Photosynthesis

• Investigating the effect of light intensity on the rate of photosynthesis

  1. Take a boiling tube

  2. Place 10 cm away from an LED light source

  3. Fill boiling tube with sodium hydrogen carbonate solution

  4. Put a piece of pond weed into the boiling tube

  5. Leave for 5 minutes to acclimatize

  6. Start stopwatch and count bubbles produced in 1 minute

  7. Repeat 2 more times and calculate mean

  8. Repeat experiment at 20 cm, 30 cm, and 40 cm
• Problems with the practical

  • Number of bubbles can be too fast to count accurately
  • Bubbles are not always the same size
• Solving the problems


  1. Measure the volume of oxygen produced instead of counting bubbles

  2. Use equipment to catch the bubbles in a measuring cylinder filled with water

  3. Use the measuring cylinder to measure the volume of oxygen produced
• Doubling the distance from the light to the pondweed

  Number of bubbles per minute falls by a factor of 4
• Inverse square law


  If we double the distance, the light intensity falls by 4 times, which causes the number of oxygen bubbles to fall by 4 times
• Higher tier students need to understand the inverse square law
• You'll find plenty of questions on this required practical in the revision workbook