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A test has discriminating value only in relation to the candidate substances. If the set contains an alcohol, alkene and carboxylic acid, a hydrogencarbonate test can identify the acid; bromine-water behaviour on a fresh portion can then distinguish the remaining suitable candidates. In a larger set, the same observations may be shared by additional compounds, so the conclusion must remain conditional.
Video by Royal Society of Chemistry. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Design an efficient fresh-portion testing sequence
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What should appear before an inference in a results table?
The actual observation and conditions. A reader needs evidence to evaluate the inference.
After watching 2: Why does dichromate reduction alone not uniquely identify an alcohol?
Aldehydes can also reduce it. Use a selective complementary test where candidates overlap.
What you'll be able to do
A test has discriminating value only in relation to the candidate substances. If the set contains an alcohol, alkene and carboxylic acid, a hydrogencarbonate test can identify the acid; bromine-water behaviour on a fresh portion can then distinguish the remaining suitable candidates. In a larger set, the same observations may be shared by additional compounds, so the conclusion must remain conditional.
Write a results grid with rows for labelled samples and columns for reagent, conditions, observation and inference. Plan fresh small portions. Adding every test to the same tube changes the unknown, introduces new ions and can make later results uninterpretable. Choose tests that are useful and suitably safe, rather than maximising the number performed.
For anions, acid and limewater can support carbonate, acidified barium chloride can support sulfate, and nitric acid followed by silver nitrate and appropriate ammonia comparisons can support chloride, bromide or iodide. The acid must not introduce the ion under investigation. Ammonium releases ammonia on warming with hydroxide, detected with damp red litmus according to the approved procedure.
For the first-year organic candidates, bromine-water decolourisation supports reactive unsaturation in the appropriate set. Acidified dichromate reduction can indicate an oxidisable alcohol or aldehyde; it is not selective for an alcohol alone. A selective aldehyde test and acid–hydrogencarbonate response can resolve useful distinctions. Record physical state as well as colour: a coloured solution is different from a precipitate.
A negative result only helps exclude a candidate when the reagent, concentration and conditions would have detected it. A known reference can check an uncertain reagent. A slowly hydrolysing compound should not be excluded by an immediate observation alone. If two candidates share the complete tested pattern, report that limitation and propose a discriminating fresh-portion test.
Teacher-approved microscale quantities, suitable warming and specified disposal reduce reagent use and exposure. Do not identify gases by direct inhalation or store reactive test residues. Report a chain of evidence: observation, chemically justified inference, then identification within the supplied set. Do not replace the chain with “it reacted”.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A sulfate test was acidified with sulfuric acid. Explain why the white barium precipitate cannot identify sulfate in the original unknown.
Repeat on a fresh portion with the specified non-sulfate acid and clean apparatus.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.