Investigate the concentrations of dilute and concentrated acids and alkalis using the pH scale.
Produce an acid–base indicator from plant material.
It is important to know how concentrated an acid or alkali is to use it safely. Concentrated acid or alkali are nearly always corrosive. To record how concentrated an acid or alkaline solution is, scientists use the pH scale.
z
Fig. 13.2.1 The pH scale
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The pH scale goes from 0 to 14. zz zz zz
Indicators
An indicator is a substance that shows whether the solution being tested is an acid or a base. Indicators contain a dye that turns a different colour in acidic or alkaline solutions.
Two commonly used indicators in the laboratory are litmus indicator and universal indicator.
z z z Fig. 13.2.2 Litmus indicator in acid zz
Litmus indicator will change colour from blue to red in acids and from red to blue in alkalis. A limitation of litmus is that it does not indicate how concentrated the acid or alkali is.
– An acid is a substance that turns blue litmus red. – A base is a substance that turns red litmus blue.
Universal indicator is a mixture of dyes that changes colour according to how concentrated an acid or alkali is. The advantage of universal indicator is that it shows the pH of the acid or alkali being tested, whereas litmus only indicates if the substance is an acid, alkali or neutral.
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A substance that has a pH of 7 on the pH scale is neutral. A substance that has a pH below 7 is acidic. A substance that has a pH above 7 is alkaline.
