Strength of Acids and Alkalis
pH scale
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pH scale is a measure of acidity and alkalinity. Practically, it is numbered between 0 and 14.
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Neutral substance has pH of 7.
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A solution of pH less than 7 is acidic The lower the pH, the more acidic the solution or the higher the concentration of H+ ions in the solution.
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A solution of pH above 7 is alkaline. The higher the pH, the more alkaline the solution or higher concentration of OH- ions presence in the solution.
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The pH of a solution can be measured by using the Universal indicator, pH paper or with a pH-meter.
Acids / Alkali Indicator
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An indicator is a substance or mixture of substances that when added to the solution gives different colours depending on the pH of the solution.
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Table below shows the colour of indicators in neutral, acidic and alkaline solution:
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The best indicator of all is the universal indicator.
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It can be used to show not only whether a substance is acidic or alkaline, but also whether it is a strong acid or a weak acid.
Colour of indicator at different pH value
Litmus paper. The colour of litmus is blue in alkali and red in acid
Colour of phenolphthalein in alkali
Strength of Acids and Alkalis
pH scale
-
pH scale is a measure of acidity and alkalinity. Practically, it is numbered between 0 and 14.
-
Neutral substance has pH of 7.
-
A solution of pH less than 7 is acidic The lower the pH, the more acidic the solution or the higher the concentration of H+ ions in the solution.
-
A solution of pH above 7 is alkaline. The higher the pH, the more alkaline the solution or higher concentration of OH- ions presence in the solution.
-
The pH of a solution can be measured by using the Universal indicator, pH paper or with a pH-meter.
Acids / Alkali Indicator
-
An indicator is a substance or mixture of substances that when added to the solution gives different colours depending on the pH of the solution.
-
Table below shows the colour of indicators in neutral, acidic and alkaline solution:
-
The best indicator of all is the universal indicator.
-
It can be used to show not only whether a substance is acidic or alkaline, but also whether it is a strong acid or a weak acid.
Colour of indicator at different pH value
Litmus paper. The colour of litmus is blue in alkali and red in acid
Methyl Orange: The colour of mthyl orange in acidic and alkaline solution. Left: Acidic; Right: Alkaline.
Colour of phenolphthalein in alkali
Methyl Red: left: acidic, middle: neutral, right: alkaline
A roll of universal indicator paper.
Bromothymol Blue pH indicator dye in an acidic, neutral, and alkaline solution (left to right)