Use flame tests, sodium-hydroxide tests and anion/gas tests to identify unknown salts. Record observations first, then infer ions from the combined evidence.Apparatus and method schematic; not to scale. Use the stated controls and measurements.
Use separate fresh portions for different tests. Contaminating a sample with chloride or sulfate reagents can create a false result in a later test.
Wear eye protection: acid/alkali, barium compounds and silver nitrate require school handling/disposal. Heat small quantities safely and never smell ammonia directly.
Use clean labelled tubes, small specified volumes, distilled water and known comparisons/controls. Negative means not detected under these conditions, not necessarily absent at every concentration.
Flames and positive ions
Clean a flame-test wire as instructed with acid/rinsing and heating until it gives no residual colour. Introduce the supplied solid into a non-luminous blue flame; contamination with sodium can mask other colours.Examples of hydroxide precipitates with sodium hydroxide, alongside a clean-loop flame-test setup. Use fresh portions and the stated reagent sequence.
Flame colours: lithium red; sodium yellow; potassium lilac; calcium orange-red; copper blue-green. Similar or weak colours need supporting evidence from other tests, not guesses.Observations need clean samples and corroborating evidence where tests overlap.
Add sodium hydroxide dropwise to fresh solutions. Copper(II) gives blue precipitate, iron(II) green and iron(III) brown/reddish-brown.
Aluminium gives a white hydroxide precipitate that dissolves in excess NaOH. Calcium gives a white precipitate that does not dissolve in excess under the school method; excess testing distinguishes the two.
For ammonium, add NaOH and warm gently. Ammonia turns damp red litmus blue; the paper must be damp and test the gas, not contact alkaline solution.
Iron(II) precipitate can turn brown on standing because of oxidation. Record the initial observation promptly.
Negative ions and interpretation
Carbonate: add dilute acid to a fresh portion. CO₂ bubbles; pass gas through limewater, which turns milky. Effervescence alone does not identify CO₂.
Sulfate: acidify a fresh portion with dilute HCl then add barium chloride. A white barium sulfate precipitate is positive; acid helps remove interfering carbonate.
Halides: acidify another fresh portion with dilute nitric acid then add silver nitrate. Chloride gives white, bromide cream and iodide yellow precipitate.
Do not use HCl before a halide test: its chloride gives a false positive. Do not use sulfuric acid before sulfate testing: it introduces sulfate.
A salt contains a positive and a negative ion. Combine both identifications, using charges to deduce a formula if asked; a cation result alone does not identify the whole salt.
Repeat uncertain tests with clean portions and known controls. Instrumental analysis can improve sensitivity and colour discrimination, but its result still requires suitable calibration and interpretation.
Explain each improvement’s purpose. Clean the loop to remove substances that could change the flame colour. Use fresh sample portions so reagents from one test do not affect another. Damp litmus lets ammonia dissolve in water before changing the indicator colour.
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 chloride introduced before a halide test.
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