Section 1 of 5Testing with Damp Litmus
Ammonia is detected by its effect on moist indicator paper, not by its smell.
1
Dampen a strip of red litmus paper with distilled water so that the gas can dissolve in it.
2
Hold the damp paper near the mouth of the test tube, without letting it touch the sides of the tube.
3
Ammonia dissolves in the water on the paper to form an alkaline solution, which changes the colour of the litmus.
4
Record the change from red to blue, which is the positive result for ammonia.
Section 2 of 5Ammonia and Its Properties
Alkaline gas
- Ammonia dissolves in water to form an alkaline solution, which is why it turns red litmus blue.
High solubility
- Ammonia is very soluble in water, so the damp paper is needed to dissolve the gas and show the colour change.
Smell
- Ammonia has a sharp, choking smell, but smell is a property of the gas and is not a formal identification test.
Reaction with acids
- Ammonia reacts with acids to form ammonium salts, which contain the NH₄⁺ ion.
Ammonium chloride
- Ammonia reacts with hydrogen chloride gas to form white fumes of solid ammonium chloride: NH₃(g) + HCl(g) → NH₄Cl(s).
Section 3 of 5Confirming Ammonia
Confirming Ammonia
Litmus test
- Damp red litmus paper turning blue confirms ammonia, provided the paper has not touched alkali left on the glass.
Acid test
- A glass rod dipped in concentrated hydrochloric acid produces white fumes near ammonia gas.
Ammonia or ammonium
- Ammonia is the gas NH₃, whereas ammonium is the ion NH₄⁺ found in salts such as ammonium nitrate.
Use in fertilisers
- Ammonia is used to make ammonium salts, which are commonly used as fertilisers.
Section 4 of 5Worked Example
Question
- A gas is suspected to be ammonia.
- Describe a test to confirm it and state the observation that shows ammonia is present.
- Dampen a piece of red litmus paper with distilled water.
- Hold the paper near the mouth of the test tube, without touching the sides.
- Observe the colour of the paper; if it turns blue, the gas is ammonia.
Answer Damp red litmus paper turns blue, which shows that ammonia is present.
Section 5 of 5Exam Checklist & Tips
4.2.4 – Key Points for the Exam
- Alkali A base that dissolves in water to form a solution with a pH above 7.
- Litmus An indicator that is red in acid and blue in alkali, used to test gases in their moist form.
- Ammonium ion The NH₄⁺ ion, found in ammonium salts such as ammonium chloride.
- Ammonium chloride A white solid, NH₄Cl, formed when ammonia reacts with hydrogen chloride gas.
Common Mistakes
- Using dry litmus paper, which does not change colour in ammonia gas.
- Touching the sides of the test tube with the litmus paper, which can give a false blue result from traces of alkali.
- Confusing ammonia, the gas NH₃, with the ammonium ion NH₄⁺.
- Stating that ammonia turns litmus red; ammonia is alkaline, so it turns red litmus blue.