Learning focus
Develop safe and reproducible practical methods, record precise observations and measurements, process evidence and evaluate experimental quality.
Ammonium
Add aqueous sodium hydroxide and warm. Ammonia is released and turns damp red litmus blue. A precipitate is not the diagnostic result.
Calcium with sodium hydroxide
Ca²⁺ gives a white precipitate insoluble in excess sodium hydroxide.
Calcium with ammonia
With aqueous ammonia, calcium gives no precipitate or a very slight white precipitate. This helps distinguish it from other white-precipitate cations.
Flame confirmation
Calcium gives an orange-red flame test. Combine flame and wet-test evidence when an unknown could contain calcium.
Practical or data skill
Construct a decision tree separating NH₄⁺, Ca²⁺, Al³⁺ and Zn²⁺.
Examination tip
Ammonium is tested by gas release on warming, not by precipitate colour.
Review questions and suggested answers
Question 1
What gas is produced from NH₄⁺ with NaOH?
Suggested answer
Ammonia.
Question 2
What is the Ca²⁺ result with NaOH?
Suggested answer
White precipitate insoluble in excess.
Question 3
What flame colour supports Ca²⁺?
Suggested answer
Orange-red.
