OpenStax-CNX module: m20514
1
To identify acids and bases with
the help of indicators
∗
Siyavula Uploaders
This work is produced by OpenStax-CNX and licensed under the
†
Creative Commons Attribution License 3.0
1 NATURAL SCIENCES
2 Reactions that occur between dierent kinds of matter: acids and bases
3 EDUCATOR SECTION
4 Memorandum
1. Coee: Acidic (4 to 5)
2. Shampoo: Acidic (4 to 5)
3. Fruit salt solution: Acidic (0 to 3)
4. Tartaric acid: Acidic (0 to 3)
5. Lemon juice: Acidic (0 to 3)
5 LEANER SECTION
6 Content
7 Activity: To identify acids and bases with the help of indicators [LO 1.2, LO
1.3]
As there are many substances that are dangerous to the taste, we will be using safer methods for determining
which substances are acids and which are alkalis.
We shall be using:
• black tea
• bromothymol blue
• litmus paper / litmus solution
When a driver plans to turn to the left, he or she uses a ickering light to signal this intention. This ickering
light serves as an indicator. The above-mentioned agents will give an indication of whether substances are
acidic, alkaline or neutral. They are called chemical indicators.
BLACK TEA
Pour approximately 5 cm3 (5 ml) of black tea to each test tube, as shown in the following sketch.
∗ Version
1.1: Mar 17, 2009 3:07 am -0500
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Figure 1
Then add:
•
•
•
•
a little vinegar to test tube no. 1
some lime juice to test tube no. 2
some bicarbonate of soda to test tube no. 3
a little milk of magnesia to test tube no. 4.
The tea in the fth test tube will be used as a reference when making observations of reactions in other test
tubes.
• Observe what happens and complete the table that is provided for recording your observations:
SUBSTANCE: DESCRIBE THE COLOUR CHANGE
Substance
Describe the colour change
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_______________________________________
• Vinegar
_______________________________________
• Lime juice
_______________________________________
• Bicarbonate of soda
_______________________________________
• Milk of magnesia
Table 1
Complete the following deductions:
• When an acid (vinegar/lime juice) is added to black tea, the solution
_____________________________________________________________________
____________________________________________________________________
• But when an alkali (bicarbonate of soda / milk of magnesia) is added to black tea, the solution
_____________________________________________________________________
____________________________________________________________________
BROMOTHYMOL BLUE
Pour approximately 5 cm3 (5 ml) pure water into each test tube as shown in the following sketch.
Add 5 drops of Bromothymol blue to the water in each test tube. The Bromothymol blue should now
have a greenish tinge. Consider the following statement when you do a test to determine whether the next
substances are acids of bases:
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Figure 2
Bromothymol blue becomes yellow in an acid solution, but looks blue when it is in contact with a base.
The colour does not change when it is added to a neutral solution.
Substance
Acid
Base
Neutral
______________ ______________ ______________
• Water
______________ ______________ ______________
• Tartaric acid
______________ ______________ ______________
• Salt
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______________ ______________ ______________
• Ammonia
Table 2
LITMUS PAPER
Use blue or red litmus paper to establish whether the following substances are acids or bases.
Place a drop of each solution on the litmus paper. If the substance is a powder, it should rst be dissolved
in water. Insoluble substances could be mixed with water to form a paste.
Baking powder
Bicarbonate of soda
Shaving cream
Milk
Salad dressing
______________
______________
______________
______________
______________
Beer
Icing sugar
Hair shampoo
Coee
Carbonated cold drink
______________
______________
______________
______________
______________
Table 3
8 Assessment
Learning Outcome 1: The learner will be able to act condently on curiosity about natural phenomena,
and to investigate relationships and solve problems in scientic, technological and environmental contexts.
Assessment Standard 1.2: We know this when the learner conducts investigations and collects data:
organises and uses apparatus/equipment or sources to gain and record information;
Assessment Standard 1.3: We know this when the learner evaluates data and communicates ndings:
generalises in terms of relevant aspects and describes how the data support the generalisation.
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