pH definition automatically introduced pH scale. As in most water solutions pH is in the 0-14 range it is commonly believed that pH scale is limited to this range, but in some cases pH values can be negative or higher than 14.

Water solutions always contain H⁺ and OH^- ions - either from water autodissociation or from introduced bases or acids. Water containing same amounts of both ions is called neutral. In neutral solution [H⁺]=[OH^-]=10^-7 (read about water autodissociation to find more), so pH of neutral solution is 7 (it can be slightly lower, especially in higher temperatures). If solution contains more H⁺ than OH^- it is acidic, and the pH value is lower than 7, if there is more OH^- than H⁺ - solution is basic - and pH value is higher than 7.

pH scale is widely used as it makes comparison of different solutions or samples very easy, besides it often gives very fast information about the sample. It is much easier to say that sea water pH is about 8, than to state H⁺ concentration is 10^-8M. Besides, knowing that sea water pH is 8 one already knows that it is slightly basic.

To get a better feeling about pH scale it is worth to remember few pH values:

Note that for a purpose some values are given with higher accuracy then others. pH of blood is kept very precisely in the given range by the carbonic buffer, when the pH of blood goes down it forces us to pant, while when it goes up it can effect in involuntary muscle contractions. pH of gastric juice varies depending on the diet and how much time elapsed from the last meal. pH of hand soap is in the range 9-10, but as natural pH of skin is slightly acidic (around pH 5) such soap lefts skin dry and rough, thus more and more soap producers modify their recipes to lower soap pH. Finally pH of lemon juice depend on the fruit ripeness (in fact first pH meters were produced for lemon planters).

For more information see pH scale lecture (and other lectures on pH) at ChemBuddy - chemical calculators site.