Practical Biology
A collection of experiments that demonstrate biological concepts and processes.
Observing earthworm locomotion
Practical Work for Learning
Published experiments
Testing leaves for starch: the technique, demonstration or class practical.
This procedure kills a leaf, disrupts the cell membranes and softens the cuticle and cell walls. This makes it possible to extract the chlorophyll with hot ethanol and also allows the iodine solution to penetrate the cells and react with any starch present.
Lesson organisation
You can run this as a teacher demonstration, or with students carrying out the procedure in pairs.
Apparatus and Chemicals
For each group of students:.
Eye protection
Beaker for boiling water, 250 cm 3
Boiling tube, 1 for each type of leaf used
Anti-bumping granules (optional)
For the class – set up by technician/ teacher:
Ethanol (IDA) ( Note 1 )
Kettles of boiling water ( Note 2 )
OR Electric water baths set at 90 °C containing a boiling tube rack
Iodine in potassium iodide, solution in dropper bottles ( Note 3 )
Beaker or jar (at least 250 cm 3 ), labelled ‘Waste ethanol’ ( Note 4 )
Leaves, different types, such as pelargonium (pot geranium) ( Note 5 )
Health & Safety and Technical notes
Ethanol (IDA), iodine solution and hot liquids require safety precautions to be taken. Wear eye protection.
Read our standard health & safety guidance
1 Ethanol (IDA) – refer to CLEAPSS Hazcard 40A and student safety sheet 60 – is highly flammable (flash point 13 °C) and harmful (because of the presence of methanol). The risks in this procedure are reduced by using hot water from kettles or in water baths rather than heating with a Bunsen burner flame. Some protocols recommend propanol (Hazcard 84A) in place of ethanol, as it removes chlorophyll more effectively. However, it has the additional risk of eye damage, its flashpoint is very similar to that of ethanol (IDA), and it may be more expensive.
2 Kettles are a safer source of hot water than heating with a Bunsen burner because of the presence of flammable ethanol (IDA) in this procedure. Students are familiar with the hazards of using kettles. Consider how to limit the movement of students around the laboratory with kettles or beakers of near-boiling water. Electrically-heated and thermostatically-controlled hot water baths may be a safer alternative.
3 Iodine solution – refer to CLEAPSS Hazcard 54B and Recipe card 39. A 0.01M solution is suitable for starch testing. Make this by 10-fold dilution of 0.1M solution. Once made, the solution is a low hazard but may stain skin or clothing if spilled, and may irritate the eyes.
4 Save the waste ethanol as a source of chlorophyll for future work. Make sure it cannot be tipped over and is in a safe place so it is not a fire hazard.
5 If the teacher or technician snips the leaves from the plants to give to the students, the plants are more likely to survive to be used again. Variegated Pelargonium (pot geranium) are good subjects for this experiment as are Tradescantia and Impatiens (busy lizzie).
6 Ensure that the plants have been well-illuminated for 24-48 hours. In winter, it might be worth using a halogen lamp to ensure the illumination is adequate.
Ethical issues
There are no ethical issues associated with this procedure.
SAFETY: Ensure the ethanol is kept away from naked flames. Students should wear eye protection when working with ethanol or iodine solution. Take care with hot liquids. Be aware that plant sap may irritate the skin.
Investigation a Collect leaves from the plants to be tested.
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VIDEO