# In the equation HCl(g) + H_2O(l) -> H_3O^+(aq) + Cl^(-)(aq), which species is a Bronsted-Lowry acid?

Jan 1, 2016

Hydrochloric acid, $\text{HCl}$.

#### Explanation:

As you know, a Bronsted - Lowry acid is a proton donor, which means that you can identify a Bronsted - Lowry acid in a reaction by looking at which chemical species lost a proton.

A proton is simply a hydrogen ion, ${\text{H}}^{+}$, so the species that has one less proton on the products' side than it does on the ractants' side will be the Bronsted - Lowry acid.

color(red)("H")"Cl"_text((aq]) + "H"_2"O"_text((l]) -> "H"_3"O"_text((aq])^(color(red)(+)) + "Cl"_text((aq])^(-)
Notice that hydrochloric acid, $\text{HCl}$, starts with a proton that ends up on a water molecule, which acts as a Bronsted - Lowry base, and forms hydronium ions, ${\text{H"_3"O}}^{+}$.
The remaining chloride anion, ${\text{Cl}}^{-}$, is called the acid's conjugate base because it can theoretically reform the acid by aaccepting a proton.