Chemistry-only: identify carbonate through its carbon dioxide
| English | 中文 | Pinyin |
|---|---|---|
| carbonate test | 碳酸根检验 | tàn suān gēn jiǎn yàn |
| effervescence/ˌefəˈvesəns/ | 冒气泡 | mào qì pào |
What would explain this observation?
- An unknown salt fizzes when dilute acid is added. The bubbles alone are not enough: testing the gas with limewater supplies the required carbon-dioxide evidence.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Carbonates react with dilute acids to release carbon dioxide. The gas turns fresh limewater milky or cloudy, supporting carbonate ions in the tested salt within the stated reference set. Name both stages: add dilute acid to the sample, then transfer or bubble the evolved gas through limewater. A carbonate ion remains part of the starting compound; it is carbon dioxide gas that reaches the limewater.
- carbonate test 碳酸根检验: Addition of dilute acid followed by identifying the evolved carbon dioxide with limewater; effervescence 冒气泡: Formation of gas bubbles observed in the stated reaction.
Which combined evidence supports carbonate in the tested salt?
For sodium carbonate and hydrochloric acid, Na₂CO₃ + 2HCl → 2NaCl + CO₂ + H₂O illustrates the source of the gas. The CO₂ test uses aqueous calcium hydroxide and forms insoluble calcium carbonate. Effervescence means gas bubbles, not a unique gas identity. A flame being extinguished is also non-specific: several gases do not support combustion, so it is weaker than the specified limewater result.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- For sodium carbonate and hydrochloric acid, Na₂CO₃ + 2HCl → 2NaCl + CO₂ + H₂O illustrates the source of the gas. The CO₂ test uses aqueous calcium hydroxide and forms insoluble calcium carbonate. Effervescence means gas bubbles, not a unique gas identity. A flame being extinguished is also non-specific: several gases do not support combustion, so it is weaker than the specified limewater result.
- RP7’s acquired student method uses a small gas transfer with a teat pipette into limewater. The teacher demonstrates this and several transfers may be needed before cloudiness is clear. Use the school-selected dilute acid and small quantities with eye protection, no mouth pipetting and no sealed gas-producing vessel. Record actual effervescence, transfer conditions and limewater change; a failed transfer should be documented as an observation limit, not renamed as another anion.
Which two habits make the investigation or model in this case more defensible?
RP7’s acquired student method uses a small gas transfer with a teat pipette into limewater. The teacher demonstrates this and several transfers may be needed before cloudiness is clear. Use the school-selected dilute acid and small quantities with eye protection, no mouth pipetting and no sealed gas-producing vessel. Record actual effervescence, transfer conditions and limewater change; a failed transfer should be documented as an observation limit, not renamed as another anion.
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: in the balanced sodium-carbonate particle model, five Na₂CO₃ formula units correspond to five CO₂ molecules and ten HCl units. The carbon count is five on each side. In a supplied reference matrix, carbonate fizzes and clouds limewater while sulfate and the three halides show no change under the same test. This supports identification from reagent-specific evidence rather than salt appearance alone.
In the stated sodium-carbonate reaction, six Na₂CO₃ formula units give one CO₂ molecule each. Count CO₂ molecules. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
In the stated sodium-carbonate reaction, six Na₂CO₃ formula units give one CO₂ molecule each. Count CO₂ molecules.
The result is 6 molecules. Known: in the balanced sodium-carbonate particle model, five Na₂CO₃ formula units correspond to five CO₂ molecules and ten HCl units. The carbon count is five on each side. In a supplied reference matrix, carbonate fizzes and clouds limewater while sulfate and the three halides show no change under the same test. This supports identification from reagent-specific evidence rather than salt appearance alone.
Check the conclusion and its limits
- The handbook’s optional alternative mentions a lit splint going out; preserve its source text, but do not treat that as unique CO₂ identification. The student method and specification explicitly use limewater. Do not confuse carbonate with carbon dioxide or infer the cation from the acid test. Both-tier Chemistry-only students need the correct two-stage interpretation.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Any gas that extinguishes a flame uniquely proves carbonate ions. This claim is false: The handbook’s optional alternative mentions a lit splint going out; preserve its source text, but do not treat that as unique CO₂ identification. The student method and specification explicitly use limewater. Do not confuse carbonate with carbon dioxide or infer the cation from the acid test. Both-tier Chemistry-only students need the correct two-stage interpretation.
Chemistry-only: identify carbonate through its carbon dioxide: For sodium carbonate and hydrochloric acid, Na₂CO₃ + 2HCl → 2NaCl + CO₂ + H₂O illustrates the source of the gas. The CO₂ test uses aqueous calcium hydroxide and forms insoluble calcium carbonate. Effervescence means gas bubbles, not a unique gas identity. A flame being extinguished is also non-specific: several gases do not support combustion, so it is weaker than the specified limewater result.
Any gas that extinguishes a flame uniquely proves carbonate ions.
The handbook’s optional alternative mentions a lit splint going out; preserve its source text, but do not treat that as unique CO₂ identification. The student method and specification explicitly use limewater. Do not confuse carbonate with carbon dioxide or infer the cation from the acid test. Both-tier Chemistry-only students need the correct two-stage interpretation.
Addition of dilute acid followed by identifying the evolved carbon dioxide with limewater: write the technical term.
carbonate test means Addition of dilute acid followed by identifying the evolved carbon dioxide with limewater.