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1. 
   An acid is a substance that accepts a lone pair of electrons.
   
2. 
   A base is a substance that donates a lone pair electrons.
   

This reaction shows a Lewis base (NH₃) donating an electron pair to a Lewis acid (H⁺) to form an adduct (NH₄⁺).

Substances capable of acting as either an acid or a base are amphoteric. Water is the most important amphoteric substance. It can ionize into hydroxide (OH^-, a base) or hydronium (H₃O⁺, an acid). By doing so, water is

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).

The presence of hydrogen ions

of hydroxide ions indicates a base.

Being neutral, water dissociates into

both equally.

hydrogen ions do not exist in water

because they bond to form hydronium.