Water molecules have a distorted tetrahedral structure with protons occupying two vertices and unshared electron pairs extending toward the other two vertices. The structure O has protons rapidly exchanging positions and hydrogen bonds forming between water molecules. An acid, such as hydrochloric acid HCl, dissociates in water, with most of the protons associating with water molecules to form hydronium ions , along with chloride ions . An acid solution has a H less than 7. A base, such as sodium hydroxide NaOH, dissociates to give hydroxide ions and sodium ions . A basic solution has a H greater than 7. The structures of the three hydrogen-oxygen species are highly dynamic, with protons continually exchanging and hydrogen bonds forming and breaking. Turn on the animation first to simulate this situation.