TITRATION finds the exact volume of one solution that reacts completely with a measured volume of another. For an acid and an alkali that means finding the volume at which neutralisation is precisely complete — no acid left over and no alkali left over.

The difficulty is that neutralisation is invisible. Both solutions are usually colourless, nothing is given off, and the moment of completion looks exactly like the moment before it. An INDICATOR is what makes it visible, and it must be a SINGLE indicator such as phenolphthalein or methyl orange rather than universal indicator, because what is needed is a sharp change at one point rather than a gradual one across a range.

Statement 2.33 — the method

  1. Use a PIPETTE to transfer a measured volume of the alkali, usually 25.0 cm³, into a conical flask. A pipette is used because it delivers one fixed volume very accurately.
  2. Add two or three drops of a single indicator such as phenolphthalein, which is pink in the alkali.
  3. Fill a BURETTE with the acid and record the starting reading, taking it from the bottom of the meniscus at eye level.
  4. Run the acid in, swirling the flask continuously so that the solutions mix as they meet.
  5. Slow to drop-by-drop as the end point is approached, and stop at the first permanent colour change — pink to colourless for phenolphthalein.
  6. Record the final burette reading and subtract to find the TITRE, the volume of acid used.
  7. Repeat until two titres agree within about 0.1 cm³, and take the mean of those CONCORDANT results.

Think of it like filling a glass to the brim in the dark with someone watching

Pouring water into a glass you cannot see, you would go quickly at first and then slow to a trickle, because the cost of one drop too many is a spill you cannot undo. If someone can tell you the instant the surface reaches the rim, the job becomes easy. The indicator is that someone. It explains both halves of the technique: why you add fast and then drop by drop, and why the indicator has to change SHARPLY — a helper who said 'getting closer... quite close now...' would be no use at all, which is exactly why universal indicator is the wrong choice.

Two words deserve precision. The TITRE is the volume delivered from the burette, found by subtracting the initial reading from the final one — burettes are read from the top down, so the numbers increase as liquid runs out. CONCORDANT results are repeats agreeing within about 0.1 cm³, and only concordant titres are averaged; a rough first run is deliberately discarded rather than included.

Now statement 2.40, which uses the titration to make a salt. There is an obvious problem: the neutralised solution contains the salt, but it also contains the indicator, so evaporating it gives a coloured, impure product. The solution is elegant and is the whole point of the second stage.

Statement 2.40 — making a pure dry salt from an acid and an alkali

  1. Carry out the titration with the indicator, and record the exact volume of acid needed to neutralise the measured volume of alkali.
  2. Now REPEAT the experiment with fresh solutions and NO INDICATOR, adding exactly that volume of acid from the burette.
  3. The solution is now neutralised and contains only the salt dissolved in water, with nothing coloured in it.
  4. Warm the solution gently in an evaporating basin to concentrate it, until crystals begin to appear at the edges.
  5. Leave it to CRYSTALLISE slowly rather than evaporating to dryness, which would spoil the crystals and drive off water of crystallisation.
  6. Filter off the crystals and dry them between filter papers or in a warm oven.

The trick in step two is worth admiring. The titration is not the preparation — it is a MEASUREMENT that makes the preparation possible. Having found the volume once with the indicator's help, you never need the indicator again, and the second run produces a solution that was neutralised blind but exactly.

Reading a titration table

A student records titres of 24.90, 23.55, 23.60 and 23.50 cm³. What mean titre should they use, and why?

  1. The first titre, 24.90 cm³, is much larger than the others — it is the rough run, in which the acid was added quickly and the end point overshot.
  2. It is DISCARDED rather than averaged in.
  3. The remaining three, 23.55, 23.60 and 23.50, all agree within 0.1 cm³, so they are concordant.
  4. Mean = (23.55 + 23.60 + 23.50) ÷ 3 = 70.65 ÷ 3 = 23.55 cm³.
  5. Including the rough run would have given 23.89 cm³, which is wrong by more than the precision of the burette — which is exactly why concordant results are specified.

Answer: 23.55 cm³, taking the mean of the three concordant titres and discarding the rough run.

Discarding data feels wrong and is correct here, because the rough run is not a measurement of the same thing. It is a deliberate approximation made to find out roughly where the end point is, so that later runs can be slowed at the right moment. Averaging it with careful results contaminates them.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

The vocabulary of a titration

Titre
The volume delivered from the burette — the final reading minus the initial reading.
End point
The moment at which the indicator changes colour permanently, showing neutralisation is complete.
Concordant results
Repeat titres agreeing within about 0.1 cm³. Only these are averaged.
Rough titration
A deliberately quick first run to find roughly where the end point is. It is discarded, not averaged.

Statement 2.33 — carrying out the titration

  1. Pipette a measured volume of alkali, usually 25.0 cm³, into a conical flask.
  2. Add two or three drops of a SINGLE indicator such as phenolphthalein.
  3. Fill the burette with acid; record the initial reading from the bottom of the meniscus at eye level.
  4. Run acid in, swirling continuously, then slow to drop-by-drop near the end point.
  5. Stop at the first permanent colour change and record the final reading; subtract to get the titre.
  6. Repeat until two titres are concordant, and take their mean.

Statement 2.40 — making the pure dry salt

  1. Titrate with indicator to find the exact volume of acid needed.
  2. Repeat with fresh solutions and NO indicator, adding exactly that volume.
  3. The solution now contains only the salt and water.
  4. Warm gently in an evaporating basin until crystals start to form at the edges.
  5. Leave to crystallise slowly, then filter off the crystals and dry them between filter papers.

Why each piece of apparatus is chosen

  • PIPETTE — delivers one FIXED volume very accurately, which is why it is used for the solution in the flask
  • BURETTE — delivers a VARIABLE volume that can be read precisely, and allows drop-by-drop addition
  • CONICAL FLASK — can be swirled vigorously without splashing, unlike a beaker
  • SINGLE INDICATOR — changes sharply at one point; universal indicator changes gradually and would give no clear end point
  • Read both burette readings at EYE LEVEL from the bottom of the meniscus, to avoid parallax error

Model answer [3 marks]

Explain why the titration is repeated without indicator when preparing a pure sample of the salt. [3]

The indicator remains in the solution after the titration, so evaporating that solution would give crystals contaminated with indicator and the salt would not be pure. Once the titration has been carried out, the exact volume of acid required to neutralise the alkali is known. The experiment can therefore be repeated with fresh solutions and no indicator, adding exactly that volume of acid, which gives a neutralised solution containing only the salt and water.

Model answer [3 marks]

A student obtains titres of 25.40, 24.20, 24.15 and 24.25 cm³. Calculate the mean titre they should use, explaining your choice. [3]

The first titre of 25.40 cm³ is the rough run and is much larger than the others, so it is discarded rather than averaged. The remaining three titres of 24.20, 24.15 and 24.25 cm³ all agree within 0.1 cm³, so they are concordant. The mean is (24.20 + 24.15 + 24.25) ÷ 3 = 72.60 ÷ 3 = 24.20 cm³.

A pipette against a burette

A. A PIPETTE delivers ONE FIXED volume, very accurately. It is used for the solution measured into the conical flask.

B. A BURETTE delivers a VARIABLE volume, read precisely and addable drop by drop. It holds the solution being measured.

Model answer [3 marks]

Explain why the flask is swirled continuously and why the acid is added drop by drop near the end point. [3]

The flask is swirled so that the acid entering mixes thoroughly with the alkali, ensuring the two react as soon as they meet and that the indicator shows the true state of the whole solution rather than a local excess of acid. Near the end point the acid is added drop by drop because a single drop can change the solution from alkaline to acidic, so adding any faster would overshoot the end point and the titre recorded would be too large.

Not this: All the titres should be averaged, because discarding a result is dishonest.

This: The rough run is not a measurement of the same quantity — it is a deliberate approximation made to locate the end point so that later runs can be slowed in time. Averaging it in contaminates careful results with a known overshoot.

Mark-losing trap. Use a SINGLE indicator, never universal indicator — a gradual change gives no end point.

Mark-losing trap. Only CONCORDANT titres are averaged. The rough run is discarded.

Mark-losing trap. Repeat WITHOUT indicator to make the salt, or the crystals are contaminated.

Mark-losing trap. Crystallise SLOWLY — evaporating to dryness spoils the crystals.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

  1. Grade 6 · State [1 mark] — Which piece of apparatus is used to add the acid in a titration?
  2. Grade 7 · Calculate [2 marks] — A burette reads 1.20 cm³ at the start and 26.75 cm³ at the end. Calculate the titre.
  3. Grade 8 · Explain [3 marks] — Explain why universal indicator must not be used in a titration.
  4. Grade 9 · Describe [6 marks] — Select every statement that belongs in a full-mark description of how to prepare a pure dry sample of sodium chloride from hydrochloric acid and sodium hydroxide solution.
  5. 9+ · Analyse [6 marks] — A student rinses the burette with distilled water before filling it with acid, and rinses the conical flask with the alkali before pipetting the alkali in. Both titres come out wrong. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who insisted a procedure be written down well enough to repeat, and the one who made chemistry a matter of measurement. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Robert Boyle — the one who insisted a procedure be written down well enough to repeat

A titration is a procedure in which every detail carries a reason — the pipette rather than a measuring cylinder, the swirling, the drop-by-drop approach, the discarded rough run — and the conviction that such details must be published in full is Boyle's contribution to how experiments are reported. He described his apparatus and method at length, including the attempts that failed, on the grounds that a reader who cannot see the whole procedure cannot judge any of it. That standard is why a school practical can be written as a repeatable list at all.

  • “Why write down every detail of how an experiment was done?”
  • “Why publish the attempts that did not work?”
  • “How do you know which details matter and which do not?”
  • “What makes a result trustworthy to someone who was not there?”
  • “How did you first come to test acids with a colour change?”

Antoine Lavoisier — the one who made chemistry a matter of measurement

A titration is chemistry done to two decimal places, and the idea that chemical questions are settled by careful measurement rather than by observation and argument is Lavoisier's. He weighed everything, sealed his vessels so that nothing could escape unnoticed, and overturned a century-old theory largely by insisting that the numbers had to balance. He is the right person to ask why precision is worth the trouble, and what changes about a subject when it starts taking measurement seriously.

  • “Why does it matter to measure chemistry precisely?”
  • “What did you find that only careful weighing could show?”
  • “How precise could you actually be with the balances of your day?”
  • “What did chemists do before they measured things?”
  • “How do you decide when a measurement is good enough?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Joseph Louis Gay-Lussac in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course