Ammonia is a chemical — a sharp-smelling gas of nitrogen and hydrogen, used in cleaning products and fertiliser. Pneumonia is an infection that inflames the lungs. They sound alike because of the shared "-monia" ending and nothing else; the words are unrelated in origin and meaning.
The core difference
- ammonia — a colourless gas with a pungent smell, chemical formula NH₃. Found in household cleaners and fertiliser.
- pneumonia — an infection causing inflammation in the lungs.
The spelling that causes the trouble
Pneumonia opens with a silent p, so both words begin, to the ear, with a vowel sound and end the same way. Written down they are not close at all — the confusion is purely spoken.
Where the words come from
Ammonia is named after the temple of Amun in Egypt, near which the salt ammonium chloride was gathered. Pneumonia comes from the Greek pneumon, lung — the same root as pneumatic, which is about air rather than lungs but shares the idea of breath.
Getting it right
If it is in a bottle under the sink, it is ammonia. If it is a diagnosis, it is pneumonia. The silent p is the spelling detail worth remembering, because it is what makes the written forms so much more distinct than the spoken ones.
