Experimental Techniques and Chemical Analysis

• If you can say which step of the loop a question is standing on, you can always start the answer. Most lost marks in Topic 12 are steps 5 and 6 collapsed into one: writing “copper is present” where the question asked what you would see.
• Five subsections. Fifteen checks in 12.1, four in 12.2, five in 12.3, eight in 12.4 and twenty-seven in 12.5 — fifty-nine in total. More than two fifths of the topic is the qualitative analysis tables, which is why they are taught here as a routine and not as a wall to memorise.
• Precipitates are described with two words — a colour and the word precipitate. “Blue solution” and “blue precipitate” are different observations, and only one of them is what happened.
• Nine of these fourteen are a pair of words that sound similar. If you can say the difference between residue and filtrate, precision and accuracy, and a precipitate and a colour change, you have already protected a large block of the marks in this topic.
• Everything in 12.5 answers one of two questions: what would I see? or what does that mean? Decide which one is being asked before you write anything.
• Marks by subsection: 12.1 — 26; 12.2 — 12; 12.3 — 14; 12.4 — 16; 12.5 — 28; practical overlay — 8. Total 104.
• If you have only ten minutes before an exam, spend them on the cation matrix and the six flame colours. They are the densest marks in the topic and the fastest to lose.

• I can name the apparatus used to measure time, temperature, mass and volume, and say which of the seven named items suits a given task.
• I can suggest an advantage and a disadvantage of a method or apparatus for the task in front of me, rather than reciting a universal claim.
• I can read an analogue scale correctly, including to the nearest half-scale division where that is required, and record the value with its unit.
• I can describe a solvent, a solute, a solution, a saturated solution, a residue and a filtrate in the exact words the syllabus uses.
• I can describe an acid-base titration that uses a burette, a volumetric pipette and a suitable indicator, and say what each item is for.
• I can describe how the end-point is identified with an indicator, including approaching it dropwise and stopping at the first permanent colour change.
• I can obtain the titre as final reading minus initial reading, and record burette readings consistently.
• I can describe how paper chromatography separates a mixture of soluble substances using a suitable solvent, including a pencil baseline above the solvent.
• I can describe why a locating agent is used when the substances are colourless.
• I can interpret a simple chromatogram to identify an unknown substance by comparison with known substances run under the same conditions.
• I can interpret a simple chromatogram as pure or impure.
• I can state and use \(R_f = \dfrac{\text{distance travelled by substance}}{\text{distance travelled by solvent}}\), measuring both from the same baseline.
• I can describe and explain separation and purification using a suitable solvent, filtration, crystallisation, simple distillation and fractional distillation.
• I can suggest a suitable technique when I am given information about the substances involved, and justify it from that information.
• I can identify a substance by comparing a measured melting point or boiling point with known values.
• I can assess purity from melting point or boiling point information, using sharpness of the value as the evidence.
• I can describe the test and the exact result for seven anions: carbonate, chloride, bromide, iodide, nitrate, sulfate and sulfite.
• I can describe what aqueous sodium hydroxide and aqueous ammonia do to eight aqueous cations, dropwise and then in excess.
• I can describe the test and the exact result for six gases: ammonia, carbon dioxide, chlorine, hydrogen, oxygen and sulfur dioxide.
• I can give the flame colour for six metal ions: lithium, sodium, potassium, calcium, barium and copper(II).
• I can combine two independent results, each on a fresh sample, to deduce an ion — and say when the evidence is not yet enough.

Why a fresh sample every time. Reagents do not politely wait their turn. Add dilute nitric acid and silver nitrate to a solution, then add sodium hydroxide to the same tube, and you are no longer testing the original substance — you are testing whatever the first test left behind. Two specific traps: a solution acidified with hydrochloric acid now contains chloride ions, so a later silver nitrate test gives a white precipitate whatever the sample was; and a solution to which sodium hydroxide has been added will produce ammonia from any ammonium ion present, which can be mistaken for a positive nitrate test if aluminium is added later. Fresh portion, clean tube, every time.

Experimental Method

The sequence of steps, apparatus and conditions chosen to obtain a particular measurement or observation. Its advantages and disadvantages are judged against the task it was chosen for: the precision the result actually needs, the capacity and range required, the speed of the method, whether a quantity is collected directly, and the risks of leakage, breakage or human reading error. The same apparatus can be an excellent choice in one experiment and a poor one in another.

Measurement Apparatus

Laboratory equipment selected to measure a stated physical quantity. Cambridge O Level Chemistry names seven items: a stop-watch for time, a thermometer for temperature, a balance for mass, and four for volume - a burette for an accurately known variable volume of liquid, a volumetric pipette for one fixed volume of liquid, a measuring cylinder for a variable volume where a coarser scale is acceptable, and a gas syringe for a volume of gas measured directly.

Indicator End-point

The point in a titration at which the indicator shows a permanent change of colour, indicating that the reaction between the two solutions in the flask is just complete. It is reached by adding the solution from the burette drop by drop while swirling, and is taken at the first colour change that does not disappear on swirling. The colour seen depends on which indicator is used and which solution is in the flask.

Gas Test

A test that identifies a gas from a single characteristic result. Six are required: ammonia turns damp red litmus paper blue; carbon dioxide turns limewater milky; chlorine bleaches damp litmus paper; hydrogen pops with a lighted splint; oxygen relights a glowing splint; and sulfur dioxide turns acidified aqueous potassium manganate(VII) from purple to colourless. The test material is held at the mouth of the vessel or the gas is bubbled through the solution; a gas is never smelled directly.

Solvent

A substance that dissolves a solute. In a salt solution the water is the solvent. Which component is the solvent depends on the mixture, not on the substance alone: a liquid that dissolves nothing present is not acting as a solvent there.

Retention Factor

The retention factor, Rf, is the distance travelled by a substance divided by the distance travelled by the solvent, with both distances measured from the same baseline. Because it is a ratio of two lengths it has no unit, and for an ordinary chromatogram its value lies between 0 and 1. Rf depends on the solvent and conditions as well as on the substance, so values are only compared between chromatograms run under the same conditions.

Solute

A substance that is dissolved in a solvent. Sodium chloride is the solute in salt solution. A solid mixed with a liquid is only a solute if it actually dissolves; sand stirred into water is a suspended solid, not a solute.

Acid-base Titration

A method in which one solution is added in a controlled way from a burette to a measured portion of another solution in a conical flask, until an indicator shows that the reaction between them is just complete. A volumetric pipette measures the fixed portion, the burette measures the variable volume added, and the indicator marks the end-point. The volume delivered is the final burette reading minus the initial reading.

Paper Chromatography

A method for separating a mixture of soluble substances. Small concentrated spots of the mixture are placed on a pencil baseline near one end of a piece of chromatography paper, and the paper is stood in a suitable solvent with the solvent level below the baseline. The solvent travels up the paper and carries the substances with it by different amounts, so they separate into individual spots. Substances that are colourless are made visible with a locating agent.

Filtration

A separation method in which a mixture is poured through filter paper held in a funnel, so that an insoluble solid is held back while liquid or solution passes through. The solid retained is the residue and the liquid collected below is the filtrate. Either can be the desired product: the residue is washed with a little distilled water and dried, and the dissolved solid in the filtrate can be recovered by crystallisation.

Chromatogram Interpretation

Reading a developed chromatogram to draw conclusions about a sample. An unknown is identified by running it alongside known substances on the same paper in the same solvent and matching the positions of the spots. The number of spots produced by a sample shows whether it is a single substance or a mixture: one spot is interpreted as pure and more than one spot as impure.

Fractional Distillation

A separation method for two or more liquids that mix completely and have different boiling points. The mixture is boiled and the vapour passes up a fractionating column, where it repeatedly condenses and re-boils on the packing. Each cycle enriches the rising vapour in the component with the lower boiling point, so that component reaches the top and distils over first while the higher-boiling component runs back into the flask. The thermometer at the top of the column shows a steady reading while one component is distilling over.

Crystallisation

A purification method that obtains a dissolved solid as crystals. The solution is heated to evaporate some of the solvent until it reaches the point of crystallisation, then allowed to cool. As the temperature falls the maximum concentration that can stay dissolved falls, so the excess solute separates as crystals. These are filtered off, washed with a little cold solvent and dried. Soluble impurities stay behind in the remaining solution.

Simple Distillation

A separation method in which a liquid is boiled, the vapour is passed through a condenser where it is cooled back to a liquid, and the condensed liquid is collected as the distillate. It separates a solvent from a dissolved solid, or a liquid from components that do not vaporise under the conditions used. The thermometer bulb is placed level with the side-arm so that it measures the temperature of the vapour leaving the flask, and the cooling water enters the condenser at the lower end and leaves at the upper end.

Filtrate

A liquid or solution that has passed through a filter. In the filtration of a mixture of sand and salt solution, the filtrate is the salt solution collected below the funnel. The material held back on the filter paper has not passed through and is the residue, not the filtrate.

Experimental Planning

Designing an investigation so that its results can answer the question asked. It means identifying the independent variable that is changed, the dependent variable that is measured, and the control variables that must be kept the same because they would otherwise also affect the measurement; choosing a sensible range and number of values; selecting and justifying apparatus; and identifying hazards with a specific precaution for each.

Flame Test

A test in which a sample is held in a hot flame and the colour the flame turns is used to identify the metal ion present. Six colours are required: lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green and copper(II) blue-green. The colour names are set by the official notes, so answers use those words rather than descriptive alternatives such as crimson, brick red or apple green.

Error and Improvement

The evaluation stage of an investigation. A random error scatters results either side of the true value and its effect is reduced by repeating and averaging; a systematic error shifts every result the same way and repeating cannot remove it; a limitation is a restriction of the method itself rather than a fault in carrying it out; and a mistake is simply something done wrongly. A creditworthy improvement names the specific problem it addresses and changes the method or apparatus so that problem no longer arises.

Anion Test

A chemical test that identifies a negatively charged ion in a sample. Seven are required: carbonate by adding dilute acid and testing the gas for carbon dioxide; chloride, bromide and iodide by acidifying with dilute nitric acid then adding aqueous silver nitrate, giving white, cream and yellow precipitates respectively; nitrate by adding aqueous sodium hydroxide then aluminium foil and warming carefully, giving ammonia; sulfate by acidifying with dilute nitric acid then adding aqueous barium nitrate, giving a white precipitate; and sulfite by adding a small volume of acidified aqueous potassium manganate(VII), which changes from purple to colourless.

Separation Technique

A physical method that separates the components of a mixture by exploiting a difference in their physical properties. The Cambridge O Level Chemistry syllabus names five: using a suitable solvent, filtration, crystallisation, simple distillation and fractional distillation. The technique is chosen from the property information given about the substances involved, and the choice is justified by naming that property difference.

Solution

A mixture of one or more solutes dissolved in a solvent. Sea water is a solution of several salts in one solvent. A solution is a mixture, so its components are not chemically combined and can be separated by physical means such as distillation or crystallisation.

Identity and Purity

Two separate conclusions drawn from the same melting point or boiling point measurement. Identity comes from comparison: a measured value that agrees with a known value for a substance supports the sample being that substance. Purity comes from sharpness: a pure substance melts and boils at a fixed characteristic temperature under stated conditions, while an impure sample changes state over a range of temperatures. A measurement can support one conclusion without supporting the other.

Saturated Solution

A solution containing the maximum concentration of a solute dissolved in the solvent at a specified temperature. The temperature is part of the definition, because raising the temperature usually allows more solute to dissolve, so the same solution can be saturated at one temperature and unsaturated at a higher one.

Residue

A substance that remains after evaporation, distillation, filtration or any similar process. Sand caught on filter paper is a residue, so is salt left in an evaporating basin, and so is the liquid left behind in a distillation flask. Restricting the word to solid on filter paper is the most common definition error in this topic.

Aqueous Cation Test

A test that identifies a positively charged ion in solution using aqueous sodium hydroxide and aqueous ammonia. The reagent is added a few drops at a time and the first observation recorded, then added until it is in excess and the second observation recorded. Most cations give a coloured or white precipitate; some of those precipitates dissolve when the reagent is in excess. The two-stage result, not the first precipitate alone, is what identifies the ion.

Aqueous Ammonia Test

The second of the two reagents used to identify aqueous cations. Aqueous ammonia is added dropwise and then to excess, exactly as aqueous sodium hydroxide is, and the two observations recorded. Its value is that it does not give the same pattern as sodium hydroxide: it separates aluminium from zinc, which sodium hydroxide cannot, and it gives copper(II) a distinctive dark blue solution in excess. For ammonium there is no required result, and for calcium the required result is no precipitate or a very slight white precipitate.

• 1. Mass and weight Fix A balance measures mass, in grams. Weight is a force. In practical chemistry you record mass, and “weigh the solid” is laboratory shorthand for “find its mass”. Write mass in the answer.
• 2. Fixed volume and variable volume Fix A volumetric pipette transfers one fixed volume it was made for. A burette delivers any volume you choose, and tells you which volume that was. A measuring cylinder also gives a variable volume, but with coarser graduations. Match the apparatus to which of these the task needs.
• 3. A reading and a volume delivered Fix A burette reading is a position on a scale. The volume delivered is final reading minus initial reading. Even when the burette starts at \(0.00\ \mathrm{cm^3}\), write the subtraction — that is the habit that stops you quoting the final reading as the titre when it starts somewhere else.
• 4. Precision and accuracy Fix Accuracy is closeness to the true value. Precision is how finely the instrument resolves and how closely repeats agree. A balance reading to \(0.001\ \mathrm{g}\) that is not zeroed is precise and inaccurate. Do not use the words as synonyms; questions ask about one or the other.
• 5. Residue and filtrate Fix The filtrate is the liquid or solution that has passed through the filter. The residue is what remains after a process — and that process may be filtration, evaporation or distillation, not only filtration. Never define residue as “the solid on the filter paper”; that is one example of a residue, not the meaning of the word.
• 6. Solute and solvent Fix The solute is dissolved; the solvent does the dissolving. In salt water the solute is salt and the solvent is water. Say which is which explicitly — a lot of definition marks are lost by writing the pair the wrong way round under time pressure.
• 7. The end-point and a colour you hoped for Fix The end-point is the first permanent colour change, reached by adding dropwise near the end and swirling. A flash of colour that swirls away is not the end-point. Which colour you are waiting for depends on the indicator you chose and on which solution is in the flask — there is no single universal end-point colour.
• 8. The baseline and the solvent front Fix The baseline is the pencil line where the spots start. The solvent front is how far the solvent reached, marked as soon as the paper comes out of the tank before it evaporates. Both \(R_f\) distances are measured from the baseline, and the solvent must start below the baseline, never above it.
• 9. Spot distance and solvent distance Fix \(R_f\) puts the substance distance on top and the solvent distance underneath. Measure the substance distance to the centre of the spot. Getting a value above 1 means you divided the wrong way round or measured from the bottom edge of the paper.
• 10. Identity and purity Fix A melting point that matches a known value supports identity. A melting point that is sharp rather than spread over a range supports purity. They are separate deductions from the same measurement, and a question often asks for only one of them.
• 11. Reagent and observation Fix “Add aqueous silver nitrate” is the reagent; “a cream precipitate forms” is the observation. A full test answer needs both, in that order, and the acidification step where the official test includes one. Naming only the reagent, or only the colour, is half an answer.
• 12. A precipitate and a colour change Fix A precipitate is an insoluble solid appearing in a liquid — the mixture goes cloudy or a solid settles. A colour change is a solution changing colour while staying clear. Sulfite decolourises acidified aqueous potassium manganate(VII); it does not precipitate. Sulfate gives a white precipitate; it is not a colour change.
• 13. Damp litmus and dry litmus Fix The ammonia and chlorine tests both specify damp litmus paper. Dry paper may give no result at all, so “litmus paper” without “damp” can cost the mark. Ammonia turns damp red litmus blue; chlorine bleaches damp litmus paper.
• 14. The dropwise result and the excess result Fix Cation tests have two stages. Add the reagent a few drops at a time and record what forms; then keep adding until the reagent is in excess and record whether the precipitate stays or dissolves. Aluminium and zinc look identical at stage one with sodium hydroxide, and identical again in excess. Only a fresh sample with aqueous ammonia separates them.
• 1. “Use a burette, because it is the most accurate.” Why it fails Precision that nothing in the task depends on is wasted, and here it makes the method slower and no better. Apparatus is chosen for the job, not ranked in order of finesse. Correct Match the apparatus to what the volume is used for, and check its capacity fits the amount. Check Would you use a burette to add \(200\ \mathrm{cm^3}\) of wash water? Four fillings, four chances of error, for a number that appears in no calculation.
• 2. “The burette has an uncertainty of \(\pm 0.05\ \mathrm{cm^3}\).” Why it fails Nothing in the question said so. Uncertainties in this course come from the scale you are shown or from a value the question supplies, not from memory. Correct Read the scale in the diagram, work out what one division is worth, and record the value to a precision that scale supports. Check A diagram shows a measuring cylinder graduated every \(5\ \mathrm{cm^3}\). Quoting “\(\pm 0.5\ \mathrm{cm^3}\)” from memory would contradict the apparatus in front of you.
• 3. “The residue is the solid on the filter paper.” Why it fails That is one example of a residue, not the definition. Defining it that way makes the salt left in an evaporating basin, and the liquid left in a distillation flask, impossible to name. Correct A residue is a substance that remains after evaporation, distillation, filtration or any similar process. Check Sea water is distilled. What is the residue? The dissolved salts left in the flask — and there is no filter anywhere in the apparatus.
• 4. “A saturated solution is one where no more will dissolve.” Why it fails Warm it and more will dissolve, so the statement describes something that changes as soon as the temperature does. Correct A solution containing the maximum concentration of a solute dissolved in the solvent at a specified temperature. Check Why do crystals form when a hot saturated solution cools? Only the full definition, with temperature in it, can answer that.
• 5. “Fill the tank so the solvent covers the spots.” Why it fails The spots dissolve straight into the solvent in the tank instead of being carried up the paper, so nothing separates. Correct The solvent level must be below the pencil baseline. Check A diagram shows the baseline \(5\ \mathrm{mm}\) above the bottom edge and the solvent \(10\ \mathrm{mm}\) deep. Two faults follow from one measurement.
• 6. “Draw the baseline in pen so it is easy to see.” Why it fails Ink dissolves in the solvent and travels up the paper, producing spots of its own and contaminating the chromatogram. Correct Pencil, which does not dissolve, so the line stays where it was drawn and can still be measured from at the end. Check Why is that a problem specifically for interpreting the result? Because extra spots would be counted as substances in the sample.
• 7. “Measure both distances from the bottom of the paper.” Why it fails The strip below the baseline was travelled by neither the substance nor the separation, and adding the same length to the top and bottom of a fraction changes its value. Correct Both distances are measured from the pencil baseline, and the substance distance goes to the centre of the spot. Check \(\frac{36}{80} = 0.45\) but \(\frac{48}{92} = 0.52\). The same paper, the same spot, a different answer.
• 8. “My \(R_f\) is \(0.45\) and the data book says \(0.45\), so it is that substance.” Why it fails \(R_f\) depends on the solvent and the conditions as well as on the substance. A match between values obtained under different conditions is not evidence. Correct Run a known sample alongside the unknown, on the same paper in the same solvent, and compare the positions directly. Check Two students use the same solvent but let it run different distances. Can they compare? Their \(R_f\) values, yes; their raw distances, no.
• 9. “One spot proves the sample is pure.” Why it fails Two substances that travel the same distance in that solvent would also give one spot. The evidence supports purity; it does not prove it. Correct Give the exam interpretation — one spot means pure — and use “supports” rather than “proves”. Running the sample again in a different solvent tests the conclusion. Check The same caution applies to the sharp melting point: it supports purity, it does not settle it beyond all doubt.
• 10. “Filter the sea water to get pure water.” Why it fails The salts are dissolved, so they pass through the filter paper with the water. Filtration removes insoluble solid only. Correct Simple distillation: the water vaporises, is condensed and collected as the distillate; the salts stay behind as the residue. Check Sea water is filtered and then distilled. Which step removed the salt? Only the second.
• 11. “The residue is what you collect at the end of the distillation.” Why it fails What is collected is the distillate. The residue is what stays behind in the heated flask. Correct Distillate: condensed vapour, in the receiver. Residue: not vaporised, in the flask. Check In a fractional distillation of ethanol and water, which one is the ethanol at the start? The distillate — and the water is the residue until it in turn distils.
• 12. “It melts at \(133\ ^\circ\mathrm{C}\) instead of \(122\ ^\circ\mathrm{C}\), so it is impure.” Why it fails Purity is judged from the sharpness, not the position. A sharp value at the wrong temperature indicates a pure substance that is something else. Correct Two sentences: it melts sharply, so it is pure; the value does not match, so it is not that substance. Check Melts from \(119\) to \(122\ ^\circ\mathrm{C}\), known value \(122\ ^\circ\mathrm{C}\). Impure, and consistent with that substance.
• 13. “Acidify with dilute hydrochloric acid, then add aqueous silver nitrate.” Why it fails The acid adds chloride ions, so a white precipitate forms whatever the sample contained. The result carries no information. Correct Acidify with dilute nitric acid. Nitrate ions give no precipitate with silver nitrate. Check Which acid must be avoided before aqueous barium nitrate? Sulfuric acid — for exactly the same reason.
• 14. “Add sodium hydroxide and aluminium foil and look for ammonia.” Why it fails The official test says warm carefully. Without warming there may be no detectable gas, and the test appears negative when it is not. Correct Add aqueous sodium hydroxide, then aluminium foil; warm carefully; test the gas with damp red litmus paper. Check Which other test in 12.5 needs warming? Ammonium with sodium hydroxide — “ammonia produced on warming”.
• 15. “A white precipitate forms with sodium hydroxide, so it is calcium.” Why it fails Aluminium and zinc also give a white precipitate at that stage. Half the test has been done. Correct Add the reagent to excess and record the second observation. Insoluble in excess points to calcium; soluble points to aluminium or zinc. Check Does the same warning apply to the light blue precipitate? For identification, no — but the excess result is still part of the required answer.
• 16. “Hydrogen relights a glowing splint.” Why it fails Both halves of the pair have been swapped. Hydrogen pops with a lighted splint; oxygen relights a glowing splint. Correct Hydrogen burns, so it needs a flame already present — a lighted splint, and the pop is the small explosion. Oxygen makes other things burn, so it revives a glowing splint. Check A gas does neither. Name two of the six it could be, and how you would tell them apart.
• 17. “It turns red litmus paper blue, so it is ammonia.” Why it fails The official test specifies damp red litmus paper, and dry paper may show nothing at all. Correct Turns damp red litmus paper blue. Check Which other gas test specifies damp paper? Chlorine — it bleaches damp litmus paper.
• 18. “Copper(II) is present.” (as an answer to “state what you would observe”) Why it fails That is an inference offered where an observation was asked for. You cannot see an ion. Correct “A light blue precipitate forms, which dissolves in excess aqueous ammonia to give a dark blue solution.” Check The reverse also fails: “a cream precipitate” is not an answer to “deduce the anion”.
• 19. “Add sodium hydroxide to the tube that already has silver nitrate in it.” Why it fails The tube no longer contains the original substance. Whatever forms may come from the first reagent rather than from the sample. Correct A fresh portion of the original substance in a clean test-tube for every test. Check Why is a solution acidified with hydrochloric acid useless for a later halide test? It now contains chloride ions of its own.
• 20. “Improvement: repeat the experiment to make it more accurate.” Why it fails Repeating reduces the effect of random error by averaging. It does nothing about a systematic error, and the phrase shows the two have been confused. Correct Name the problem, then give a change that addresses it: “gas escaped while the bung was fitted, so use a sealed vessel with the acid added through an inlet”. Check Three titres agreeing closely but all \(1\ \mathrm{cm^3}\) too high. Would a fourth titration help? No.
• 21. “The syllabus names a volumetric flask as apparatus for measuring volume.” Why it fails Statement 12.1.1 names seven items, and a volumetric flask is not one of them: stop-watches, thermometers, balances, burettes, volumetric pipettes, measuring cylinders and gas syringes. Correct A volumetric flask may appear in a method you are given, and you can use it in an answer; it is simply not one of the seven you are required to name. Check Asked to “name the apparatus in 12.1.1 for measuring a fixed volume”, the answer is the volumetric pipette.
• 22. “Statement 12.1.3 is about collecting and drying gases.” Why it fails Statement 12.1.3 is the six definitions — solvent, solute, solution, saturated solution, residue and filtrate. Gas collection and drying methods are not part of Topic 12 for this cycle. Correct Learn the six definitions for 12.1.3, in the official wording. Check If a revision resource lists upward delivery and drying agents under 12.1.3, it is describing an older specification.
• 23. “The final reading was \(28.60\ \mathrm{cm^3}\), so the titre is \(28.60\ \mathrm{cm^3}\).” Why it fails A reading is a position on a scale, not a volume delivered. Some liquid had already left the burette before the titration began. Correct Titre = final reading − initial reading. Write the subtraction even when the initial reading is \(0.00\ \mathrm{cm^3}\). Check Initial \(3.10\), final \(28.60\). Titre \(25.50\ \mathrm{cm^3}\), not \(28.60\).
• 24. “The end-point is when the solution turns pink.” Why it fails Which colour appears depends on the indicator chosen and on which solution is in the flask. There is no universal end-point colour. Correct The end-point is the first permanent colour change of the indicator — one that does not disappear on swirling. Name a colour only if the indicator was named. Check Adding acid to alkali containing phenolphthalein means watching pink disappear, not appear.
• 25. “Titres must agree within \(0.10\ \mathrm{cm^3}\).” Why it fails No such universal tolerance is stated in this syllabus. Quoting one as a rule asserts something you were not told. Correct Repeat until the careful titres agree closely, and use whatever tolerance the question or the instructions give you. Check Titres of \(23.10\), \(24.60\) and \(23.15\ \mathrm{cm^3}\): you can say the second is out of line with the other two without quoting any fixed number.
• 26. “You must spray the paper with ninhydrin.” Why it fails The syllabus explicitly states that knowledge of a specific locating agent is not required. Naming one earns nothing and naming the wrong one can cost. Correct “Treat the dried paper with a locating agent, which makes the colourless spots visible so their positions can be measured.” Check When is a locating agent needed at all? Only when the separated substances are colourless.
• 27. “Use a separating funnel to separate the mixture.” Why it fails A separating funnel is not one of the five methods named in statement 12.4.1, so it is not the expected answer — and it only works at all for liquids that do not mix. Correct Choose from a suitable solvent, filtration, crystallisation, simple distillation and fractional distillation, and justify the choice from the property data. Check Ethanol and water are miscible, so no layers form; fractional distillation is the method.
• 28. “Chromium(III) gives a grey-green precipitate; copper(II) in excess ammonia gives a royal blue solution.” Why it fails Both are embellishments of the official wording. The notes say green and dark blue. Correct Chromium(III): green precipitate, soluble in excess. Copper(II) with excess ammonia: soluble, giving a dark blue solution. Check Iron(II) is also green — the same word, and the excess result is what separates them.
• 29. “Chlorine turns damp litmus red and then bleaches it.” Why it fails The required result is simply that chlorine bleaches damp litmus paper. Making the red stage compulsory risks writing an answer that does not match the result being marked. Correct “Bleaches damp litmus paper.” Check Ammonia's result is a colour change and chlorine's is bleaching — two different kinds of observation on the same kind of paper.
• 30. “Lithium is crimson, calcium is brick red, barium is apple green.” Why it fails None of those three words appears in the official flame-colour list, and a marker working from that list may not accept them. Correct Lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green, copper(II) blue-green. Check Which two pairs are easiest to confuse? The two reds and the two greens — and in each pair the qualifying word carries the whole distinction.

• The qualitative analysis notes are provided. The syllabus states that notes for use in qualitative analysis are supplied for both Paper 3 and Paper 4. That does not make 12.5 optional learning. Under timed conditions you have to read a result, match it to a row and combine it with a second result — and you will not do that fluently from a table you are meeting for the first time in the exam. Learn the rows so that the supplied notes become a check, not a lifeline. On Papers 1 and 2 they are not supplied at all.
• Decimal places carry meaning. A burette reading is recorded as \(22.35\ \mathrm{cm^3}\), not \(22.4\ \mathrm{cm^3}\), and an initial reading of exactly zero is recorded as \(0.00\ \mathrm{cm^3}\). Every reading in the same column of a table is written to the same number of decimal places, because the number of decimal places tells the reader what the instrument could resolve.
• On the rough run and on agreement. The first, quick titration overshoots on purpose — its job is to find roughly where the end-point lies so the later runs can be slowed down near it. Careful runs are then repeated until they agree closely, and it is those that are used. How close “closely” has to be is set by the question or the instructions you are working to; do not quote a fixed tolerance as though it were a universal rule, because none is stated in the syllabus.
• On locating agents. The syllabus asks you to describe the use of a locating agent when a mixture contains colourless substances — and states that knowledge of a specific locating agent is not required. So the creditworthy answer is about the job, not the name: “the paper is treated with a locating agent, which makes the colourless spots visible so their positions can be seen and measured”. Naming one is not wrong, but it earns nothing extra, and getting the name wrong can cost you.
• Two habits worth a mark each. Write the equation in words before substituting, and finish with “no unit” when the answer is an \(R_f\). Both are quick, and both are things markers look for. If a question gives distances in different units, convert first and say so in the working — an examiner cannot award a conversion you did in your head.
• The three-part answer. A full-credit 12.4.2 answer names the method, says which property difference makes it work, and states what you keep. “Fractional distillation” on its own is one third of an answer. “Fractional distillation, because the two liquids are miscible but have different boiling points, and the one boiling at \(65\ ^\circ\mathrm{C}\) is collected first” is all of it.
• What is reliable, and what is not. The dependable evidence for purity is the spread: pure substances change state at a fixed temperature, impure ones over a range. The direction in which an impurity shifts the value depends on the system, so build your answer on the range rather than asserting that an impurity always lowers a melting point or always raises a boiling point. If a question gives you both a known value and a measured range, use the range for purity and the position for identity, and say which you used for which.
• Two words that are worth marks on their own. Precipitate — not “it goes cloudy”, not “a solid”, and never “a blue solution” when a blue solid appeared. And excess — every cation test has two stages, and the second one is what separates aluminium from calcium and copper(II) from iron(II). If your answer to a cation test has only one observation in it, it is not finished.
• On chromium(III). The official result is “green precipitate, soluble in excess”. That is what to write. The notes do not specify a colour for the solution formed in excess, so do not invent one — and do not embellish the precipitate as “grey-green” or “dirty green” in a core answer. The word required is green, and the same word is used for iron(II).
• Two entries that are not ordinary precipitates. For ammonium, the official table has a dash: aqueous ammonia is not used to test for it, and inventing a result loses the mark. For calcium, the required result is “no precipitate or a very slight white precipitate”, and the qualification matters — write it in full, because “a white precipitate” would put calcium alongside aluminium, and “no reaction” on its own drops the alternative the notes allow for.
• Why the wording matters. The right-hand column is not a list of wrong observations — some of those words describe the flames rather well. It is a list of words that are not in the official notes, and a marker working from those notes may not accept them. There is no cost to writing the official word, and there is a real risk in writing anything else. This is one of the few places in chemistry where the exact vocabulary is the answer.
• What a full deduction answer contains. One line per test, in the order observation then inference; a sentence naming the cation and a sentence naming the anion, each pointing at its evidence; and only then the compound. If the evidence names an ion but not a formula — for example when the charge on the metal has not been established — name the ion and stop there. Inventing a formula the evidence does not support turns a correct deduction into a wrong answer.
• Control variables need a reason. Listing three things you kept the same earns little. Saying why each one had to be kept the same — because it would otherwise also affect the dependent variable — is what the mark is for. A quick test: if changing it would not change your measurement, it is not worth naming.
• Spotting an anomaly without guessing. Compare a result with the pattern the rest of the data makes, not with what you expected. Here, the means rise by about \(15\ \mathrm{cm^3}\) for each \(0.40\ \mathrm{mol/dm^3}\) step, so a value near \(60\ \mathrm{cm^3}\) is expected at \(1.60\) — which makes run 1's \(72\) the suspect, not run 2's \(58\). Naming which reading is anomalous, and saying what the pattern led you to expect, is worth more than the word “anomaly” on its own.

• Every candidate takes three papers: Paper 1, Paper 2, and one practical paper chosen from Paper 3 or Paper 4. The two practical papers require the same experimental skills and the same experimental contexts, and both test assessment objective AO3 — the difference is whether you carry the experiments out yourself.
• The qualitative analysis notes are provided. The syllabus states that notes for use in qualitative analysis are supplied for both Paper 3 and Paper 4. That does not make 12.5 optional learning. Under timed conditions you have to read a result, match it to a row and combine it with a second result — and you will not do that fluently from a table you are meeting for the first time in the exam. Learn the rows so that the supplied notes become a check, not a lifeline. On Papers 1 and 2 they are not supplied at all.
• Across the whole qualification the assessment objectives are weighted AO1 50%, AO2 30% and AO3 20%. Within components, Papers 1 and 2 are 63% AO1 and 37% AO2, and the practical papers are 100% AO3. So one fifth of your grade is earned by experimental skill and reasoning — the material in this chapter and the overlay at the end of it.
• Most lost marks in practical questions come from answering the wrong one of these three. They are not interchangeable, and a question asks for exactly one of them.
• A colour, a precipitate, a gas, a temperature, a scale reading. Written as if you were standing at the bench. “A light blue precipitate formed. It dissolved when more aqueous ammonia was added, giving a dark blue solution.”
• The ion, the gas, the substance, the conclusion. It names something you never actually saw. “The solution contains copper(II) ions.”