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Chemistry
for Secondary Schools

6. Acids are mostly present in liquid or gaseous forms, but there are some
which exist in solid form such as oxalic acid.
7. Acids change the colour of phenolphthalein indicator to colourless.
8. Acids change the colour of methyl orange indicator to red.
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Chemical properties of acids
Acids have various chemical properties, including the following:
1. Acids have a pH less than 7 on a pH scale. Low pH values indicate high
acidity, for example, hydrochloric acid (HCl) has a pH between 1.5 and
3.5, nitric acid (HNO ) has pH around 3, and sulfuric acid (H SO ) has pH
4
2
3
between 2 and 3.
2. Solution of an acid turns blue litmus paper red (Figure 5.3). This is used in
the identification of acids.

Figure 5.3: Blue litmus paper immersed in an acid solution

3. Acids react with carbonates and hydrogencarbonates to produce salt, water,
and carbon dioxide gas as shown in the following chemical equations:

One test of the produced gas (carbon dioxide) is to bubble it through
limewater. The limewater will turn milky or cloudy white. The laboratory
experimental set-up for the reactions of acids and carbonates is shown in
Figure 5.4.

102 Student’s Book Form Two

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