What does hydrogen chloride do?
Hydrogen chloride (HCl) is a gas. It has two atoms, hydrogen and chlorine. It dissolves very easily in water, where it breaks down into a positively charged hydrogen nucleus (a proton) and a negatively charged chlorine ion. The proton immediately reacts with a water molecule to form the hydronium ion H3O+.
The result of dissolving the gas in water is called hydrochloric acid. It is a very useful acid – it cleans tarnish and rust off of metals, it etches concrete, and it is used in swimming pools to neutralize the water (which is often slightly basic due to other chemicals in the water).
But hydrogen chloride gas is also interesting by itself. It will react with ammonia vapors in the air to form ammonium chloride, which is a solid. A fun thing to do is to put a little hydrochloric acid on a paper towel, and some ammonium hydroxide solution on another paper towel and bring them together. The vapors combine in the air, and the solid ammonium chloride particles look like smoke.
The chlorine atom in hydrogen chloride holds onto electrons very tightly. The electron that would normally be associated with the hydrogen is strongly attracted to the chlorine, and ends up spending most of its time on that side of the molecule.
This behavior leaves one side of the molecule positive, and the other side negative, forming what we call being a polar molecule. Water is also a polar molecule, and polar molecules attract one another because of their electrical charges. Hydrogen chloride gas combines with water vapor in the air to form hydrochloric acid vapor.