What are the steps for testing starch in a leaf, and why does the leaf turn different colors?

here are the answers to questions 3 and 4:

3. The diagrams below illustrate the steps a student used to test for starch in a leaf.

3. a) Write out the procedure used by the student in each of the steps:

• I. The leaf is dipped in boiling water to kill the cells and break down cell membranes.
• II. The leaf is placed in a test tube with hot ethanol (alcohol) and heated in a water bath to decolorize it by dissolving the chlorophyll.
• III. The decolorized leaf is rinsed in cold water to rehydrate it and remove any remaining ethanol.
• IV. Iodine solution is added to the leaf using a dropper to test for the presence of starch.

3. b) Give two (2) reasons why the leaf is dipped in boiling water.

• To kill the leaf cells and stop all enzyme activity.
• To break down the cell walls and membranes, making it easier for the chlorophyll to be extracted by ethanol and for the iodine solution to penetrate the cells.

3. c) i) Why did the leaf in the dark turn pale brown?
The leaf kept in the dark turned pale brown because it did not undergo photosynthesis, and therefore, no starch was produced or stored in its cells. Iodine solution gives a pale brown color in the absence of starch.

3. c) ii) Why did the leaf in light turned blue-black?
The leaf kept in the light turned blue-black because it underwent photosynthesis, producing and storing starch in its cells. Iodine solution reacts with starch to produce a blue-black color.

4. The diagrams below show some hazard symbols usually displayed on chemical containers.

4. a) Identify each of the symbols labelled A, B, C, D and E.

• A: Flammable (specifically, highly flammable gas or aerosol)
• B: Flammable (specifically, highly flammable liquid or solid)
• C: Toxic / Poison
• D: Irritant / Harmful
• E: Oxidizer

4. b) Name two (2) chemicals which can be associated with each symbol A, B, C, D and E.

• A (Flammable): Propane, Butane
• B (Flammable): Ethanol, Acetone
• C (Toxic): Cyanide, Mercury compounds
• D (Irritant / Harmful): Ammonia, Bleach
• E (Oxidizer): Hydrogen peroxide, Potassium permanganate

4. c) Name two (2) places where symbol B can be used.

• Petrol stations
• Chemical storage facilities