Page 126 - Chemistry_Form_2

Chemistry
for Secondary Schools

3. A bee stung a gardener while working in the garden. What household material
can be used to relieve the pain in the stung area? Explain your answer.

Acid-base indicators
FOR ONLINE READING ONLY
Acid-base indicators are chemicals that are used to determine the chemical
nature of a substance; whether it is acidic or basic. The acid-base indicators are
also known as pH indicators because acidity and alkalinity relate to pH range.
Indicators normally change colour to indicate the presence or absence of acids or
bases. The pH indicators operate efficiently over a certain pH range.

pH scale
pH scale is a scale of numbers, from 0 to 14 which is used to express acidity,
neutrality, and alkalinity. The pH 7 indicates neutrality of a substance. Any
substance that has a pH below 7 is acidic, while a substance with pH above 7
is basic. The smaller the number below 7 in the scale, the more acidic is the
substance. Similarly, the greater the number above 7, the more basic is the
substance. This means that an acid with pH 1 is more acidic than that with pH 6,
and the base with pH 14 is more basic than that with pH 8. Figure 5.12 shows the

trends in acidity and alkalinity in the pH scale.

pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Neutral
Increasing acidity Increasing alkalinity

Figure 5.12: Trends in acidity and alkalinity

Occurrence of pH indicators
pH indicators can occur naturally or be synthesised.

Natural pH indicators
A natural pH indicator is a substance which is found naturally and can be used
to determine whether the substance is acidic or basic. Examples of natural pH
indicators are turmeric, grape juice, red cabbage, cassava leaves, onion, beetroot,
and coloured flowers like that of rosella and hibiscus. For example, red cabbage
juice is pink in acids and green in bases. An extract of hibiscus flowers has a

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