Page 20 - Curriculum Visions Dynamic Book

Testing for acidity (pH)
An important way to measure acidity is to measure
the concentration of the hydrogen ions in the solution. The concentration of hydrogen ions is measured by a special scale known as the pH scale. A pH of 0 is strongly acidic, pH 7 is neutral, and pH 14 is strongly alkaline.
The concentration of hydrogen ions can be measured with a pH meter or a chemical indicator. At a specific pH an indicator will change colour. There are many indicators, so that it is possible to find out the pH of a solution by seeing which indicator changes colour.
To make this process as easy as possible, a number of indicator solutions are mixed together to give a Universal Indicator. In the bottle, the Universal Indicator is deep green, but when a few drops are added to a solution of acid or alkali, it changes colour. Deep red indicates a very acidic solution, whereas bright blue indicates a very alkaline solution.
Indicators
Most indicators are complicated organic substances that change colour in different chemical environments, such as acidic or basic solutions. (For example, red cabbage leaves soaked in water are sometimes recommended as indicators in home-based chemistry experiments). The most common acid–base indicator is litmus paper, which is paper treated with a chemical that is red in acidic solutions and blue in basic solutions.
Also... the meaning of pH
The acidity of a solution depends on the relative amounts of hydrogen (H+) ions and hydroxide
(OH–) ions in the solution. This number varies enormously between very acid and very alkaline,
so it is not possible to use a simple (linear) scale to measure the concentration. Instead, the pH scale is a logarithmic scale where each number on the scale represents a ten-fold change in concentration. Thus a solution of pH 5 has ten times the concentration of hydrogen ions as a solution of pH 6. On this scale a pH of 1 is strongly acid, the pH of 14 is strongly alkaline, and a pH of 7 is neutral.
 This chart shows the range of pH from a strong acid at pH 1 to a strong alkali at pH 14.
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pH
Strong acid
 This chart shows the colour change for Universal Indicator in relation to pH.
Weak acid
Neutral Weak alkali Strong alkali
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