What is pH?
The pH scale measures how acidic, or how alkali a given solution is.
The term pH can be broken down into two parts; the first is the [p], this represents the mathematical symbol for -log (negative logarithm) of the number in question. The “H” stands for Hydrogen, and is represented by the chemical symbol [H] So the correct way to write this down is a small p, and a capital H (pH).
The pH scale is basically a rough guide as to how many Hydrogen ions are present in any given substance; the more hydrogen ions are present, the more acidic the substance becomes. The pH of distilled water is 7.0, this is neutral. Any solution with a pH below 7.0 (i.e. pH 1.0 to pH 6.9) is an acid and any solution with a pH above 7 (i.e. pH 7.1 to pH 14) is an alkali. The pH scale is logrithmic, that is, a pH 6.0 solution is 10 times more acidic than a pH 7.0 solution (pH 5.0 is 100x more than 7.0!).
Acidic solutions have a pH between 1 and 6.9 (your stomach contains HCl it is pH2).
Alkaline solutions have a pH between 7.1 and 14. (your small intestine is pH 9).
Neutral solutions are neither acidic nor alkaline so their pH is 7.
Acids all produce Hydrogen ions (H+). Acids like Hydrochloric acid produce lots of Hydrogen ions; this is because when Hydrogen Chloride gas dissolves in water the molecules of Hydrogen Chloride dissociate into Hydrogen ions and Chloride ions.
HCl = H+ + Cl-
Water also dissociates to produce ions, this time it is Hydrogen ions and Hydroxyl ions.
H2O = H+ + OH-
Sodium Hydroxide also dissociates to produce ions when it is dissolved in water, this time it is Sodium ions and Hydroxyl ions.
NaOH = Na+ + OH-
In each case, we can measure or calculate the concentration of Hydrogen ions present. We use the symbol [H+], we use square brackets to mean that it is the concentration of Hydrogen ions.
In HCl Hydrogen Chloride solution or Hydrochloric acid [H+] = 0.01
In H2O water [H+] = 0.0000001
In NaOH Sodium Hydroxide solution [H+] = 0.00000000000001
We count the decimal places from the first number, and that is where the pH scale is derived from.
HCl = pH2
H2O = pH7
NaOH = pH14
So to recap if the pH is low, it means that there is a high concentration of Hydrogen ions and if the pH is high it means that there is very low concentration of Hydrogen ions or none at all. Water and other neutral solutions are in the middle at pH7.