Required practical 7: identify both ions in an actual unknown salt
| English | Français |
|---|---|
| unknown salt | unknown salt |
| reference matrix | reference matrix |
What would explain this observation?
- A lilac flame tells you about the positive ion, while acidified barium chloride tells you about the negative ion. A single ionic compound needs both identities before its formula can be justified.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- RP7 requires chemical tests on unknown single ionic compounds covering the specified flame, hydroxide, carbonate, halide and sulfate evidence. The acquired handbook’s basic activities establish known flame and anion results, then repeat them on the unknown. Teachers also ensure the 4.8.3.2 hydroxide comparisons are covered in the school programme. A written table of predicted colours is useful preparation but is not an actual practical record.
- unknown salt 未知盐: The school-selected single ionic compound whose cation and anion are to be identified; reference matrix 参照检验表: A recorded set of known reagent-and-observation results used to interpret the unknown.
Why use separate fresh unknown portions for different tests?
Establish reference results with labelled known cation compounds and carbonate/sulfate/chloride/bromide/iodide salts. Use fresh unknown portions for different tests. Keep flame-wire cleaning, dilute-acid identities and precipitating reagents exact. Record carbonate gas and limewater, sulfate with dilute HCl/barium chloride, and halides with dilute nitric acid/silver nitrate. Compare subtle silver-halide colours side by side. The handbook suggests potassium sulfate as a convenient unknown; the school can choose another suitable single compound.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Establish reference results with labelled known cation compounds and carbonate/sulfate/chloride/bromide/iodide salts. Use fresh unknown portions for different tests. Keep flame-wire cleaning, dilute-acid identities and precipitating reagents exact. Record carbonate gas and limewater, sulfate with dilute HCl/barium chloride, and halides with dilute nitric acid/silver nitrate. Compare subtle silver-halide colours side by side. The handbook suggests potassium sulfate as a convenient unknown; the school can choose another suitable single compound.
- Use the approved station arrangement, separate pipettes, current school risk assessment and supervised small quantities. The teacher demonstrates pipette gas transfer to limewater and controls Bunsen use, caustic reagents, soluble barium salts and silver waste. Keep raw observations, test conditions, any no-change/inconclusive result and justified inference. Do not erase contradictory results; check contamination, repeat under approved conditions and retain the explanation. No proprietary reagent-making or concentrated-acid cleaning is assigned to students.
Which two habits make the investigation or model in this case more defensible?
Use the approved station arrangement, separate pipettes, current school risk assessment and supervised small quantities. The teacher demonstrates pipette gas transfer to limewater and controls Bunsen use, caustic reagents, soluble barium salts and silver waste. Keep raw observations, test conditions, any no-change/inconclusive result and justified inference. Do not erase contradictory results; check contamination, repeat under approved conditions and retain the explanation. No proprietary reagent-making or concentrated-acid cleaning is assigned to students.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: an original unknown gives a lilac flame, no carbonate response, white precipitate in the acidified barium-chloride test and no halide result. Within the stated references this supports K⁺ and SO₄²⁻. Two +1 potassium ions balance one −2 sulfate ion, giving K₂SO₄. Five formula units contain ten potassium ions and five sulfate ions. Those quantities explain the inferred formula, not observations supposedly collected by the student.
Seven K₂SO₄ formula units contain two potassium ions each. Count potassium ions. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Seven K₂SO₄ formula units contain two potassium ions each. Count potassium ions.
The result is 14 ions. Known: an original unknown gives a lilac flame, no carbonate response, white precipitate in the acidified barium-chloride test and no halide result. Within the stated references this supports K⁺ and SO₄²⁻. Two +1 potassium ions balance one −2 sulfate ion, giving K₂SO₄. Five formula units contain ten potassium ions and five sulfate ions. Those quantities explain the inferred formula, not observations supposedly collected by the student.
Check the conclusion and its limits
- Do not reuse an HCl-treated portion for the silver-nitrate halide test, or infer the anion from a flame. A matrix can leave ambiguity or conflicting evidence; report that honestly instead of inventing a unique salt. The practical uses actual supervised tasks and both-ion reasoning, not an invented written performance exam or an automatically certified completion checkbox.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A written list of correct ion-test colours proves RP7 was performed. This claim is false: Do not reuse an HCl-treated portion for the silver-nitrate halide test, or infer the anion from a flame. A matrix can leave ambiguity or conflicting evidence; report that honestly instead of inventing a unique salt. The practical uses actual supervised tasks and both-ion reasoning, not an invented written performance exam or an automatically certified completion checkbox.
Required practical 7: identify both ions in an actual unknown salt: Establish reference results with labelled known cation compounds and carbonate/sulfate/chloride/bromide/iodide salts. Use fresh unknown portions for different tests. Keep flame-wire cleaning, dilute-acid identities and precipitating reagents exact. Record carbonate gas and limewater, sulfate with dilute HCl/barium chloride, and halides with dilute nitric acid/silver nitrate. Compare subtle silver-halide colours side by side. The handbook suggests potassium sulfate as a convenient unknown; the school can choose another suitable single compound.
A written list of correct ion-test colours proves RP7 was performed.
Do not reuse an HCl-treated portion for the silver-nitrate halide test, or infer the anion from a flame. A matrix can leave ambiguity or conflicting evidence; report that honestly instead of inventing a unique salt. The practical uses actual supervised tasks and both-ion reasoning, not an invented written performance exam or an automatically certified completion checkbox.
The school-selected single ionic compound whose cation and anion are to be identified: write the technical term.
unknown salt means The school-selected single ionic compound whose cation and anion are to be identified.