Written by the Fiveable Content Team • Last updated September 2025
Verified for the 2026 exam
Verified for the 2026 exam•Written by the Fiveable Content Team • Last updated September 2025
Definition
A hydronium ion is a water molecule (H2O) that has gained an extra hydrogen ion (H+), making its formula H3O+. It's often present in solutions where water is interacting with acids.
Related terms
Proton Donor: This term refers to any substance that can donate a proton, or H+, to another substance. In our analogy, this would be like the bucket of water being dumped onto the sponge.
pH Scale: The pH scale measures how acidic or basic a solution is based on its concentration of hydronium ions. It's like rating how wet our sponge is after it's soaked up some water.
An acidic solution has more hydronium ions than hydroxide ions (OH-). If our sponge was soaking up more and more water without releasing any, it would be similar to an acidic solution getting more and more hydronium ions.