The scale is based on the concentration of hydrogen ions present in the substance. An acidic solution contains a high concentration of hydrogen ions while a basic solution has a low concentration of hydrogen ions. Because acids and bases are opposite ends of the pH scale, mixing the two together will result in a more intermediate pH- if you get the quantities right you will get a neutral solution with a pH of 7- the same as water. Try this out yourself by mixing together substances of different pHs.
The pH scale was invented by someone called Sören Sörensen in 1909 and is described by the formula : pH = -log [H+] (Note: the square brackets around the H+ (hydrogen ions) means concentration). pH is inversely related to the concentration of hydrogen ions- i.e. As the pH goes up ,the hydrogen ions go down.
The concentration of hydrogen ions is measured in moles/liter- moles in just another was of describing how much of something is there without taking into account weight (i.e. a mole of lead blocks would weigh a lot more than a mole of feathers but would have exactly the same number of blocks as a mole of feathers- 6 × 10^23 in fact). A solution of pH 0 has 1 × 10^0 moles of hydrogen ions in it and a solution of pH 14 has 1 × 10^-14 moles of hydrogen ions. Test the equation out yourself on a calculator and work out the concentrations of hydrogen ions in the substances you tested.
Some pH examples:
pH 14- NaOH (sodium hydroxide)
pH 11- Ammonia
pH 8- Baking soda
pH 5- Rain water (-how acid is your rain?)
pH 2- Lemon Juice
pH 0- HCl (Hydrochloric acid)
For a closer look at the pH calculations and an explanation of
why pH only goes up to 14, click
here (if you're crazy and like maths).
