Chemistry-only: colour and excess alkali identify metal hydroxides
| English | 中文 | Pinyin |
|---|---|---|
| precipitate/prɪˈsɪpɪteɪt/ | 沉淀 | chén diàn |
| excess reagent/ekˈses rɪˈeɪdʒənt/ | 过量试剂 | guò liàng shì jì |
What would explain this observation?
- Three different metal-ion solutions can initially make white precipitates 沉淀 with sodium hydroxide. Adding excess reagent 过量试剂 supplies another observation: the aluminium precipitate dissolves.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Adding sodium hydroxide solution forms insoluble hydroxides with the specified cations. Aluminium, calcium and magnesium give white precipitates, but only the aluminium hydroxide precipitate dissolves in excess sodium hydroxide in this reference set. Copper(II) gives blue, iron(II) green and iron(III) brown precipitates. A precipitate is an insoluble solid formed by the reaction, not a solution that merely changes colour.
- precipitate: An insoluble solid formed in the stated reaction from dissolved substances; excess reagent: More reagent than needed for the stated initial reaction, allowing subsequent behaviour to be observed.
Which result supports aluminium within the supplied six-cation reference set?
Record initial precipitation separately from what happens on adding excess reagent. A white precipitate that dissolves in excess supports aluminium; one remaining supports calcium or magnesium within the supplied set. Use additional suitable evidence, such as calcium’s orange-red flame, when a single observation leaves alternatives. The Roman numerals in iron(II) and iron(III) distinguish ion charges and their different hydroxides. Colour should be compared with known results in similar conditions.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Record initial precipitation separately from what happens on adding excess reagent. A white precipitate that dissolves in excess supports aluminium; one remaining supports calcium or magnesium within the supplied set. Use additional suitable evidence, such as calcium’s orange-red flame, when a single observation leaves alternatives. The Roman numerals in iron(II) and iron(III) distinguish ion charges and their different hydroxides. Colour should be compared with known results in similar conditions.
- Carry out only the approved supervised small-scale comparison using labelled solutions, separate clean portions and selected dilute sodium hydroxide. Add the reagent as instructed, observe the precipitate, then add excess and record whether it dissolves. Sodium hydroxide is corrosive at sufficient concentration; school controls govern eye protection, handling and waste. Students need not write an equation for producing sodium aluminate: the specification requires the observation but excludes that equation.
Which two habits make the investigation or model in this case more defensible?
Carry out only the approved supervised small-scale comparison using labelled solutions, separate clean portions and selected dilute sodium hydroxide. Add the reagent as instructed, observe the precipitate, then add excess and record whether it dissolves. Sodium hydroxide is corrosive at sufficient concentration; school controls govern eye protection, handling and waste. Students need not write an equation for producing sodium aluminate: the specification requires the observation but excludes that equation.
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: supplied unknown A forms white precipitate then clears in excess alkali; B forms white and remains; C forms brown. A supports Al³⁺, B leaves Ca²⁺/Mg²⁺ unresolved, and C supports Fe³⁺. If four of eight white-precipitate samples dissolve in excess, their count proportion is 4/8×100=50%. It does not mean an individual sample is half aluminium hydroxide.
Six of eight supplied white-precipitate samples dissolve in excess sodium hydroxide. Calculate their percentage. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Six of eight supplied white-precipitate samples dissolve in excess sodium hydroxide. Calculate their percentage.
The result is 75 %. Known: supplied unknown A forms white precipitate then clears in excess alkali; B forms white and remains; C forms brown. A supports Al³⁺, B leaves Ca²⁺/Mg²⁺ unresolved, and C supports Fe³⁺. If four of eight white-precipitate samples dissolve in excess, their count proportion is 4/8×100=50%. It does not mean an individual sample is half aluminium hydroxide.
Check the conclusion and its limits
- Do not treat all white precipitates as aluminium, mix up iron hydroxide colours or call an original blue solution a blue precipitate without observing a solid. Excess reagent and initial addition are different test stages. The classification applies on both tiers; the separate equation lesson teaches the required balanced formation equations.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
All white hydroxide precipitates identify the same metal ion. This claim is false: Do not treat all white precipitates as aluminium, mix up iron hydroxide colours or call an original blue solution a blue precipitate without observing a solid. Excess reagent and initial addition are different test stages. The classification applies on both tiers; the separate equation lesson teaches the required balanced formation equations.
Chemistry-only: colour and excess alkali identify metal hydroxides: Record initial precipitation separately from what happens on adding excess reagent. A white precipitate that dissolves in excess supports aluminium; one remaining supports calcium or magnesium within the supplied set. Use additional suitable evidence, such as calcium’s orange-red flame, when a single observation leaves alternatives. The Roman numerals in iron(II) and iron(III) distinguish ion charges and their different hydroxides. Colour should be compared with known results in similar conditions.
All white hydroxide precipitates identify the same metal ion.
Do not treat all white precipitates as aluminium, mix up iron hydroxide colours or call an original blue solution a blue precipitate without observing a solid. Excess reagent and initial addition are different test stages. The classification applies on both tiers; the separate equation lesson teaches the required balanced formation equations.
An insoluble solid formed in the stated reaction from dissolved substances: write the technical term.
precipitate means An insoluble solid formed in the stated reaction from dissolved substances.