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A precipitate is produced when an alkaline solution is added to a solution containing some metal ions - Edexcel - GCSE Chemistry - Question 8 - 2022 - Paper 1
Question 8
A precipitate is produced when an alkaline solution is added to a solution containing some metal ions. (i) Which of these is evidence of a precipitate being produce... show full transcript
Worked Solution & Example Answer:A precipitate is produced when an alkaline solution is added to a solution containing some metal ions - Edexcel - GCSE Chemistry - Question 8 - 2022 - Paper 1
Step 1
Which of these is evidence of a precipitate being produced?
Answer
The only correct answer is B: solid forms in the solution. A precipitate is defined as an insoluble solid that emerges from a liquid solution during a chemical reaction. Hence, the formation of a solid indicates a precipitate has formed. Options A, C, and D are incorrect because:
- A (fizzing): This may indicate a gas is produced rather than a solid.
- C (the solution turns purple): This does not necessarily relate to precipitate formation, as it is more about the color change of the solution, not the solid phase.
- D (the solution gets hot): This indicates an exothermic reaction but does not confirm the formation of a precipitate.
Step 2
Devise a plan to identify which solution is which.
Answer
To differentiate between the calcium ions (Ca²⁺) and aluminum ions (Al³⁺) in the two solutions, follow these steps:
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Add a small amount of dilute sodium hydroxide (NaOH) to each solution: Aluminum hydroxide will form a precipitate, while calcium hydroxide may form only a slight precipitate or remain dissolved.
- Observation: If a white precipitate forms, the solution likely contains Al³⁺ ions.
- If only slight cloudiness occurs, it may suggest the presence of Ca²⁺ ions.
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Add excess NaOH to the solution that formed a precipitate: Aluminum hydroxide precipitate will dissolve in excess NaOH, confirming its identity.
- Observation: If the precipitate dissolves, the solution contains Al³⁺ ions.
- If it does not dissolve, the solution contains Ca²⁺ ions.
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For further confirmation, you may add a solution containing sulfate ions (like BaSO₄) to test for calcium ions, as a white precipitate will form if Ca²⁺ is present.