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10.3 Acid-alkali indicators
i Words to know:
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English term
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Write the words again
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Chinese term
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1
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Natural
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天然的
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2
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Indicators
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指示劑
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3
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Litmus
paper / solution
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石蕊試紙 / 溶液
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4
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Acidic /
acidity
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酸性的 / 酸度
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5
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Alkaline
/ alkalinity
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鹼性的 / 鹼度
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6
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Neutral
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中性的
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7
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Universal
indicator
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通用指示劑
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8
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Degree
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程度
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9
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pH paper
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pH試紙
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10
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pH value
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pH值
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11
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pH scale
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pH標度
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i Fill in the blanks:
1
An
acid-alkali indicator shows different colours in acids and
alkalis. It can be used to distinguish acids from alkalis.
2
Some deeply coloured plants show different colours
in acids and alkalis. They can be used to make
natural acid-alkali indicators.
3
Litmus is a common acid-alkali
indicator and has two forms: red and blue.
Both
of them are available in solution and paper form.
4
Acids turn blue litmus red, and alkalis turn red litmus paper blue. Distilled water
does not change the colours
of both blue and red litmus paper. It is said to be neutral.
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Red litmus paper
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Blue litmus paper
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Acidic
substances
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no colour change
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turns red
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Alkaline substances
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turns blue
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no colour change
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Neutral substances
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no colour change
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no colour change
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5. To show the
degree of acidity or alkalinity of a substance, we can use
universal indicator.
6. Universal indicator is a mixture of
several indicators. It shows different colour in
substances with different degree of acidity or alkalinity.
It
is available in solution and paper form. Its paper form is called pH paper.
7. Acidity or alkalinity is measured in terms of pH value. The scale for
measuring acidity or
alkalinity in terms of pH values is called pH scale. Most
substances have pH values between 0 to 14.
9. An acid has a pH value smaller than 7. The lower the pH value,
the more acidic it is.
An alkali has a pH value greater
than 7. The higher the pH value, the more alkaline it is.
A neutral substance has a pH value
equal to 7.
i Matching:
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Red Litmus paper
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•
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turns blue by
alkaline substance.
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/ can be used
to test alkaline substance only.
/ cannot be
used to test for neutral substance.
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pH paper
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•
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turns light
green by distilled water.
/turns blue by
alkaline substance.
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pH value
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•
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•
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is used to
express the degree of acidity or alkalinity.
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Universal
indicator
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•
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is a solution
which shows different colours in substances with different
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degrees of
acidity or alkalinity.
/ turns light
green by distilled water.
/ turns blue by
alkaline substance.
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i Concept checking:
Decide whether the following statements are
true or false, and try to rewrite the true statements.
1. Some plant extracts show different colours in acids or alkalis, hence
they can be used as acid-alkali indicators. True / False
__________________________________________________________________________________________
2. Table salt solution can turn red litmus paper blue. True
/ False
Table salt solution keeps red litmus paper red.
3. If red litmus paper remains red, the substance under test should be
acidic. True / False
If red litmus paper remains red, the substance under test should be
acidic or neutral.
4. A neutral substance has a pH value equal to 8. True
/ False
A neutral substance has a pH value equal to 7.
5. The lower the pH value of a solution, the higher its alkalinity is. True
/ False
The lower the pH value of a solution, the higher its acidity is /
The higher the pH value of a solution, the higher its alkalinity is.
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