<P> pH is an example of an acidity function . Other acidity functions can be defined . For example, the Hammett acidity function, H, has been developed in connection with superacids . </P> <P> The concept of "Unified pH scale" has been developed on the basis of the absolute chemical potential of the proton . This scale applies to liquids, gases and even solids . </P> <P> Pure water is neutral . When an acid is dissolved in water, the pH will be less than 7 (25 ° C). When a base, or alkali, is dissolved in water, the pH will be greater than 7 . A solution of a strong acid, such as hydrochloric acid, at concentration 1 mol dm has a pH of 0 . A solution of a strong alkali, such as sodium hydroxide, at concentration 1 mol dm, has a pH of 14 . Thus, measured pH values will lie mostly in the range 0 to 14, though negative pH values and values above 14 are entirely possible . Since pH is a logarithmic scale, a difference of one pH unit is equivalent to a tenfold difference in hydrogen ion concentration . The pH of neutrality is not exactly 7 (25 ° C), although this is a good approximation in most cases . Neutrality is defined as the condition where (H) = (OH) (or the activities are equal). Since self - ionization of water holds the product of these concentration (H) × (OH) = K, it can be seen that at neutrality (H) = (OH) = √ (K), or pH = pK / 2 . pK is approximately 14 but depends on ionic strength and temperature, and so the pH of neutrality does also . Pure water and a solution of NaCl in pure water are both neutral, since dissociation of water produces equal numbers of both ions . However the pH of the neutral NaCl solution will be slightly different from that of neutral pure water because the hydrogen and hydroxide ions' activity is dependent on ionic strength, so K varies with ionic strength . </P> <P> If pure water is exposed to air it becomes mildly acidic . This is because water absorbs carbon dioxide from the air, which is then slowly converted into bicarbonate and hydrogen ions (essentially creating carbonic acid). </P>

The term for a solution that has an equal concentration of h+ and oh- is
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