Question

describe an experiment to prove that oxygen gas is envolved during photosynthesis​

Answer 1

I think you are mistaken. Oxygen is not used in photosynthesis. Carbon dioxide is used instead. Actually oxygen is given out after photosynthesis.

Answer 2

Here is your answer

Experiment to prove the necessity of carbon dioxide in photosynthesis

To prove the necessity of carbon dioxide in photosynthesis following steps need to do
Insert a part of the leaf of a destarched plant into a conical flask containing potassium hydroxide.
Potassium hydroxide solution absorbs the carbon dioxide gas from the air present in the glass bottle.
Leave the plant sunlight.
After few hours, perform a starch test to this and another leaf of the same plant.
Observation-
Leaf exposed to the atmosphere are bluish black.
Leaf exposed to KOH does not change to blue-black colour
Result- Starch is present in the green part of the leaf.
Conclusion- Presence of starch proves that chlorophyll is necessary for photosynthesis as it traps sunlight.