Describe a control for the above experiment. The factor, the presence of starch (if the process of photosynthesis has taken place or not) needs to be left out. How would you do this?​

Hi their here is the Answer

During photosynthesis a plant absorbs light energy using the pigment chlorophyll. This allows it to convert carbon dioxide and water into glucose. This glucose is:

transported to the growing parts of the plant for use in respiration
transformed into cellulose, proteins and oils
turned into starch for storage
Therefore, to test if a plant has been photosynthesising, you can test the leaf to see if starch is present.

Iodine solution is used to test leaves for the presence of starch. You need to:

heat a plant leaf in boiling water for 30 seconds (this kills the leaf, stopping any chemical reactions)
add the leaf to boiling ethanol in a water bath for a few minutes (the boiling ethanol dissolves the chlorophyll and removes the green colour from the leaf - it turns white so it is easy to see the change in colour)

wash with water to rehydrate and soften the leaf and spread onto a white tile
add iodine solution from a dropping pipette
After a few minutes, the parts of the leaf that contain starch turn the iodine from brown to blue/black.

Hope this helps :)