Natural sciences
Reactions that occur between different kinds of matter: acids and bases
Educator section
Memorandum
1. Coffee: 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)
Leaner section
Content
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 flickering light to signal this intention. This flickering 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.
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 fifth 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 |
|
_____________________________________________ |
|
_____________________________________________ |
|
_____________________________________________ |
|
_____________________________________________ |
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:
Substance | Acid | Base | Neutral |
|
______________ | ______________ | ______________ |
|
______________ | ______________ | ______________ |
|
______________ | ______________ | ______________ |
|
______________ | ______________ | ______________ |
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 first be dissolved in water. Insoluble substances could be mixed with water to form a paste.
Baking powder | ______________ | Beer | ______________ |
Bicarbonate of soda | ______________ | Icing sugar | ______________ |
Shaving cream | ______________ | Hair shampoo | ______________ |
Milk | ______________ | Coffee | ______________ |
Salad dressing | ______________ | Carbonated cold drink | ______________ |
Assessment
Learning Outcome 1: The learner will be able to act confidently on curiosity about natural phenomena, and to investigate relationships and solve problems in scientific, 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 findings: generalises in terms of relevant aspects and describes how the data support the generalisation.