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What is pH?

pH means 'power of hydrogen' and is the measure of the concentration of hydrogen ions in a solution. pH can determine whether a solution is acidic, basic or neutral.
 
The pH Scale
 
The pH scale is a scale from 0 to 14, which shows how acidic or basic a substance is. Acidic substances have a pH less than 7, basic substances have a pH over 7, and neutral substances have a pH of 7. To measure the pH of a solution you can use either a pH indicator or pH paper. 
 
Indicators
 
Not all indicators determine the pH of a substance, but they can determine whether a substance is acidic, basic or neutral. Acid-base indicators indicate when an acid or base is present by changing their colour. Some indicators occur naturally in dyes in plants, while others are synthetic. Litmus for example is a natural indicator. It comes from lichens. In an acidic solution, litmus turns red and in a basic solution it turns blue. Phenol red is a synthetic indicator. In acidic solutions it turns yellowy-orange and in a basic solution it turns red.
 
Most indicators have only two colours. Universal indicators are a mixture of several different indicators. It can determine the pH of a solution by the variety of colours it can turn. For example if a substance with a pH of 7 was tested with a universal indicator it would turn green, and if the substance had a pH of 4 the universal indicator would turn orange.
 
pH paper can also be used to test the pH of a substance. The pH paper is simply dipped into the solution and the colour of the paper is matched the pH chart.

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phscale.gif
The pH scale and the pH of some common substances

Intel Young Scientist 2004