The leaf discs were placed in a beaker of sodium hydrogencarbonate (NaHCO₃) solution. Explain why sodium hydrogencarbonate solution was used instead of water.

Photosynthesis requires two main reactants: water and carbon dioxide. The leaf discs are already in an aqueous solution, so water is readily available. However, the amount of dissolved CO₂ in tap water can be low and variable, and it would quickly be used up by the photosynthesising leaf discs. Sodium hydrogencarbonate (NaHCO₃) solution releases carbon dioxide when dissolved in water. By using this solution, the student ensures that there is a constant and plentiful supply of carbon dioxide for the leaf discs. This is important for making it a fair test, as it prevents CO₂ concentration from becoming the limiting factor, allowing the effect of light colour to be investigated properly.