Here's a demonstration common in chemistry classes. Two chemicals, hydrochloric acid and ammonia, are placed next to each other. They don't touch. But one suddenly starts smoking. What happened?
This is the famous "white smoke" demonstration. It's a simple thing, but it's interesting to look at, and it demonstrates a very good chemical principle. Unfortunately, it involves a vial full of hydrochloric acid, which isn't an easy thing to handle, so it's mostly demonstrated in chemistry classrooms. Put a stoppered beaker of hydrochloric acid next to a stoppered beaker of ammonia. Nothing happens. Yank out the stoppers, and suddenly, the beaker of acid will start giving off plumes of white smoke.
It doesn't take too much thinking to guess that fumes from the ammonia have traveled through the air and reacted to the hydrochloric acid, but given the two materials are out in the open, why does no smoke form over the ammonia bottle?
The smoke isn't really the result of combustion. It's a fine salt powder that forms in the air when the ammonia and the acid combine. Here's the equation the describes the process:
NH3 + HCl —> NH4Cl
A look at the chemical equation gives us the reason that only the hydrochloric acid starts "smoking." The ammonia is one nitrogen atom and three hydrogen atoms. Since hydrogen is such a lightweight, ammonia has a molecular mass of a mere seventeen grams per mole. The hydrochloric acid is the heavyweight. Chlorine is about thirty-five and a half grams per mole, and the hydrogen brings the acid up to thirty-six grams per mole.
We all know that a pot of liquid left on a counter will evaporate eventually. Its molecules will turn to gas and diffuse through the air. Studies show the diffusion rate of a substance is inversely proportional to the square root of the molecular mass. In other words - the heavier the molecular mass, the slower the gas will diffuse through a given space. Hydrochloric acid, then, is a lumbering gas. Ammonia is quick. Anyone who has dealt with it knows that an unstoppered bottle of ammonia smells up a room. The hydrochloric acid beaker appears to be smoking because the ammonia gets to it before it can even clear the rim of its own beaker.