Potted plants. Image credit nubolo.net

Potted plants. Image credit nubolo.net

ZIMSEC O Level Combined Science: Carbon Dioxide and Photosynthesis

Aim: To test whether carbon dioxide is necessary for photosynthesis.

Materials: Potted green plants, potassium hydroxide solution (soda lime), two polythene bags/two bell jars, string, apparatus and reagents for the start test.


  1. Destarch the two plants by placing them in a dark place for 24 hours.
  2. Test one leaf from each plant using the starch test to make sure they have no starch.
  3. Set both plants into the light and cover them with polythene bags tied with strings at the bottom or just cover them with jar bells as shown in the diagram above.
  4. Inside one bag/jar place a container with soda lime/Potassium hydroxide solution. This absorbs the carbon dioxide. Leave the other plant as the control.
  5. Leave the plants in the light for a few hours.
  6. Test one leaf from each plant for starch.


Image credit rollitup.org

Leaf with carbon dioxide left, without carbon dioxide right .Image credit rollitup.org

  • The leaf with carbon dioxide tests positive for starch by turning blue-black in colour.
  • The one without carbon dioxide remains unchanged as shown to the right.


  • Carbon Dioxide is required in order for photosynthesis to take place.

To access more topics go to the Combined Science Notes page.