Chemical analysis

A sealed sample contains only solid sulfur. Decide whether it is a pure substance in the chemical sense and justify your decision.

Reference data give the melting point of substance P as $64\,^\circ\mathrm{C}$. Batch A melts sharply at $64\,^\circ\mathrm{C}$, while batch B melts from $58$ to $61\,^\circ\mathrm{C}$. Which batch is more likely to be pure? Explain.

A drink is advertised as 'pure fruit juice'. Analysis shows water, sugars, acids and flavour compounds. Explain why the label may be reasonable in everyday language but the drink is not pure in chemistry.

A cleaning spray is made from measured amounts of solvent, detergent, fragrance and dye, each with a stated purpose. State why the spray is a formulation.

A $250\,\mathrm{g}$ fertiliser formulation is $18\%$ nitrogen compound, $12\%$ potassium compound and the remainder filler. Calculate the mass of filler and explain one purpose of measuring the components accurately.

Two batches of a medical cream contain the same chemicals. In batch X the components were weighed precisely; in batch Y their proportions varied. Evaluate which batch better fits the definition of a formulation.

On a chromatogram, a pigment centre is $4.2\,\mathrm{cm}$ above the origin and the solvent front is $7.0\,\mathrm{cm}$ above the origin. Calculate the pigment's $R_f$ value.

Describe how to use paper chromatography to find out whether a purple pen ink contains more than one soluble dye. Include two setup details that prevent misleading results.

A solvent front moves $8.0\,\mathrm{cm}$. An unknown gives spots at $2.4\,\mathrm{cm}$ and $5.6\,\mathrm{cm}$. Reference $R_f$ values in this solvent are: J $0.30$, K $0.55$, L $0.70$. Identify the substances present and state why this evidence does not prove the sample contains only those substances.

A colourless gas makes a pop when tested at the mouth of its tube with a lit splint. Identify the gas.

Give the procedure and positive observation for confirming that gas collected from a metal-acid reaction is hydrogen.

A student tests a gas using a glowing splint and sees no change, then concludes that hydrogen is absent. Evaluate the conclusion and describe the correct confirmation test.

A glowing splint returns to flame inside a jar of colourless gas. Name the gas.

Describe a test that distinguishes oxygen from a gas that does not support combustion. State the positive result.

Three observations are reported for one gas: a lit splint gives no pop, limewater stays clear, and a glowing splint relights. Use all three results to identify the gas and explain which observation is decisive.

An unknown gas is shaken with clear limewater, which becomes cloudy. Identify the gas.

State the reagent and the expected visible change when testing a gas sample for carbon dioxide.

A learner writes: 'Carbon dioxide is confirmed because it extinguishes a flame.' Improve this claim to give the specification test, its reagent and its positive observation.

Damp litmus paper loses its colour and becomes white in an unknown gas. Identify the gas.

Describe how litmus paper is used to test for chlorine and state the observation that confirms a positive result.

Two students test the same gas. One uses dry blue litmus and sees no change; the other uses damp litmus and it turns white. Explain which result should be used and what conclusion follows.

A compound produces a crimson flame. Identify the metal ion present.

Samples M and N give a green flame and an orange-red flame respectively. Identify the metal ion in each sample.

A mixture known to contain two metal ions gives only an intense yellow flame. State one ion supported by the observation and explain why the second ion cannot be identified confidently from this test alone.

Adding sodium hydroxide solution to an unknown ionic solution forms a brown precipitate. Identify the metal ion.

Solutions A and B each form a white precipitate when a little sodium hydroxide is added. The precipitate from A dissolves after excess sodium hydroxide is added, but B's remains. What can be concluded about A, and why is B not fully identified?

Three solutions are tested with sodium hydroxide. W gives a blue precipitate, X gives a green precipitate, and Y gives a white precipitate that dissolves in excess sodium hydroxide. Identify the metal ion in W, X and Y.

A solid fizzes when dilute acid is added, and the gas makes limewater cloudy. Name the ion detected in the solid.

Describe a complete chemical test for carbonate ions in an unknown powder, including how the gaseous product is identified.

Powders C and D both fizz when dilute acid is added. The gas from C turns limewater cloudy, but the gas from D leaves limewater clear. Explain which powder has tested positive for carbonate ions and why fizzing alone is insufficient evidence.

After dilute nitric acid and silver nitrate are added to a solution, a yellow precipitate forms. Identify the halide ion.

Give the reagents, in order, used to test an aqueous sample for halide ions, and state the result for chloride ions.

Samples R, S and T form white, cream and yellow precipitates respectively after the correct halide test. Identify all three ions and state the two reagents that must have been added, in order.

An acidified solution forms a white precipitate when barium chloride solution is added. Identify the ion being tested.

Describe the reagent sequence and positive observation for testing an unknown solution for sulfate ions.

A student adds barium chloride directly to an unknown solution, sees a white precipitate and reports sulfate ions. Evaluate the procedure and give the complete test needed before accepting the identification.

State one advantage of an instrumental method over a chemical test based on a visible colour change.

A laboratory must screen $180$ water samples in one day for a contaminant present at very low concentration. Give two reasons for choosing an instrumental method.

For a certified $10.0\,\mathrm{mg\,dm^{-3}}$ standard, method A reports $12\,\mathrm{mg\,dm^{-3}}$ after $25$ minutes and misses very dilute samples. Method B reports $10.1\,\mathrm{mg\,dm^{-3}}$ after $40$ seconds and detects every sample. Compare the methods using the three specification advantages of instrumental analysis.

Reference lines occur at $589\,\mathrm{nm}$ for sodium and $671\,\mathrm{nm}$ for lithium. A sample has one line at $671\,\mathrm{nm}$. Identify the metal ion.

A sodium calibration is linear through the origin. An intensity of $24$ units corresponds to $3.0\,\mathrm{mg\,dm^{-3}}$. A sample gives $40$ units under identical conditions. Calculate its sodium-ion concentration.

A solution produces emission lines at $589\,\mathrm{nm}$ and $766\,\mathrm{nm}$. References assign these to sodium and potassium. At $766\,\mathrm{nm}$, standards of $2.0$ and $6.0\,\mathrm{mg\,dm^{-3}}$ give intensities $15$ and $47$ units; the sample gives $31$ units. Identify both ions and estimate the potassium-ion concentration, assuming a linear response.