3.21 To identify acids and bases with the help of indicators

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

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:

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.

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.