Experimental chemistry
Measurement, apparatus, gas collection, purity and separation techniques, with practical skills for G3 Pure and O-Level Chemistry.
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Before you begin
Every result in chemistry comes from an experiment, and an experiment is only as good as its measurements and methods. The lessons start with choosing and reading apparatus and drawing it, then planning a fair test, presenting results and evaluating them. They go on to the core laboratory techniques: titration, collecting and drying gases, separating mixtures by filtration, crystallisation, distillation and chromatography, and checking whether a substance is pure.
Be comfortable with: reading a scale and using units such as cm³, g and °C. For practical exam preparation, see the Chemistry practical guide.
Learning goals
- Name apparatus for measuring time, temperature, mass and volume, including burettes, pipettes, measuring cylinders and gas syringes
- Suggest suitable apparatus for simple experiments, including drying and collecting gases and measuring reaction rates
- Describe separation and purification using a suitable solvent, filtration, and crystallisation or evaporation
- Describe separation by sublimation
- Describe separation by distillation and fractional distillation
- Describe separation using a separating funnel
- Describe separation by paper chromatography
- Suggest how to separate and purify a mixture of solids from information about its substances
- Suggest how to separate and purify a solid–liquid mixture from information about its substances
- Suggest how to separate and purify mixtures of miscible or immiscible liquids from information about their substances
- Interpret paper chromatograms by comparing them with known samples and using Rf values
- Explain why locating agents are needed to chromatograph colourless compounds
- Deduce a substance's identity and purity from melting and boiling point data
- Explain why the purity of everyday substances, such as foods and drugs, matters
- Follow a sequence of practical instructions
- Use practical techniques, apparatus and materials
- Make and record observations, measurements and estimates in experiments
- Interpret and evaluate observations and results from experiments
- Plan an investigation and choose its techniques, apparatus and materials
- Evaluate an experimental method and suggest improvements
Syllabus statements covered
- name appropriate apparatus for the measurement of time, temperature, mass and volume; including burettes, pipettes, measuring cylinders and gas syringes
- suggest suitable apparatus, given relevant information, for a variety of simple experiments, including drying and collection of gases and measurement of rates of reaction (drying agents will be limited to calcium oxide, concentrated sulfuric acid and fused calcium chloride).
- describe methods of separation and purification for the components of mixtures, to include: — use of a suitable solvent, filtration and crystallisation or evaporation
- describe methods of separation and purification for the components of mixtures, to include: — sublimation
- describe methods of separation and purification for the components of mixtures, to include: — distillation and fractional distillation (see also 11.1(b))
- describe methods of separation and purification for the components of mixtures, to include: — use of a separating funnel
- describe methods of separation and purification for the components of mixtures, to include: — paper chromatography
- suggest suitable separation and purification methods, given information about the substances involved in the following types of mixtures: — solid-solid
- suggest suitable separation and purification methods, given information about the substances involved in the following types of mixtures: — solid-liquid
- suggest suitable separation and purification methods, given information about the substances involved in the following types of mixtures: — liquid-liquid (miscible and immiscible)
- interpret paper chromatograms including comparison with ‘known’ samples and the use of Rf values
- explain the need to use locating agents in the chromatography of colourless compounds (knowledge of specific locating agents is not required)
- deduce from given melting point and boiling point data the identities of substances and their purity
- explain the importance of measuring the purity in substances used in everyday life, e.g. foodstuffs and drugs.
- follow a sequence of instructions
- use techniques, apparatus and materials
- make and record observations, measurements and estimates
- interpret and evaluate observations and experimental results
- plan investigations, select techniques, apparatus and materials
- evaluate methods and suggest possible improvements.
Lessons
Work through them in order.
Measurement and recording
Planning and evaluation
- Planning a chemistry investigationPlan a fair test, with variables, a method someone else could follow, repeats and safety.
- Presenting practical results in tables and graphsRecord results in tables, draw graphs with best-fit lines and find a rate from a gradient.
- Evaluating practical results and improving a methodTell random from systematic errors and accuracy from precision, and suggest specific improvements.
Collection and separation
- Titration technique and concordant resultsCarry out a titration, read a burette and choose concordant titres for the mean.
- Collecting gases and measuring their volumesChoose how to collect a gas from its solubility and density, and measure its volume.
- Choosing a drying agent for a gasChoose a drying agent that removes water without reacting with the gas.
- Choosing separation methodsChoose filtration, crystallisation, sublimation or a separating funnel from how the substances differ.
- Simple and fractional distillationSeparate liquids by simple and fractional distillation, and explain the job of each piece of apparatus.
- Paper chromatography and Rf valuesRun a chromatogram, compare an unknown with known substances and calculate Rf values.
- Using evidence to assess purityJudge purity and identity from melting points, boiling points and chromatograms, and explain why purity matters.
Practise and check
Or choose
Topic reference
Quick reference
| Item | Quick rule / reminder |
|---|---|
| Recording readings | stopwatch: nearest 1 s; thermometer: nearest 0.5 °C; balance: 0.01 g; burette: nearest 0.05 cm³ (2 d.p.); pipette: 25.0 cm³ |
| Volume conversions | 1 cm³ = 1 mL; 1 dm³ = 1000 cm³ |
| Meniscus | read the bottom of the meniscus, with the eye level with it |
| Concordant titres | at least two titres within 0.20 cm³; leave the rough titre out of the mean |
| Collecting a gas | insoluble → over water; soluble and less dense than air → upward delivery; soluble and denser → downward delivery; to measure → gas syringe |
| Drying a gas | ammonia: quicklime; acidic gases: concentrated sulfuric acid or fused calcium chloride |
| Distillation | thermometer bulb level with the side arm; condenser water in at the bottom |
| Chromatography | pencil start line above the solvent; Rf = (distance moved by spot)/(distance moved by solvent), between 0 and 1 |
| Purity | pure: sharp melting and boiling points, one spot; impure: melts lower and over a range, boils over a range |
Core knowledge to remember
- Accuracy: how close a result is to the true value. Precision: how close repeated readings are to each other.
- Random error: readings that scatter unpredictably; reduced by repeating and taking a mean. Systematic error: every reading out by the same amount; not reduced by repeating.
- Parallax error: a misreading caused by viewing a scale from an angle.
- Residue: the solid left on the filter paper. Filtrate: the liquid that passes through. Distillate: the liquid collected by distillation.
- Concordant titres: titres that agree closely, within 0.20 cm³.
- Locating agent: a substance sprayed on a chromatogram to make colourless spots visible.
- Pure substance: a single element or compound, with a sharp melting point and boiling point.
Common mistakes
- Burette readings: record to two decimal places, ending in 0 or 5.
- Soluble gases: never collect them over water.
- Ammonia: never dry it with concentrated sulfuric acid or fused calcium chloride.
- Distillation: the thermometer bulb goes at the side arm, not in the liquid.
- Chromatography: draw the start line in pencil and keep it above the solvent.
- Purity: one spot, or a sharp melting point alone, does not prove a substance is pure.
- Evaluation: name a specific cause and a specific improvement, not “human error”.