Ammonia turns moist red litmus blue and gives white fumes with hydrogen chloride: NH3(g) + HCl(g) → NH4Cl(s). Chlorine is a greenish-yellow gas that turns moist blue litmus red and then bleaches it white.
Ammonia and chlorine are two gases that are easy to confuse in a hurry but behave oppositely: ammonia is alkaline, while chlorine is acidic and bleaches. Both are examined in Form 4, and the exam rewards the exact observation and, for ammonia, the white-fume reaction. Learn the litmus behaviour first, then the confirmatory detail for each.
Test for ammonia
Ammonia, NH3, is the only common alkaline gas, and this is the basis of its test.
- Hold a piece of moist red litmus paper in the gas: it turns blue, because ammonia dissolves to form an alkaline solution.
- Ammonia also has a pungent, choking smell.
- Confirmatory test: hold a glass rod dipped in concentrated hydrochloric acid near the gas. Dense white fumes of ammonium chloride form:
NH3(g) + HCl(g) → NH4Cl(s)
Ammonia is often produced in the lab by warming an ammonium salt with an alkali, for example:
NH4Cl(s) + NaOH(aq) → NaCl(aq) + H2O(l) + NH3(g)
This reaction is itself the confirmatory test for the ammonium ion: warming a salt with sodium hydroxide and detecting ammonia (which turns red litmus blue) shows the ammonium ion is present.
Test for chlorine
Chlorine, Cl2, is a greenish-yellow gas with a sharp, choking smell, and it is acidic and an oxidising bleach.
- Hold a piece of moist blue litmus paper in the gas: it turns red and then is bleached white. The turning red shows chlorine is acidic; the bleaching to white is the feature unique to chlorine.
- Chlorine also turns moist starch-iodide paper blue-black, because it displaces iodine from the iodide, and starch shows the iodine as a blue-black colour:
Cl2(g) + 2KI(aq) → 2KCl(aq) + I2(aq)
For the standard test, the bleaching of damp litmus is the key observation to quote.
Telling them apart
The single cleanest way to distinguish the two is the litmus paper. Ammonia turns red litmus blue (alkaline) and never bleaches it. Chlorine turns blue litmus red then white (acidic, then bleaches). If a gas bleaches the litmus, it is chlorine, not ammonia; if a gas turns red litmus blue, it is ammonia. Adding the white-fume test for ammonia and the colour and smell for chlorine makes the identification watertight.
Common mistakes to avoid
State the colour of the litmus you start with: red litmus for ammonia (turns blue), blue litmus for chlorine (turns red then bleaches). Do not forget that chlorine bleaches, stopping at “turns red” loses the point that separates it from any other acidic gas. Write NH3(g) + HCl(g) → NH4Cl(s) with the solid state symbol for the white fumes. Do not describe ammonia as bleaching; it does not.
How it appears in the SPM exam
In Paper 1 (4541/1), objective items test the litmus results, the colour of chlorine and the white-fume reaction. In Paper 2 (4541/2) you may identify a gas from its test, write the ammonium-ion confirmatory equation, or explain how to distinguish ammonia from chlorine. In Paper 3 (4541/3), testing a gas with litmus and recording the change is a standard practical step. Anchor ammonia on “alkaline, red-to-blue, white fumes” and chlorine on “greenish-yellow, blue-to-red-then-bleached”, and the two never get mixed up.
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