Your revision route
What you’ll learn
- Distinguish a chemically pure substance from a mixture using melting or boiling point data.
- Explain why a formulation uses measured components with particular purposes.
- Explain paper chromatography, calculate an Rf value and report it with sensible significant-figure precision.
- Recall the tests and positive observations for hydrogen, oxygen, carbon dioxide and chlorine.
- Separate Chemistry: identify selected cations and anions using flame, precipitation and gas tests.
- Separate Chemistry: compare instrumental methods with chemical tests and interpret flame emission data.
Build the big picture
Key ideas
Pure has a strict chemical meaning
Chemical purity is about composition: exactly one elemental or compound substance, regardless of whether a product is natural or premium.
- Under fixed conditions, a single substance has characteristic melting and boiling temperatures.
- Impurities usually produce a melting or boiling range and shift the measured value.
- Compare measured data with reference values to judge whether a sample is pure.
The punchline: Use composition and physical data, not everyday advertising language.
A formulation is a mixture with a job description
Paint, fuel and medicine work because deliberately proportioned ingredients contribute chosen properties.
- Manufacturers combine ingredients in controlled proportions so the finished material behaves as intended.
- Every ingredient is selected to perform a defined job.
- Foods, fertilisers and alloys are formulations, as are medicines, paints, cleaning products and fuels.
The punchline: Identify each component's function and the property it helps create.
Chromatography separates by divided loyalties
Each substance is pulled by the mobile solvent and held by the stationary paper to a different extent.
- The stationary phase stays in place while the mobile phase moves through it.
- A mixture may form several spots; a pure compound gives one spot in every solvent tested.
- Rf values identify substances only when compared under the same solvent and conditions.
- Measure from origin to spot centre and origin to solvent front, then use appropriate significant figures.
The punchline: Same solvent, same conditions, same measuring origin—or the comparison is weak.
From pencil line to chemical evidence
A chromatogram is a race in which each substance divides differently between a stationary phase and a moving solvent.
- OriginPlace small sample spots on a pencil line above the solvent; ink would dissolve and contaminate the result.
- SeparationThe solvent rises while substances travel different distances according to their distribution between the two phases.
- Solvent frontMark it before the paper dries, then measure both distances from the same origin line.
- InterpretCompare spot number, positions and Rf values obtained using the same solvent and conditions.
Gas tests turn invisible samples into visible evidence
The gas is unseen, but its behaviour with a carefully chosen splint or reagent leaves a signature.
- Hydrogen pops with a burning splint; oxygen relights a glowing splint.
- Carbon dioxide turns limewater milky; chlorine bleaches damp litmus paper white.
- Do not replace ‘glowing’ with ‘burning’ or omit that litmus must be damp.
- Chlorine is toxic: do not inhale it. Use only small, teacher-controlled quantities, follow the school risk assessment and provide suitable ventilation.
The punchline: Name the test material, its condition and the positive observation.
Separate Chemistry: metal ions colour a flame
A clean flame becomes a rapid colour code, though mixtures can let one bright signal mask another.
- Lithium is crimson, sodium yellow, potassium lilac, calcium orange-red and copper green.
- Clean the wire loop between samples and use a non-luminous flame.
- A mixture can mask weaker colours, so a flame test is not always conclusive.
The punchline: Learn only the five specified colours and recognise the mixture limitation.
Make the model move
Interactive checkpoint
Touch the science. Change a state, build a route or test a relationship.
Match evidence to gas
Which positive result belongs to which gas?
Pair each gas with its diagnostic observation, including the correct test material.
A reliable identification states both what was done and what was observed; vague claims such as ‘it changed colour’ are not diagnostic.
Measure the chromatogram
Explore the Rf ratio
Change the spot and solvent-front distances. Both must be measured from the origin in the same unit.
Rf = spot distance / solvent distance
Retention factor0.6 no unit
Rf compares a spot's travel from the origin with the solvent front's travel. Matching length units cancel, so the ratio has no unit.
Separate Chemistry: hydroxide precipitates reveal cations
Add sodium hydroxide and a dissolved ion may build an insoluble, coloured solid in front of you.
- Al³⁺, Ca²⁺ and Mg²⁺ all give white solids; excess sodium hydroxide redissolves the aluminium precipitate but leaves the calcium and magnesium precipitates.
- Cu²⁺ gives blue, Fe²⁺ green and Fe³⁺ brown precipitates.
- Examples: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) and Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s).
- To balance, use the cation charge to choose the number of OH⁻ ions, then check atoms and total charge on both sides.
The punchline: Colour identifies possibilities; a balanced ionic equation explains how the solid forms.
Separate Chemistry: acid choice protects the logic
The preliminary acid is not decoration: the wrong one can import the very ion you hope to detect.
- Carbonate plus dilute acid releases carbon dioxide, confirmed with limewater.
- For halides, use dilute nitric acid then silver nitrate: iodide gives yellow, chloride white and bromide cream.
- For sulfate, add dilute hydrochloric acid before barium chloride; formation of a white solid is positive.
The punchline: Pair each ion with its exact acid, reagent and observation.
Four gases, four diagnostic observations
The reagent or splint is part of the answer. A colour or sound without the test conditions is only half an identification.
- HydrogenA burning splint at the tube mouth gives a rapid pop.
- OxygenOxygen is identified when the gas makes a glowing splint flare back into flame.
- Carbon dioxideBubble or shake with limewater; it turns milky or cloudy.
- ChlorineChlorine bleaches moist litmus white. Toxic gas: do not inhale. Use small, teacher-controlled amounts, suitable ventilation and the school risk assessment.
Separate Chemistry: combine tests without contaminating the evidence
Use fresh portions of the unknown for complementary cation and anion tests, then combine both observations to identify the salt.
- Split the samplePlace the unknown solution into separately labelled fresh portions so one reagent cannot interfere with a later test.
- Test cation: portion AUse a flame test or add sodium hydroxide; record flame colour or precipitate colour and behaviour in excess.
- Test anion: portion BUse a fresh portion and choose the carbonate, halide or sulfate sequence; never continue with portion A.
- Protect the logicUse nitric acid before silver nitrate for halides, or hydrochloric acid before barium chloride for sulfate.
- Combine evidenceJoin the supported cation and anion conclusions—for example, Na⁺ plus Cl⁻ identifies sodium chloride.
Separate Chemistry: identify the lines, then interpolate intensity
Line wavelength identifies the ion. With a linear calibration under matching conditions, line intensity estimates concentration.
- Read line positionsA sample has a strong line at 589 nm and another at 766 nm.
- Identify ionsReference data match 589 nm to Na⁺ and 766 nm to K⁺, so both ions are present.
- Bracket sodium dataAt 589 nm, 0.10 mol/dm³ gives intensity 40 and 0.20 mol/dm³ gives intensity 80.
- Place the unknownIts 589 nm intensity is 60, exactly halfway between 40 and 80 on the stated linear calibration.
- InterpolateHalfway between 0.10 and 0.20 mol/dm³ gives sodium concentration ≈ 0.15 mol/dm³.
Separate Chemistry: instruments hear chemical whispers
Where a bench test sees a broad colour, an instrument can detect a smaller signal quickly and precisely.
- Compared with these wet tests, instruments can return results sooner, detect smaller quantities and produce measurements closer to accepted values.
- Sensitivity means detecting very small amounts; accuracy concerns closeness to the accepted value.
- Interpret the supplied output against reference data rather than guessing from appearance.
The punchline: State the concrete advantage in context: speed, a smaller detection limit or closeness to the accepted value.
Separate Chemistry: a spectrum is a metal ion's barcode
Light from a flamed sample is split into lines whose positions identify ions and whose intensity can reveal concentration.
- Feed the solution into a flame; a spectroscope separates its emitted light into a line pattern.
- Compare line positions with reference spectra to identify metal ions.
- Use calibrated intensity data to determine concentration when the question supplies it.
The punchline: Line position identifies; calibrated intensity measures concentration.
Words worth knowing
Key definitions
- pure substance
- Material consisting solely of one element or one compound, with no additional substances present.
- formulation
- A deliberately proportioned blend whose ingredients give the finished product chosen properties.
- stationary phase
- The phase that remains fixed during chromatography.
- mobile phase
- The solvent or phase that moves through the stationary phase.
- precipitate
- An insoluble solid formed from substances in solution.
- Separate Chemistry: instrumental method
- An analytical technique using an instrument to detect, identify or measure substances.
Calculate with confidence
Equations
Retention factor
Rf = distance moved by substance / distance moved by solvent
A ratio comparing movement of a spot with movement of the solvent front in the same chromatogram.
| Symbol | Meaning | Unit |
|---|---|---|
| Rf | retention factor | no unit |
| distance | distance from origin to spot centre or solvent front | same length unit for both |
Exam tip: Measure both distances from the origin, then round the ratio sensibly for the measurement precision.
Follow it step by step
Processes to remember
How to analyse a chromatogram
- Count the spots in each sample lane.
- Compare spot heights only where the solvent and conditions match.
- Measure from the origin to each spot centre and to the solvent front.
- Calculate Rf and compare it with reference values from the same conditions.
Exam tip: Repeat with a different suitable solvent: mixture components that overlap once may then separate, whereas a pure sample will not split into components.
How to report an identification test
- Name the sample preparation or preliminary acid where required.
- Name the reagent or splint and its condition.
- Record the exact positive observation.
- Conclude which gas or ion the result supports.
Exam tip: Observation and conclusion are different marks: ‘white precipitate’ is evidence; ‘chloride ions’ is the inference.
Separate Chemistry: balance a hydroxide precipitation equation
- Write the metal ion with its charge and write OH⁻.
- Use the metal-ion charge to choose how many hydroxide ions are needed in the neutral precipitate.
- Write the solid formula in brackets where more than one OH group is present.
- Check each atom and the total charge on both sides.
Exam tip: Include states: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s); Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s).
See the thinking
Worked example
Worked example: calculate and interpret Rf
A spot centre moves 4.2 cm from the origin while the solvent front moves 7.0 cm. Calculate Rf. A reference substance has Rf = 0.60 in the same solvent and conditions. What does the result suggest?
- Start with the Rf ratio stated above: spot travel divided by solvent-front travel.
- Substitute: Rf = 4.2 cm / 7.0 cm.
- Calculate: Rf = 0.60; the centimetres cancel.
- Compare only because the reference used the same solvent and conditions.
Answer: Rf = 0.60 with no unit. The matching value supports the suggestion that the spot may be the reference substance.
The match is evidence, not absolute proof on its own. A different solvent or conditions could change Rf, and another substance could coincide under one set of conditions.
Protect the marks
Common mistakes
Watch out: Measuring an Rf distance from the bottom of the paper.
Do this instead: Measure both spot and solvent distances from the pencil origin line.
Watch out: Giving Rf a unit.
Do this instead: It is a ratio of two distances in the same unit, so the units cancel.
Watch out: Assuming a product is chemically pure because it is natural.
Do this instead: Chemical purity requires a single elemental or compound substance; being unadulterated in everyday language is not enough.
Watch out: Using a burning splint for oxygen.
Do this instead: Use a glowing splint; it relights in oxygen.
Watch out: Separate Chemistry: calling every white hydroxide precipitate aluminium.
Do this instead: All three are white, but excess sodium hydroxide removes the aluminium precipitate while the calcium and magnesium solids remain.
Watch out: Separate Chemistry: using hydrochloric acid before testing for chloride.
Do this instead: Use dilute nitric acid so the acid does not add chloride ions.
Plan it like the exam
Required practicals
Separate and identify coloured substances by paper chromatography
Combined Science and separate Chemistry
Aim: Separate coloured mixtures by paper chromatography and calculate Rf values from the finished chromatogram.
Method
- Draw a pencil origin line above the solvent. Use a separate clean capillary for each sample; apply a small, concentrated spot and let each application dry before re-spotting.
- Stand the paper in a covered container with the solvent below the origin; keep it upright and avoid touching the sides.
- Allow the solvent to rise, remove the paper before it reaches the top and mark the solvent front immediately in pencil.
- Dry the paper, mark spot centres, measure distances from the origin and calculate Rf values.
- Repeat with standards or another solvent if the identification needs stronger evidence.
Variables
- Independent
- sample or solvent being compared
- Dependent
- spot pattern, distance moved and calculated Rf
- Controls
- paper type and dimensions
- origin and solvent depth
- spot volume and drying
- development time and temperature
Analysis: Compare the number and positions of spots. Calculate Rf for each spot using the same origin and solvent front; compare only with reference data collected using the same solvent and conditions.
Safety
- Wear eye protection and follow the school hazard guidance for the selected solvent and samples.
- Keep volatile or flammable solvents covered and away from flames, hot equipment and ignition sources.
- Avoid skin contact and use the solvent only in the teacher-approved quantity and ventilation.
Improvements
- Use capillary tubes to make small, repeatable spots.
- Run standards beside the unknown on the same paper.
- Repeat the chromatogram and test a second solvent if spots overlap.
Separate Chemistry: identify ions in unknown single ionic compounds
Separate Chemistry only
Aim: Use flame, hydroxide and anion tests to identify the ions in teacher-provided unknown single ionic compounds.
Method
- Plan a test sequence from the possible ions and label a fresh portion of unknown for each branch.
- Carry out a flame test on a clean wire loop where appropriate and record the colour.
- Add sodium hydroxide solution dropwise, then in excess where needed; record precipitate colour and solubility.
- Test fresh portions for carbonate, halide or sulfate using the correct acid and reagent sequence.
- Compare observations with known controls and report evidence before the final identity.
Variables
- Independent
- identity of the teacher-provided unknown
- Dependent
- flame colour, gas result or precipitate colour and behaviour
- Controls
- sample amount and concentration
- reagent identity, concentration and volume
- clean apparatus and separate sample portions
- flame conditions where used
Analysis: Use a decision table to connect each exact positive result with the supported ion. An absent or ambiguous result should trigger a repeat with fresh apparatus, not a forced conclusion.
Safety
- Wear splash-proof eye protection and follow the school's risk assessment throughout.
- Use only small, teacher-approved quantities; acids, alkalis and several test reagents can irritate or damage skin and eyes.
- Keep hair and loose clothing secured around a lit Bunsen burner; extinguish it when flame testing is complete.
- Treat barium and silver reagents and unknowns as hazardous laboratory chemicals; avoid contact and dispose of them in labelled school waste, not the sink.
- Do not deliberately inhale gases; use the specified test at the tube mouth or by transferring gas as directed.
Improvements
- Use known positive and negative controls.
- Use fresh portions to prevent one reagent contaminating the next test.
- Repeat uncertain colours under consistent lighting.
Try it before you move on
Quick check
Say your answer first, then open the card to check it.
A sample melts over 78–82 °C. What does the range suggest?
Answer: The sample is likely impure or a mixture.
A narrow transition at the expected temperature supports purity; a melting interval suggests additional substances.
A spot moves 3.0 cm and the solvent front 6.0 cm. What is Rf?
Answer: 0.50
Rf = 3.0 / 6.0; the matching distance units cancel.
Which gas relights a glowing splint?
Answer: Oxygen.
Hydrogen instead gives a pop with a burning splint.
Separate Chemistry: what colour is silver bromide?
Answer: Cream.
Silver iodide is yellow, silver chloride is white and silver bromide is cream.
Separate Chemistry: which white hydroxide dissolves in excess sodium hydroxide?
Answer: Aluminium hydroxide.
Calcium and magnesium hydroxides remain as white precipitates in this test.
Separate Chemistry: reference: Na⁺ 589 nm; K⁺ 766 nm. Sample has both. Linear calibration: 0.10 mol/dm³→40, 0.20→80; sample→60. Give ions and Na⁺ concentration.
Answer: Na⁺ and K⁺ are present; sodium concentration is about 0.15 mol/dm³.
589 nm identifies sodium and 766 nm potassium. On the stated linear calibration, intensity 60 is halfway between 40 and 80, so concentration is halfway between 0.10 and 0.20.
Good questions, clear answers
Frequently asked questions
Does one chromatography spot prove a substance is pure?
One solvent can hide two components that travel together. Repeat with a different suitable solvent: separation then reveals the mixture, whereas a genuinely single component will not split into several spots.
Why must the solvent begin below the origin line?
If the sample is submerged, it can dissolve directly into the solvent reservoir instead of travelling up the paper with a measurable starting point.
Are flame and ion tests in Combined Science?
The four gas tests and chromatography are shared. The specified flame tests, hydroxide and anion tests, instrumental methods and flame emission spectroscopy are separate Chemistry content.
Separate Chemistry: why are instrumental methods useful?
They can work quickly, detect small quantities and give measurements close to accepted values. Flame emission spectroscopy identifies metal ions from line positions and measures concentration from calibrated signal intensity.
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