What are the substances that form hydronium ions in water?

According to Elmhurst College, an acid can be defined as any substance that results in either hydrogen ions or hydronium ions when that substance is dissolved in water. They also claim that a hydrogen atom cannot exist without an electron, so it bonds to a water molecule, resulting in hydronium.

There are many different types of acids, according to Elmhurst College, ranging in use from toilet bowl cleaners and batteries to fruit juice preservatives and naturally occurring blood pH buffers. In each of these cases, acid molecules self-ionize, breaking apart into a hydrogen atom and the rest of the original molecule.