Page 127 - Chemistry_Form_2

Chemistry
for Secondary Schools

red colour in an acid and greenish yellow colour in a base. Onion is one of the
olfactory indicators which diminishes its smell in a base and remains as it is in
an acid. Vanilla essence has similar characteristics to onion. The smell of vanilla
essence disappears when it is added to a base, but persists when it is added to an
acid.
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The most commonly used natural indicator is litmus which is extracted from
lichens (Figure 5.13). Litmus has purple, red, and blue colours in distilled water,
acidic solution, and basic solution, respectively. Litmus is available either in
solution, or in the form of strips of paper.

Figure 5.13: Lichen
Activity 5.4

Aim: To prepare a pH indicator from hibiscus flowers

Requirements: Hibiscus flowers, water, beakers, droppers, mortar and pestle,
knife, 50% ethanol solution, heat source, evaporating dish,
dilute hydrochloric acid, and dilute sodium hydroxide
Procedure

1. Cut the hibiscus flowers into small pieces and then crush them by using
mortar and pestle.

2. Add about 20 cm of ethanol solution into the crushed hibiscus flowers and
3
continue crushing. Decant the mixture obtained and then keep the solution
obtained aside.

3. Boil the solution obtained to make sure that all the ethanol evaporates.
Leave the residue to cool.

120 Student’s Book Form Two

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