Overview
What you’ll learn
- Plan an investigation with a clear hypothesis, one independent variable, a measured dependent variable and controlled variables.
- Choose apparatus that matches both the quantity and the precision needed, and read scales correctly with appropriate significant figures.
- Select a suitable purification or separation technique from the physical properties of the substances in a mixture.
- Describe filtration, evaporation and crystallisation, simple and fractional distillation, and justify the details of each set-up.
- Interpret paper chromatograms, calculate Rf values, and use chromatography to judge purity and identify substances.
- Use melting point and boiling point data to decide whether a sample is pure.
- Choose a collection method and a drying agent for a gas from its density and solubility.
In 30 seconds
- 1A fair test changes one factor (the independent variable) and measures one outcome (the dependent variable) while every other condition stays fixed.
- 2Repeat readings and average them — repeats expose anomalies and make results more reliable.
- 3Match the apparatus to the precision needed: a 50 cm³ measuring cylinder reads to about 1 cm³, while a pipette or burette measures to 0.1 cm³.
- 4Filtration separates an insoluble solid from a liquid; evaporation keeps the dissolved solid; crystallisation gives pure crystals instead of a crust of impurities.
- 5A simple still separates a solvent from dissolved solids; a fractionating column separates miscible liquids with different boiling points.
- 6In chromatography the stationary phase is the paper and the mobile phase is the solvent; Rf = distance moved by substance ÷ distance moved by solvent front.
- 7A pure substance melts and boils at one sharp temperature; impurities lower and widen the melting range.
- 8Collect gases by upward or downward delivery, or over water, depending on density and solubility — and never dry ammonia with concentrated sulfuric acid.
Notes · 1 of 6
Designing a fair investigation
One thing changes, one thing is measured, everything else is held constant.
Every good investigation starts with a hypothesis — a testable statement predicting how one quantity affects another. For example: "the rate at which sugar dissolves increases as the temperature of the water increases". The experiment then exists to test that statement, not to wander.
| Variable | Meaning | Sugar-dissolving example |
|---|---|---|
| Independent | The one factor you deliberately change | Temperature of the water |
| Dependent | The one factor you measure as the outcome | Time taken for the sugar to dissolve |
| Controlled | Factors kept the same so the test is fair | Volume of water, mass of sugar, grain size, stirring |
Reliability comes from repeats: do the whole run again (ideally three times) and average, ignoring any obviously anomalous result — after checking it really is anomalous. Precision is about how finely a quantity can be measured, and accuracy is about how close a measurement is to the true value: a balance can be precise but inaccurate if it was not zeroed.
Notes · 2 of 6
Apparatus, precision and readings
Pick the instrument whose scale matches the size and precision of the quantity.
Choose an instrument that suits the size of the quantity and how precise you need to be. A measuring cylinder only a little bigger than the volume you need gives a more precise reading than a large one, because its scale divisions are smaller.
Beaker
Rough (tens of cm³)
Measuring cylinder
About 1 cm³
| Apparatus | Typical precision | Use it for |
|---|---|---|
| Pipette | 0.1 cm³ at one fixed volume | Delivering one exact volume |
| Burette | 0.1 cm³ at any volume | Adding variable volumes, titrations |
| Volumetric flask | One exact volume | Making up a standard solution |
For heating, the Bunsen burner gives a hot, blue, non-luminous flame when the air hole is open. Heat liquids in a beaker on a tripod and gauze; never heat a measuring cylinder or a graduated pipette — heat changes their calibration.
Notes · 3 of 6
Choosing a separation technique
Every technique exploits a difference in physical properties.
A mixture is separated by finding a physical property in which its components differ — solubility, boiling point, density, or how strongly each substance sticks to paper. Pick the property first; the technique follows.
| Mixture | Difference used | Technique |
|---|---|---|
| Insoluble solid + liquid | Solubility (solid insoluble) | Filtration |
| Dissolved solid + solvent | Solubility (solid soluble) | Evaporation to dryness, or crystallisation |
| Solvent + dissolved solid | Boiling point (solvent much lower) | Simple distillation |
| Two miscible liquids | Boiling points close but different | Fractional distillation |
| Substances that travel differently on paper | Solubility on paper | Paper chromatography |
- 1Filtration
The residue stays on the filter paper; the filtrate passes through. Works because the solid particles are too large for the pores of the paper.
- 2Evaporation to dryness
Heat the solution until all the solvent has gone. Fast, but heat-sensitive solids decompose and impurities are trapped in the crust.
- 3Crystallisation
Heat gently until the solution reaches the point of crystallisation (dip a glass rod: small crystals form on cooling), then leave it to cool slowly and filter off the crystals. Pure, well-formed crystals; gentle on delicate salts.
Details that earn marks: in simple distillation the thermometer bulb sits level with the side-arm, so it reads the temperature of the vapour entering the condenser; cooling water enters the bottom of the condenser and leaves at the top, so the jacket stays full of cold water and the vapour meets progressively colder glass; a few anti-bumping granules stop violent, uneven boiling.
Notes · 4 of 6
Paper chromatography and Rf values
Substances separate because they differ in solubility in the solvent and attraction to the paper.
A spot of mixture is placed on a start line drawn in pencil (pencil marks do not dissolve or move). The paper stands in a shallow layer of solvent below the line. As the solvent soaks up the paper — the mobile phase moving through the stationary phase — each substance dissolves and travels at its own speed: more soluble substances, and those less attracted to the paper, travel further.
| Term | Meaning |
|---|---|
| Start line (baseline) | Pencil line where spots are placed; must be above the solvent level |
| Solvent front | The furthest point the solvent has reached |
| Rf value | Distance moved by substance ÷ distance moved by solvent front (always between 0 and 1) |
| Locating agent | Spray used to make colourless spots visible (e.g. ninhydrin for amino acids) |
A chromatogram is also a purity test: a pure substance gives exactly one spot, because there is nothing else to separate. A mixture gives one spot per component. To identify the spots, run known pure samples alongside and compare heights — or compare Rf values with published tables for that solvent.
Notes · 5 of 6
Judging purity from melting and boiling
Pure substances have sharp melting and boiling points; impurities blur them.
A pure substance changes state at one fixed temperature: pure ice melts at exactly 0 °C and pure water boils at exactly 100 °C at standard pressure. While it changes state, the temperature stays constant — the energy goes into the change of state, not into raising the temperature.
| Sample | Melting behaviour | Conclusion |
|---|---|---|
| Melts sharply at 156 °C (all of it, at once) | One temperature, narrow range | Pure — 156 °C is indium’s accepted melting point, and the sharp single value is the key |
| Starts softening at 150 °C, fully liquid at 154 °C | Melts over a range, below the true value | Impure |
So impurities lower the melting point and spread it over a range, and they raise and spread the boiling point. This is the standard evidence question: given a table of temperature against time for two samples, the one with the flat, longer plateau at the correct value is the pure one.
Notes · 6 of 6
Collecting and drying gases
Density decides the direction of delivery; solubility decides whether water is allowed.
| Method | Use when the gas… | Examples |
|---|---|---|
| Collection over water | Is insoluble or only slightly soluble in water | Hydrogen, oxygen (carbon dioxide works but dissolves a little) |
| Upward delivery (air displaced upwards) | Is denser than air | Carbon dioxide, chlorine |
| Downward delivery (air displaced downwards) | Is less dense than air | Ammonia, hydrogen |
To dry a gas, pass it through a drying agent: concentrated sulfuric acid for most gases, or fused calcium oxide (quicklime) and calcium chloride for others. The choice matters — ammonia is alkaline and reacts with concentrated sulfuric acid, so it must be dried with quicklime instead.
The full tables of tests for cations, anions and gases are examined constantly and are developed in full in Chapter 7 (Salts and Qualitative Analysis) — start memorising the gas tests now, since this chapter already asks for them by name.
Safety
Flammable liquids
Ethanol and other fuel vapours ignite easily: heat with an electric mantle or a water bath, never a naked flame.
Safety
Corrosive reagents
Concentrated acids and alkalis blister skin and blind eyes: wear goggles, and add acid to water when diluting.
Safety
Toxic gases
Chlorine and ammonia irritate lungs even at low levels: generate them in a fume cupboard.
Revise
Key terms
The words examiners expect you to use precisely.
- Hypothesis
- A testable statement predicting how one quantity affects another.Stated before the experiment and judged by the results.
- Independent variable
- The single factor deliberately changed in an investigation.
- Dependent variable
- The single factor measured as the outcome of an investigation.
- Controlled variables
- The conditions kept identical so that a test is fair.
- Filtrate
- The liquid that passes through the filter paper during filtration.
- Residue
- The solid left behind on the filter paper during filtration.
Show 7 more key termsShow fewer key terms
- Point of crystallisation
- The concentration at which crystals just form on cooling; found by dipping a glass rod into the hot solution.
- Fractional distillation
- Separation of miscible liquids using their different boiling points and a fractionating column.
- Stationary phase
- The phase that does not move in chromatography — the chromatography paper.
- Mobile phase
- The solvent that moves up the chromatography paper, carrying the substances with it.
- Rf value
- Distance moved by a substance divided by distance moved by the solvent front; a pure-substance fingerprint for a given solvent.
- Locating agent
- A spray that converts colourless chromatography spots into visible ones.
- Drying agent
- A substance that absorbs water vapour from a gas without reacting with it, e.g. quicklime for ammonia.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Changing two variables at once, e.g. different grain size and different temperature, then blaming temperature.
Two changed variables make the result unattributable — the test is not fair.
Instead: Change the independent variable only; control everything else.
Mistake: Reading a measuring cylinder above or below eye level.
An angled view shifts the meniscus against the scale (parallax error).
Instead: Eyes level with the bottom of the meniscus.
Show 12 more mistakesShow fewer mistakes
Mistake: Quoting a burette reading as 21.3 cm³.
Burettes are read to 0.1 cm³, so the reading has two decimal places: 21.30 cm³.
Instead: The trailing zero carries information about precision.
Mistake: Choosing evaporation to dryness when the question wants pure crystals.
Heating to dryness traps impurities and decomposes heat-sensitive solids.
Instead: Concentrate to the point of crystallisation, cool slowly, then filter and dry between filter papers.
Mistake: Saying fractional distillation separates ethanol and water because they have "different densities".
Density has nothing to do with a still; both liquids are miscible and stay mixed.
Instead: Miscible liquids separate because of different boiling points.
Mistake: Putting the thermometer bulb into the liquid in a distillation flask.
The liquid is hotter than the vapour; the reading would not show when the pure vapour arrives.
Instead: The bulb sits level with the side-arm, in the vapour stream.
Mistake: Running cooling water into the top of the condenser.
The water jacket would drain partly empty and the cooling would be uneven.
Instead: Water enters at the bottom and leaves at the top, against the vapour flow.
Mistake: Drawing the chromatography start line in ink.
Ink dissolves and travels, adding its own spots.
Instead: Use pencil — graphite does not dissolve in the solvent.
Mistake: Letting the solvent start above the pencil baseline.
The samples would dissolve into the bulk solvent instead of travelling up the paper.
Instead: The start line must sit above the solvent level.
Mistake: Writing Rf = 0.80 cm.
Rf is a ratio of two distances; the units cancel.
Instead: Rf values carry no units and lie between 0 and 1.
Mistake: Claiming a substance is pure because its melting point is "high".
Purity is shown by a sharp melting point at the accepted value, not by a high one.
Instead: Pure: one fixed temperature; impure: lower, over a range.
Mistake: Collecting ammonia over water.
Ammonia is very soluble, so it dissolves before it can be collected.
Instead: Collect ammonia by downward delivery and dry it with quicklime.
Mistake: Drying any gas with concentrated sulfuric acid.
Ammonia is alkaline and neutralises the acid — the gas is destroyed, not dried.
Instead: Match the drying agent to the gas: quicklime for ammonia.
Mistake: Calling anomalous readings "wrong" and quietly deleting them.
Anomalies are identified and excluded with a reason, or the run is repeated.
Instead: Repeat, average the consistent values, and comment on the anomaly.
Check yourself
Quick quiz
20 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Plan a fair investigation
In an investigation into how temperature affects how fast sugar dissolves, which set-up makes the test fair?
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Practise
Topical questions
Exam-style practice in three tiers, from recall to data questions.
- 4Foundation
- 5Standard
- 3Challenging
12 questions worth 50 marks, each with a model answer, mark scheme and examiner tips.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a school paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 1 Practice Paper
30 marks · 40 min · 4 pages
5 multiple-choice questions and structured questions with answer lines. Chapter 1 (experimental design, separation and purification, chromatography, purity, gas collection).
Mark scheme
Chapter 1 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Where this chapter is tested
Each school sets its own scope — check your assessment outline.
Questions
Frequently asked questions
What does Sec 3 Chemistry Chapter 1 cover?
It covers the experimental-chemistry section of O-Level Pure Chemistry 6092: planning fair investigations with variables and precision, separation and purification techniques (filtration, evaporation, crystallisation, simple and fractional distillation), paper chromatography with Rf values, purity from melting and boiling points, and collecting and drying gases. The full ion and gas test tables follow in Chapter 7.
How do I choose between evaporation and crystallisation?
If the solid decomposes on heating or you want pure, well-formed crystals, use crystallisation: concentrate the solution until a crust forms on a glass rod, then cool slowly, filter and dry. Evaporation to dryness is faster but traps impurities and can destroy heat-sensitive solids, so it is the weaker answer in exams unless the question only wants the solvent gone.
Why do Rf values have no units?
An Rf value is one distance divided by another, in the same units, so the units cancel: Rf = distance moved by the substance ÷ distance moved by the solvent front. Values always sit between 0 and 1, and they are only comparable when the same solvent is used.
How is purity judged in this chapter?
Three ways: a pure substance melts and boils at one sharp temperature (impurities lower and widen the melting range and raise the boiling range); a pure substance gives a single spot on a chromatogram; and in distillation the thermometer holds steady at the accepted boiling point while the pure fraction comes over.
When is this chapter usually tested in school?
Experimental chemistry opens the Sec 3 year, so it features in Weighted Assessment 1 (Term 1) and again in the end-of-year examination — and its techniques reappear inside practical-style questions all the way to the O-Level papers. Scope and weighting vary by school, so check your school’s assessment outline.
Up next
Chapter 2: Kinetic Particle Theory