The Difference Between Hydrogen Chloride and Hydrochloric Acid
Hydrochloric acid is basically a solution of hydrogen chloride gas in water.
The Bronsted-Lowry Theory
Bronsted and Lowry defined acids and bases as the following:
-An acid donates a proton.
-A base accepts a proton.
How is this related? Well, when hydrogen loses its only electron, it becomes a hydrogen ion (H+). In other words, it is also a proton, because it has lost all of its electrons (it only has one remember?).
When hydrogen chloride dissolves in water, a proton (the hydrogen ion) is transferred to the water. This gives us the equation:
H2O(l) + HCl(g) -> H3O+(aq) + Cl–(aq)
The H3O+ ion is called a hydroxonium ion. We normally write it as H+(aq). You can think of it as a hydrogen ion riding on a water molecule.
So in this example, HCl is an acid because it donates a proton (the hydrogen ion) to water.
So the real differences? Hydrogen chloride is NOT an acid and is a gas. Hydrochloric acid is an aqueous solution of hydrogen chloride.
Hydrogen Chloride and Methylbenzene
Explaining Water Being a Polar Molecule
Water is a polar molecule. Electrons in water are attracted towards the oxygen end of the bond, which leaves it slightly negative. This leaves hydrogen slightly short of electrons, and therefore, making it slightly positive, just like the picture to the left. Because of this electrical distortion, water is described as a polar molecule.
When something such as sodium chloride is being dissolved in water, the slightly positive hydrogens cluster around the chlorine, whereas, the slightly negative oxygen cluster around the sodium. The water molecules then literally pull the sodium chloride crystal apart.
This pull doesn’t work on every molecule. Magnesium oxide isn’t soluble in water because the water molecules aren’t strong enough to break the magnesium-oxygen attractions.
What’s so special about methylbenzene?
Well, methylbenzene is not a polar molecule. It is unable to pull the hydrogen and chlorine apart and therefore, hydrochloric acid won’t be formed.