Natural Sciences Grade 9 Practical Task / Investigation

Marks: 35 converted to 20 Worksheet

Acids, bases and the use of an indicator

PART 1

Aim:

To determine the colour of an indicator in an acid, a base and a neutral substance.

Materials:

Universal indicator or red cabbage water (see instructions for preparation below)

pH scale and colours for universal indicator or red cabbage water (Table 1)

Bicarbonate of soda (Baking soda, NaHCO3 or sodium hydrogen carbonate)

Vinegar (Ethanoic acid)

Water (Distilled water if possible)

Test tubes, glass containers or transparant plastic cups

Test tube rack

Droppers

Plastic teaspoon

How to prepare red cabbage indicator:

Chop up a few (2 to 3) red cabbage leaves.

Cover the leaves with boiling water and boil for a few minutes.

Let the mixture cool down.

Instead of boiling the mixture, liquidise the leaves and water with an electric food processor.

Strain the mixture to remove the solid pieces from the indicator (red cabbage water).

How to prepare the bicarbonate of soda solution:

Dissolve 2 teaspoons of bicarbonate of soda in 100 m (20 teaspoons) of water.

Stir until all the solid has dissolved.

Table 1: Colour scale and pH for universal indicator and red cabbage water

pH

Colour of

universal

indicator

Colour of red

cabbage water

0 Dark red Red

1 Red

2 Red Light pink

3 Orange red

4 Orange Dark pink

5 Orange yellow

6 Greenish yellow Purple

7 Green

8 Greenish blue Blue

9 Blue

10 Navy blue Blue green/Cyan

11 Purple

12 Dark purple Green

13 Violet

14 Violet Yellow

Copyright FS DoE 1Natural Sciences Grade 9 Worksheet Practical Task 2 Term 2

Method:

1. Put 3 test tubes in a test tube rack and number them 1, 2 and 3.

2. Pour 5 to 10 m of vinegar in test tube 1.

3. Pour 5 to 10 m of (distilled) water in test tube 2.

4. Pour 5 to 10 m of the bicarbonate of soda solution in test tube 3.

5. Add a few drops of universal indicator or red cabbage water to each test tube.

6. Observe the colour of each solution.

7. Determine the pH of each solution. Use Table 1.

8. Decide whether the solution in each of test tubes 1, 2 and 3 is an acid, a base or neutral.

9. Keep the solutions to use as a reference in Part 2 of the investigation.

Test tube 1 Test tube 2 Test tube 3

Test tube Colour pH Acid, base or neutral

1

(Vinegar)

2

(Water)

3

(Bicarbonate

of soda)

Table of results:

Conclusion:

1. Which substance is neutral? ____________________________________ (1)

2. What is the pH of a neutral substance? ____________________________ (1)

3. Which substance is an acid? ____________________________________ (1)

4. The pH of an acid is (below / above) seven. (1)

5. Which substance is a base?

____________________________________ (1)

6. The pH of a base is (below / above) seven. (1)

7. What is the function of an indicator?

________________________________________________________________

________________________________________________________________ (2)

Copyright FS DoE 2Natural Sciences Grade 9 Worksheet Practical Task 2 Term 2

PART 2

Aim:

soda.

To investigate how the pH changes when vinegar is treated with a solution of bicarbonate of

Investigative question:

How will the pH change when vinegar is treated with a solution of bicarbonate of soda?

Hypothesis: (2)

_________________________________________________________________________

Method:

1. Pour a small amount (5 m or 1 teaspoon) of vinegar in a test tube or glass jar.

2. Add a few drops of universal indicator or red cabbage water to the vinegar.

3. Record the colour and the pH of the vinegar, using Table 1.

4. Use a medicine dropper or propette to transfer 10 drops of the original bicarbonate of

soda solution, prepared in Part 1, to the vinegar-indicator mixture.

5. Stir the solution gently or swirl the test tube.

6. Record the colour and the pH of the solution.

7. Add another 10 drops of bicarbonate of soda and record the colour and pH.

8. Repeat steps 4 to 6 until the pH of the solution is well above 7.

Table of results:

Total number of drops of

bicarbonate of soda Colour pH

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

(6)

Copyright FS DoE 3Natural Sciences Grade 9 Worksheet Practical Task 2 Term 2

Graph of the results:

Draw a graph of the results in the table. Use the graph paper given below. Provide the graph

with a heading and label each axis clearly. (6)

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Conclusion:

Were you able to confirm or reject your hypothesis? Use your experimental results to

motivate your answer. (2)

_________________________________________________________________________

_________________________________________________________________________

Copyright FS DoE 4Natural Sciences Grade 9 Worksheet Practical Task 2 Term 2

Questions:

1. When the vinegar and bicarbonate of soda (NaHCO3) react with each other, gas

bubbles are observed. Which gas is liberated? (1)

2. 3. ___________________________________________________________________

Complete the general word equation for this reaction: (3)

Acid + metal carbonate

____________ + ______________ + ______________

How many drops of bicarbonate of soda neutralised the vinegar in this

investigation? (1)

4. ___________________________________________________________________

Explain how you determined your answer in question 3. (2)

___________________________________________________________________

___________________________________________________________________

5. Identify the:

5.1 Independent variable in this investigation. (1)

___________________________________________________________________

5.2 Dependent variable (1)

6. ___________________________________________________________________

Bicarbonate of soda helps to calm the burning sensation after a bee sting. Explain

how this is possible. (2)

___________________________________________________________________

___________________________________________________________________