Required practical 6: photosynthesis

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

  1. Take a boiling tube and place it 10 cm away from an LED light source

  2. Fill the boiling tube with sodium hydrogen carbonate solution

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

  4. Leave for 5 minutes to acclimatize

  5. Start a stopwatch and count the number of bubbles produced in 1 minute

  6. Repeat the experiment at 20 cm, 30 cm, and 40 cm from the light source
• 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 a measuring cylinder to catch the bubbles and measure the volume of oxygen
• Doubling the distance from the light source

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


  If the distance is doubled, the light intensity falls by a factor of 4, which causes the number of oxygen bubbles to fall by 4 times