A hydrogen atom that is in a hydrogen bond between two water molecules will occasionally shift from one molecule to the other. The hydrogen atom leaves its electron behind when this happens, and a hydrogen ion is transferred. A hydrogen ion is a single proton with a charge of 1+. The water molecule that lost a proton becomes a hydroxide ion (OH-). Hydroxide ions have ion a charge of 1-. One water molecule dissociates into a hydrogen ion and a hydroxide ion. A substance that increases the hydrogen ion concentration of a solution is an acid. A substance that reduces the hydrogen ion concentration of a solution is a base. When hydrogen ion concentration increases, the pH of a solution decreases. When hydrogen ion concentration decreases, the pH of a solution increases. When a solution has an equal concentration of OH- and H+, it is said to be neutral.
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