• The iodine test for starch can be used to investigate the requirement for carbon dioxide in photosynthesis
• Before testing for starch, complete the following procedure:

Apparatus

• Conical flasks
• Potassium hydroxide solution
• Clamps
• Clamp stands
• A plant
• Beakers
• Bunsen burner
• Tripod
• Gauze platform
• Prongs
• Ethanol
• Apron
• Safety goggles
• Gloves
• Iodine solution
• White tile

Method

• Destarch the plant by placing it in a dark cupboard for 24 hours
  • This ensures that any starch already present in the leaves will be used up and will not affect the results of the experiment

   

• Following destarching, enclose 1 leaf with a conical flask containing potassium hydroxide
  • The potassium hydroxide will absorb carbon dioxide from the surrounding air

   

• Enclose another leaf with a conical flask containing no potassium hydroxide (control experiment)
• Place the plant in bright light for several hours
• Test both leaves for starch using iodine solution
  • Drop the leaf in boiling water
  • Transfer the leaf into hot ethanol in a boiling tube for 5-10 minutes
  • Dip the leaf in boiling water
  • Spread the leaf out on a white tile and cover it with iodine solution

   

Photosynthesis cannot occur without carbon dioxide

Results

• The leaf from the conical flask containing potassium hydroxide will remain orange-brown as it could not photosynthesise due to lack of carbon dioxide
• The leaf from the conical flask not containing potassium hydroxide should turn blue-black as it had all necessary requirements for photosynthesis

Applying CORMS evaluation to practical work

• When working with practical investigations, remember to consider your CORMS evaluation

CORMS evaluation

• In this investigation, your evaluation should look something like this:
  • C - We are changing whether there is carbon dioxide or no carbon dioxide
  • O - The leaves will be taken from the same plant or same species, age and size of plant
  • R - We will repeat the investigation several times to ensure our results are reliable
  • M1 - We will observe the colour change of the leaf when iodine is applied
  • M2 - ...after 1 day
  • S - We will control the temperature of the room and the light intensity

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