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Identify acids and alkali

The substance is acidic when:
•    the blue Litmus paper turns red and the red Litmus paper is unaffected  
The substance is alkaline when:
•    the red Litmus paper turns blue and the blue Litmus paper is unaffected  
If neither red nor the blue paper changes colour, the substance is neutral.

The pH scale measures how acidic or alkaline a substance is. Substances with a pH of 7 are neutral, substances with a pH greater than 7 are alkaline (or 'basic') and substances with a pH lower than 7 are acidic. Alkalis are 'bases' that are soluble in water. (All alkalis are bases but not all bases are alkalis.)
Acids and alkalis
The pH scale
The chemical properties of many solutions enable them to be divided into three categories – acids, alkalis and neutral solutions. The strength of the acidity or alkalinity is expressed by the pH scale.
•    Solutions with a pH of 7 are neutral.
•    Solutions with a pH greater than 7 are alkaline. The higher the pH, the stronger the alkali.
•    Solutions with a pH less than 7 are acidic. The lower the pH, the stronger the acid.
If universal indicator is added to a solution it changes to a colour that shows the pH of the solution.

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