# How was it concluded that the H3O+ rather than H+ is the “acid” ion?

I suspect that initially, scientists believed that the acid ion was $\ce{H^+}$ since $\ce{H2}$ is released through electrolysis, right? But what experiment was done to change the standpoint to assume that it is instead the $\ce{H3O^+}$ ion?

Or perhaps there's both $\ce{H^+}$ and $\ce{H3O^+}$ ions?

It should be noted that $\ce{H+}$ notation is still useful for reaction balancing, since it's much easier to count atoms in $\ce{H+}$ instead of $\ce{H3O+}$, even if the former ion may not really exist.
Actually, the initial theories before Lewis suggested that $\ce{H+}$ is the cause of acidity. However, it soon turned up that an ion as small as the nucleus of hydrogen (you may simply call it a proton) can't be created in low energy reactions due to its high polarising power. So, $\ce{H+}$ is though the cause of acidic nature in aqueous solutions, $\ce{H+}$ never exists as $\ce{H+}$ but as $\ce{H3O+}$, $\ce{H9O4+}$, etc. It is because the proton is heavily hydrated. Though we are not very sure of what is the actual hydrated form, we usually refer it to as $\ce{H3O+}$ or $\ce{H+ (aq)}$.