In the reaction of an acid with a base in aqueous solution, the hydrogen ions of the acid react with the hydroxide ions of the base to give water. The second product is a salt, which is composed of the positive metal ion from the base and the negative ion from the acid. For example,

  • HCl ( aq ) + KOH ( aq ) → H 2 O ( l ) + KCl ( aq )

Since HCl ( aq ) and KOH ( aq ) are fully ionized in solution, the preceding equation can be written as

  • H + ( aq ) + Cl − ( aq ) K + ( aq ) + OH − ( aq ) → H 2 O ( l ) K + ( aq ) + Cl − ( aq )

Ions common to both sides can be canceled to yield

  • H + ( aq ) + OH − ( aq ) → H 2 O ( l )

This is referred to as the net ionic equation for the neutralization reaction. If H 3 O + is substituted for H + ( aq ), the neutralization equation becomes

  • H 3 O + ( aq ) + OH − ( aq ) → 2 H 2 O ( l )

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