Ammonia and Ammonium Salts
Ammonia and ammonium ions: the damp-red-litmus gas test, the ammonium-ion test and the balanced ionic equations.
On this page
This lesson connects two easily confused ideas: identifying ammonia gas and testing a solution for ammonium ions.
1. Definition
A. Ammonia
Ammonia, NH₃, is a colourless, pungent gas. It dissolves in water and produces hydroxide ions, so the solution is alkaline.
B. Ammonium ion
The ammonium ion, NH₄ +, is a positive ion found in ammonium salts such as ammonium chloride, NH₄Cl. It is not ammonia gas.
| Species | Formula | Charge | Where it appears in these tests |
|---|---|---|---|
| Ammonia | NH₃ | No net charge | Gas released on warming; also dissolves in the damp paper |
| Ammonium ion | NH₄ + | +1 | In the original ammonium-salt solution |
| Hydroxide ion | OH⁻ | −1 | Supplied by aqueous sodium hydroxide; also formed when ammonia reacts with water |
2. Key Ideas
- Ammonia gas turns damp red litmus paper blue.
- The paper must be damp so ammonia can dissolve and produce OH⁻(aq).
- Test an ammonium ion by adding aqueous sodium hydroxide and warming.
- A positive ammonium-ion test releases ammonia gas, confirmed with damp red litmus paper.
- Write observations separately from conclusions.
3. Detailed Explanations
Ammonium-ion test: add aqueous sodium hydroxide and warm → ammonia is released → damp red litmus paper turns blue.
A. Why ammonia is alkaline in water
NH₃(aq) + H₂O(l) ⇌ NH₄ + (aq) + OH⁻(aq)
The hydroxide ions make the solution alkaline. Ammonia is a weak base: only some dissolved ammonia molecules react with water, as shown by the reversible arrow. The molecule contains no hydroxide ion to release directly. Dry litmus may give no change because water is needed to form these aqueous ions.
B. Identifying ammonia gas
| Test | Positive observation | Conclusion |
|---|---|---|
| hold damp red litmus paper at the mouth of the test tube, in the evolved gas | red litmus turns blue | ammonia is present |
Do not identify ammonia by smell. A pungent smell may be noted as a property, but directly inhaling a gas is unsafe and the prescribed conclusion relies on the litmus test.
C. Testing for ammonium ions
Add aqueous sodium hydroxide to the sample and warm gently. Hold damp red litmus paper in the evolved gas at the mouth of the tube, without touching the alkaline solution or droplets. Otherwise, sodium hydroxide could turn the paper blue even if no ammonia were released. Ammonium ions react with hydroxide ions:
NH₄ + (aq) + OH⁻(aq) → NH₃(g) + H₂O(l)
Confirm the released gas by showing that it turns damp red litmus paper blue.
“Add aqueous sodium hydroxide and warm. The gas evolved turns damp red litmus paper blue, so ammonium ions are present.”
D. Link to ammonia manufacture
The Haber process produces ammonia from nitrogen and hydrogen. Its feedstocks, reversible equation, separation and recycling are covered in Manufacturing Ammonia.
4. Common Mistakes
- Using dry litmus paper.
- Writing “blue litmus” without stating the starting colour and direction.
- Forgetting to warm the sample during the ammonium-ion test.
- Concluding that ammonium ions are present before confirming the released ammonia.
- Writing hydrogen gas as the product of NH₄ + reacting with OH⁻.
- Relying on smell as the chemical test.
5. Exam Tips
- State reagent → action → observation → conclusion.
- Distinguish the two tasks: direct ammonia test uses damp red litmus; ammonium-ion test first needs sodium hydroxide and warming.
- Balance both atoms and charge in the ionic equation.
6. Worked Examples
Modelled example 1
Identify ammonia
Problem
Study the worked solution
Read the indicator evidence
Method
Use damp red-to-blue litmus change.Reason
This is the prescribed positive observation for an alkaline gas.Working
Damp red litmus → blue.Identify the gas
Method
Name ammonia.Reason
Ammonia dissolves in the moisture and produces an alkaline solution.Working
The gas is NH₃.
Guided practice 2
Test an ammonium salt
Problem
Assemble the complete prescribed test
Hints
Hint 1: release the gas
Hint 2: test the alkaline gas
View solution step by step
Release ammonia
Method
Add aqueous sodium hydroxide and warm.Reason
Hydroxide ions release ammonia from ammonium ions.Working
Ammonia gas evolves.Confirm the gas
Method
Test with damp red litmus paper.Reason
The ammonia solution formed on the paper is alkaline.Working
Damp red litmus turns blue, confirming NH₄ + in the original sample.
Common misconception 3
Explain damp litmus
Learner claim
A student says dry red litmus works just as well for ammonia because the gas is alkaline. Explain why the paper must be damp.
Connect water to the indicator change
View solution step by step
Dissolve the gas
Method
Use moisture to dissolve ammonia.Reason
Indicator behaviour requires an aqueous acid-base environment.
Working
NH₃ enters the water film.Explain the colour change
Method
Link dissolved ammonia to hydroxide-ion formation.
Reason
Hydroxide ions make the solution alkaline and turn red litmus blue.
Working
Dry paper can give a false negative; damp paper turns blue.
Challenge 4
Write the ionic equation
Symbolic transfer
Represent reagent and evolved gas
Hints
Hint 1: reacting ions
Hint 2: products
View solution step by step
Choose reacting species
Method
Combine aqueous ammonium and hydroxide ions.Reason
Other solution ions are spectators.Working
NH₄ + (aq) + OH⁻(aq).Balance products and states
Method
Form ammonia gas and liquid water.Reason
N, H, O and total charge balance.Working
NH₄ + (aq) + OH⁻(aq) → NH₃(g) + H₂O(l).
7. Mind Stretchers
Mind stretcher 1: Explain a false negativeExtension
Question: A student uses dry red litmus and sees no colour change. Why can the conclusion “not ammonia” be wrong?
Show Answer
Without water, ammonia cannot form enough aqueous hydroxide ions at the paper. The test must use damp red litmus.
Mind stretcher 2: Separate evidence and conclusionExtension
Question: In an ammonium-ion test, identify the observation and the conclusion.
Show Answer
Observation: the gas evolved turns damp red litmus paper blue. Conclusion: ammonia was released, so ammonium ions were present in the original sample.
Try independently: A learner dips damp red litmus into the mixture after adding sodium hydroxide. It turns blue. Why is that observation insufficient evidence for ammonium ions?
Show answer and reasoning
Sodium hydroxide itself is alkaline and turns red litmus blue. The paper must test the evolved gas, away from the liquid and droplets, after warming. A red-to-blue change there supports the release of ammonia and hence the presence of ammonium ions in the original sample.
Try independently: A fertiliser contains ammonium salts. Explain why warming it with an alkali can release a different nitrogen-containing species. Name that species and write the net ionic equation.
Show answer and reasoning
Hydroxide ions remove a hydrogen ion from ammonium ions, forming ammonia. Warming helps ammonia leave the solution as a gas:
NH₄ + (aq) + OH⁻(aq) → NH₃(g) + H₂O(l)
The nitrogen atom remains in the ammonia molecule; it has not become nitrogen gas, N₂.
Practise and check
Use the Acid–Base Chemistry topic check to practise base reactions and ammonia chemistry. For the wider set of gas and ion tests, continue to the Qualitative Analysis topic.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
Last reviewed: