JC2 H2 Chemistry Tutor | Preparing a Qualitative-Analysis Decision Tree with Unresolved Cases

State that the unknown contains one cation from the three listed possibilities. This is a deliberate simplification for teaching decision-making. It excludes mixtures and unlisted ions from the first task, so the student can make a justified choice within a known set.

Ask for the cation and the observations supporting it. Do not ask for the complete salt identity unless relevant anion evidence is also supplied. Identifying Zn²⁺ does not establish whether the original solution was zinc nitrate, zinc sulfate or another suitable salt.

Provide the relevant qualitative notes beside the task. The intended skill is reading and applying the evidence, rather than testing whether the student can recall every entry without access to information.

Write the endpoint of each branch before drawing the student version. Some branches should end with a candidate set rather than a single name. That makes an incomplete conclusion visible instead of encouraging guesses.

Keep the actual practical context separate from the paper exercise. Laboratory use of reagents, sample handling and waste disposal follows the institution’s supervised arrangements. The tutor’s question can teach the logic without supplying an improvised operating procedure.

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For the three candidates, the notes describe white precipitates with aqueous sodium hydroxide. In excess sodium hydroxide, the aluminium and zinc precipitates dissolve, while the magnesium precipitate remains.

With aqueous ammonia, these candidates also give white precipitates. In excess ammonia, the zinc precipitate dissolves, while the aluminium and magnesium precipitates remain. The excess-reagent observation provides a useful distinction.

Ask the student to read both the initial and excess entries before concluding. White precipitate is only the first part of the record. The later behaviour can change which candidates remain.

Keep aqueous ammonia and sodium hydroxide as distinct reagent conditions. A similar first observation does not make their full patterns identical. The tutor key should preserve the reagent name beside each result.

Explain the word excess in relation to the supplied observation pattern. It is not merely a decorative instruction after adding a little reagent. A report that omits whether excess was used may leave an important distinction unresolved.

A final clear solution can represent two different histories: no precipitate formed, or a precipitate formed and then dissolved in excess reagent. Ask students to preserve the sequence in the written record. Otherwise a decisive observation may be lost even when the final appearance is described accurately.

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Start with the candidate set {Al³⁺, Mg²⁺, Zn²⁺}. A white precipitate with sodium hydroxide leaves all three candidates possible in this restricted task. No unique identity follows from that observation alone.

If the precipitate remains in excess sodium hydroxide under the suitable supplied conditions, the remaining candidate is Mg²⁺. The inference uses both the observation and the stated three-ion set.

If the precipitate dissolves in excess sodium hydroxide, the remaining set is {Al³⁺, Zn²⁺}. A student should write those two possibilities rather than choose the more familiar ion.

Ask what the first branch has achieved. It can isolate magnesium or reduce the remaining problem to aluminium versus zinc. A decision tree is useful because each observation changes the candidate set in an explainable way.

Prepare the answer key with the initial set, the observation and the reduced set on separate lines. This makes it easier to see whether an error came from reading the notes, tracking candidates or overclaiming a conclusion.

A useful student annotation is: after this result, magnesium is excluded but aluminium and zinc remain. Naming an exclusion as well as a candidate set can make the reasoning easier to inspect, especially when a student tends to jump directly from one reagent name to an ion.

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For the unresolved aluminium-or-zinc branch, use the supplied ammonia pattern from a fresh portion of the original unknown. Dissolution in excess ammonia supports Zn²⁺ within the candidate set; a precipitate remaining supports Al³⁺.

The fresh-portion condition matters. A sample already treated with excess sodium hydroxide has a different chemical environment. Its response after ammonia is added cannot simply be read as though it were the original solution in the ammonia column.

On the paper worksheet, label the results as separate portions A and B. This prevents a student from treating the logical sequence of decisions as a claim that every reagent is added consecutively to the same tube.

Ask why ammonia helps at this point. It produces different excess-reagent outcomes for the two remaining candidates. The choice is justified by discrimination, rather than by adding another reagent because a practical list includes it.

Keep complex-ion equations at the appropriate taught depth. The candidate-reduction task can first use the supplied empirical patterns, then add precipitation or complex-formation explanations when those are part of the student’s current learning.

If the sodium-hydroxide result has already left only aluminium or zinc, an ammonia precipitate that remains selects aluminium in that restricted branch. Without the earlier restriction, the same ammonia result would also fit magnesium. An observation’s discriminating value depends on which candidates are still under consideration.

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Unknown X gives a white precipitate with sodium hydroxide that dissolves in excess. A fresh portion gives a white precipitate with ammonia that also dissolves in excess. The first result leaves aluminium or zinc, and the second selects zinc within the stated set.

Unknown Y gives a white precipitate that remains in excess sodium hydroxide. Under the same suitable restricted assumptions, magnesium is supported. An ammonia result showing a white precipitate remaining is consistent with that identification.

Unknown Z dissolves in excess sodium hydroxide, while the precipitate from a fresh ammonia portion remains in excess. The combined pattern selects aluminium within the supplied candidate set.

The table below records the verified reference patterns. Ask students to explain why one row fits and why another does not. Matching a row is more useful when the student can articulate the decisive comparison.

Do not describe the table as a universal identification of every white precipitate. Its scope is the selected candidates and the conditions of the supplied notes. Expanding the candidate set may require additional observations.

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Supply only white precipitate, soluble in excess sodium hydroxide for another unknown. The correct endpoint is aluminium or zinc still possible. The student should identify the next useful observation rather than make a forced identification.

Now supply only a white precipitate with ammonia, without an excess observation. All three selected candidates can fit that initial result. The missing information is scientifically meaningful.

Ask for a complete sentence: the current evidence does not distinguish these candidates; a fresh-portion excess-ammonia observation would distinguish aluminium from zinc after the sodium-hydroxide result. That sentence connects limitation to a next decision.

Do not penalise the student for preserving uncertainty that the task genuinely leaves unresolved. A precise candidate set is a stronger answer than an unsupported single ion.

Prepare at least one incomplete record in the first practice set. If every question always ends in an obvious unique name, students may learn that identification is compulsory even when the evidence is insufficient.

The candidate sets can be checked by intersection. The sodium-hydroxide result may permit {Al³⁺, Zn²⁺}, while the ammonia result permits {Al³⁺, Mg²⁺}; their shared candidate is Al³⁺. This is a compact reasoning aid, not an additional formula students must memorise.

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Consider a separate fictional record: white precipitate remains in excess sodium hydroxide, but a fresh ammonia portion gives a white precipitate that dissolves in excess. No one of the three listed candidates fits both supplied results.

Label this as an audit task. The student should state that the record is inconsistent with the restricted model and conditions, rather than choose magnesium from one result and ignore the other.

Possible checks include sample labels, reagent labels, whether excess was actually reached, contamination, concentration and the assumption of one cation. These are hypotheses to investigate; the paper data do not establish which error occurred.

Preserve the original observations when investigating a real record. A verified correction should be documented with its reason. Quietly rewriting a result to match the expected ion removes evidence the next teacher may need.

After the audit, give a fresh consistent record. Students should leave the lesson with the valid patterns and the rule for handling conflict clear, rather than remember only an impossible combination.

If the actual unknown contains more than one cation, the single-cation decision tree is no longer sufficient. A mixture can produce a combined pattern that should not be forced into one reference row. Return to the composition assumption before interpreting an apparent contradiction as an incorrect observation.

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Where appropriate, use Mg²⁺ + 2OH⁻ → Mg(OH)₂(s) to connect the white precipitate to hydroxide formation. Check the atom and charge balance before using the equation in the answer key.

For aluminium, Al³⁺ + 3OH⁻ → Al(OH)₃(s) describes the initial precipitate. Its behaviour in excess hydroxide involves further chemistry; do not infer that the precipitate never formed merely because the final mixture is clear.

Likewise, dissolving in excess ammonia should be recorded as a change after the initial precipitate. A final colourless solution without the sequence can be less informative than the complete observation.

Students can first use the supplied table, then explain the underlying chemistry at the taught level. Requiring an unfamiliar complex formula as an unannounced prerequisite would obscure the decision skill the task was designed to assess.

Ask the student to separate observation from inference. White precipitate dissolved in excess ammonia is an observation; zinc is supported within this candidate set is the inference. This distinction strengthens both practical reporting and written reasoning.

Ask why a colour description should include precipitate or solution. A white precipitate and a colourless solution describe different physical observations. Clear language helps the student follow whether a solid formed, remained or dissolved, rather than reducing the record to isolated colour words.

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Give the same candidate information in a different order and ask the student to design an alternative first branch. Excess ammonia can first isolate zinc from the other two, after which suitable sodium-hydroxide evidence can distinguish aluminium and magnesium.

The two valid decision orders illustrate that a tree need not have one memorised layout. Its quality comes from justified candidate reduction and clear sample conditions.

Remove the completed row labels for an independent attempt, while retaining the reference notes appropriate to the task. Ask for the candidate set after each observation, not just the final answer.

In group teaching, collect each student’s first branch before discussing the class tree. A shared final diagram can conceal who selected the discriminating observation and who copied the structure.

A parent update can say that the student now combines excess-reagent observations and leaves an unsupported identity unresolved. That is visible progress in scientific reasoning, without implying that one worksheet establishes full practical readiness.

A second independent task can supply only two observations in prose and ask for a minimal candidate table. The student must translate the wording into constraints before giving a name. This varies the presentation while retaining the same reference chemistry and appropriate resource access.

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Verify every pattern against the actual examination-year qualitative notes. Keep reagent, initial observation, excess observation and sample identity together. A reversed excess-solubility entry can corrupt every branch built from it.

Audit whether the student version supplies the same candidate restrictions and fresh-portion conditions as the key. The conclusion should not depend on information visible only to the tutor.

Choose repair for misunderstood observation language, stabilise through another consistent candidate-reduction task and extend through an incomplete or conflicting record. Add broader candidate sets only when the relevant interpretation is secure.

The 2027 H2 notes are supplied in the practical paper as stated in that syllabus. Use the actual documents for another year rather than assuming every resource condition is identical.

The finished preparation should help the student answer three questions: what was observed, which candidates remain and what evidence would distinguish them. That gives a qualitative-analysis tree a practical purpose beyond drawing a neat branching diagram.

The most useful first branch is one that the student can carry out or interpret reliably and that distinguishes relevant candidates under suitable conditions. A tree with fewer theoretical steps is not automatically the best practical plan if its observations are difficult to establish. Keep those two planning judgments visible.

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