To investigate if a plant needs carbon dioxide for photosynthesis, we need to create conditions for the plants where carbon dioxide is present in one test and absent in another. The air contains 0.04% carbon dioxide. Scientists can use sodium hydroxide to absorb carbon dioxide from the air so that it is unavailable for the plants to use in photosynthesis.
In this particular experiment, a de-starched plant is covered using an airtight transparent plastic bag. The chemical sodium hydroxide is placed in the bag with the plant to absorb the carbon dioxide. The plant is left for 24 hours and the leaves are tested for starch using iodine. The leaves will show that no starch has been made as no photosynthesis occurred without carbon dioxide.
What is the purpose of the control?
The control is kept in exactly the same conditions as the experiment except the independent variable (sodium hydroxide) has been replaced with water. This means no carbon dioxide has been absorbed from the air.
The control is a comparison to show that the plant would make starch by photosynthesis if the sodium hydroxide were not absorbing the carbon dioxide.
This is the evidence that it is the independent variable which is responsible for the result.