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Amphoterism and Water[edit]

1. 
   Increasing the H⁺ or OH^- concentration (Arrhenius),
   
2. 
   Donating or accepting a proton (Brønsted-Lowry), and
   
3. 
   Accepting or donating an electron pair (Lewis).
   

Important A bare proton (H+ ion) cannot exist in water. It will form a hydrogen bond to the nearest water molecule, creating thehydronium ion (H3O+). Although many equations and definitions may refer to the "concentration of H+ ions", that is a misleading abbreviation. Technically, there are no H+ ions, only hydronium (H3O+) ions. Fortunately, the number of hydronium ions formed is exactly equal to the number of hydrogen ions, so the two can be used interchangeably.

H⁺ ions actually exist as hydronium, H₃O⁺.

Water will dissociate very slightly (which further explains its amphoteric properties).

{\displaystyle {\hbox{H}}_{2}{\hbox{O}}\leftrightarrow {\hbox{H}}^{+}+{\hbox{OH}}^{-}}		The presence of hydrogen ions indicates an acid, whereas the presence of hydroxide ions indicates a base. Being neutral, water dissociates into both equally.
{\displaystyle 2{\hbox{H}}_{2}{\hbox{O}}\leftrightarrow {\hbox{H}}_{3}{\hbox{O}}^{+}+{\hbox{OH}}^{-}}		This equation is more accurate—hydrogen ions do not exist in water because they bond to form hydronium.
	Helpful Hint!
	Although the other halogens make strong acids, hydrofluoric acid (HF) is a weak acid. Despite being weak, it is incredibly corrosive—hydrofluoric acid dissolves glass and metal!

Most acids and bases are weak. You should be familiar with the most common strong acids and assume that any other acids are weak.

Common strong bases are the hydroxides of Group 1 and most Group 2 metals. For example, potassium hydroxide and calcium hydroxide are some of the strongest bases. You can assume that any other bases (including ammonia and ammonium hydroxide) are weak.