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How does the hydroxide ion form water when it reacts with a hydrogen ion?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — chemical compound Methane, in which four hydrogen atoms are bound to a single carbon atom, is an example of a basic chemical compound. The structures of chemical compounds are influenced by complex factors, such as bond angles and bond length.

In chemistry, the formation of water from a hydrogen ion and a hydroxide ion is a fundamental neutralization reaction.12

While the term "hydrogen ion" is often used to represent a proton ( $H^{+}$ ), it does not exist in isolation in aqueous solutions; instead, it exists as a hydronium ion ( $H_{3} O^{+}$ ), which is a hydrogen ion combined with a water molecule. When an acid and a base react in water, the neutralization process involves the combination of these ions:321

$H_{3} O^{+} + O H^{-} ⇌ 2 H_{2} O$

In this reaction, the hydroxide ion ( $O H^{-}$ ) acts as a base by accepting a proton from the hydronium ion. This transfer results in the formation of two water molecules. This process is essential for maintaining the chemical balance of aqueous solutions, as it occurs to the extent necessary to satisfy the equilibrium constant of water, where the product of the concentrations of hydrogen and hydroxide ions remains constant at a given temperature.14