Many of the consumer products that we use everyday are either acidic or basic. Because of this we often say that an acid is a hydrogen ion donor . It is often thought of as a hydrogen ion acceptor.
Pure water is defined as neutral and is described as neither an acid nor a base.
If the concentration of hydronium ions is greater than this value the solution is acidic. M then has a pH of 7 and is defined as a neutral solution. When using the pH scale one must keep in mind that it is a log function. There are 10 times as many hydronium ions in a solution with a pH 7 verse a solution with the pH 8. Likewise there is a 100-fold difference between a pH of 7 and a pH of 9.