Ace the Chemical FE Exam 2026 with NCEES – Engineer Your Success!

The hydroxide ion is represented by the formula OH-. This ion consists of one oxygen atom and one hydrogen atom, with an overall negative charge. This negative charge arises because the hydroxide ion has one extra electron compared to the number of protons in its constituent atoms.

Hydroxide ions play a crucial role in various chemical reactions, particularly in acid-base chemistry, where they can react with protons (H+) to form water (H2O). They are also significant in maintaining pH levels in solutions.

The other options do not correctly represent the hydroxide ion: OH+ indicates a positively charged ion, which does not exist in common chemistry contexts. O2- corresponds to the peroxide ion, indicating two oxygen atoms with a negative charge, and H2O is the formula for water, which does not include the distinct charge or composition of the hydroxide ion.