Indigo carmine is a water-soluble indicator. When oxygen is bubbled through a solution of this indicator, the colour changes from colourless to blue. A student used indigo carmine solution to investigate some of the factors that may limit the rate of photosynthesis. Fig. 2.1 shows the apparatus used by the student. [Figure 2.1] The student prepared four sets of this apparatus. For each set of apparatus, the student used a different concentration of 5cm³ sodium hydrogencarbonate solution: 0.25%, 0.50%, 0.75% and 1.00%. Sodium hydrogencarbonate solution is a source of carbon dioxide for photosynthesis. After adding indigo carmine solution and sodium hydrogencarbonate solution to the alginate beads, the apparatus was illuminated with a bench lamp from a distance of 20 cm. To minimise heating effects, the bench lamp used an LED bulb. For each concentration of sodium hydrogencarbonate solution, the time for the indigo carmine solution to turn from colourless to blue was recorded. The investigation was repeated with four new sets of apparatus illuminated with the same bench lamp at a distance of 10cm, instead of 20cm. The same concentrations of sodium hydrogencarbonate solution were used as previously. All other variables in this investigation were standardised.