The second student placed a watch glass containing potassium hydroxide in one bell jar as:

As the student wants to show that carbon dioxide is essential for photosynthesis, he sealed both the bell jars with Vaseline (- petroleum product) and placed KOH in one bell jar as KOH absorbs CO₂ gas present in the bell jar. The plant (in this bell jar) will not be able to perform photosynthesis or making food as carbon dioxide has been removed from the bell jar.