An acid is defined primarily by its ability to produce hydrogen ions (H⁺) when dissolved in water. This release of H⁺ ions is what makes a substance an acid
. More specifically:
- According to the Arrhenius definition, an acid increases the concentration of hydrogen ions (or hydronium ions, H₃O⁺) in aqueous solution
- Acids typically contain at least one hydrogen atom that can dissociate as a proton (H⁺) in water, such as hydrochloric acid (HCl) or acetic acid (CH₃COOH)
- The presence of these free hydrogen ions in solution is responsible for characteristic acid behaviors, such as a sour taste, the ability to turn blue litmus paper red, and reactions with bases and certain metals to form salts
- The strength of an acid depends on how readily it donates these protons, measured by its acid dissociation constant (Ka). Strong acids dissociate more completely, releasing more H⁺ ions
- On a molecular level, acids are substances that can give up a proton without destabilizing themselves, making the conjugate base more stable after proton loss
In summary, what makes an acid an acid is its capacity to donate hydrogen ions (protons) into a solution, thereby increasing the solution's acidity and lowering its pH below 7
