Separate coloured dyes in ink using paper chromatography, then collect water from water-based (aqueous) ink by simple distillation. Chromatography separates dissolved substances. Distillation collects a solvent that can evaporate (a volatile solvent).Apparatus and method schematic; not to scale. Use the stated controls and measurements.
Chromatography uses paper, pencil, ruler, capillary/dropper, suitable solvent and a covered container. Distillation uses a heated flask, thermometer, condenser and receiving vessel.
Use eye protection and secure glassware. Heating hot glass and solvent handling require the school procedure. Never seal a heated distillation system or heat a flask to dryness.
The solvent must dissolve the ink components. Use water-based ink/water for the aqueous distillation part; flammable organic solvents must not be heated over a naked flame.
Chromatography method and analysis
Draw a pencil baseline and place small concentrated ink spots on it. Allow spots to dry; repeated tiny applications can concentrate a spot without making it excessively wide.
Suspend paper with the baseline above solvent level. Cover the vessel where appropriate to limit evaporation; keep paper away from container sides and allow solvent to rise.
Remove before the front reaches the top, mark the solvent front immediately in pencil and let the chromatogram dry. An unmarked evaporated front prevents a valid distance measurement.
Measure from baseline to each spot centre and from baseline to front in the same direction. Rf = spot distance/front distance; it has no unit and normally lies between 0 and 1.
A mixture can give multiple spots; a single spot supports purity in that system but does not prove purity absolutely. Overlapping components or insoluble material may conceal a mixture.
Compare unknown and known samples on the same paper under identical conditions. Equal Rf supports identification, but one matching value alone is not conclusive.
Different dyes have different attractions to the stationary phase in the paper and the moving solvent. A dye that is more strongly attracted to the solvent compared with the paper generally travels further.
Distillation method and evaluation
Gently heat aqueous ink in the flask with apparatus clamped securely. A Bunsen burner is used where specified for the aqueous core procedure; add boiling chips before heating, if instructed, to reduce sudden, uneven boiling (bumping).Simple distillation: vapour passes through the inner condenser tube; cooling water flows around it from lower inlet to upper outlet. The receiver remains open.
Position the thermometer bulb by the side-arm vapour path, not immersed in the liquid. Vapour passes into a condenser, cools and becomes liquid collected in an open receiving vessel.
Supply condenser cooling water at the lower inlet and let it leave at the upper outlet so the jacket fills. Keep joints secure and check the cooling flow.
Water evaporates and is collected, while dyes that do not evaporate (non-volatile dyes) stay in the flask. Distillation uses boiling and condensation. Filtration cannot remove dissolved dye from water.
A collected boiling-temperature range and other tests can assess purity, but not every impurity is detectable by colour alone. Do not claim every distillate is safe to drink.
Improve chromatography by small dry spots, pencil marks, solvent below baseline and prompt front marking. Improve distillation by correct bulb position, controlled heating, cooling and secure joints.
Measure distances with a suitable ruler and use consistent spot-centre criteria. Wider spots increase uncertainty; repetitions do not repair using a pen baseline that dissolves.
Exam skills: planning, precision and evaluation
State what you change (the independent variable), what you measure (the dependent variable) and what you keep the same (control variables). Explain how you keep each control variable constant, rather than just saying “make it fair”.
Accuracy means how close a result is to the true value. Precision means how close repeated measurements are to each other. Resolution is the smallest change an instrument can show. More digits on a display do not automatically mean a more accurate result.
Repeat measurements for each condition, calculate a mean and describe how spread out the results are. This helps assess and reduce the effect of random errors. Repeating cannot fix an error that pushes results consistently in one direction (a systematic error), such as a soluble ink baseline contaminating chromatograms.
Repeatability means getting similar results when the same person repeats the same method with the same equipment. Reproducibility means getting similar results when someone else, or different suitable equipment, repeats the experiment. Results can be consistent but still inaccurate.
Check that instruments read zero correctly and are calibrated where needed. Read scales at eye level: looking from an angle can give a wrong reading (parallax error). Choose suitable ranges, measurement intervals and scale divisions (resolution).
Write down the original readings straight away in a table, with units in the headings. Use decimal places that match the instrument’s resolution. Keep the original data and round only when needed. Do not discard a result just because it differs from your prediction.
An anomalous result does not fit the pattern of the other results. Repeat that measurement and check the method. Only leave it out of a mean if you have a clear reason; state which result you excluded and why.
For continuous variables, plot the independent variable on the horizontal axis and the dependent variable vertically. Use sensible scales, units and a best-fit line or curve; do not automatically join every point or force the graph through zero.
Find the gradient of a straight best-fit line using a large triangle: vertical change ÷ horizontal change. For a curve, draw a tangent to estimate the gradient at one point. Explain what the gradient shows in this experiment, include its units and use measured values to support your conclusion.
Uncertainty describes the possible range around a measurement. For one reading on a scale, half the smallest division is a useful classroom estimate unless the question says otherwise. If you subtract two readings, both have uncertainty. Percentage uncertainty = absolute uncertainty ÷ measured value × 100. Follow the method specified in the question.
Use results as evidence and then explain what they mean. A pattern linking variables (a correlation) does not prove that one causes the other. If the ranges of repeat results overlap, a claimed difference may be less convincing. Keep conclusions within the range tested and suggest an improvement that tackles a specific error.
Mr Kidman's Classroom