photosynthesis

• 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. Count the number of bubbles produced in 1 minute
• Repeating the experiment at different distances


  1. Repeat the experiment at 20 cm, 30 cm, and 40 cm from the light source

  2. Calculate the mean number of bubbles produced per minute at each distance
• Problems with counting bubbles


  • Number of bubbles can be too fast to count accurately
  • Bubbles are not always the same size
• Measuring the volume of oxygen produced

  1. Place the pond weed in a measuring cylinder filled with water

  2. Measure the volume of oxygen produced
• 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
• Higher tier students need to understand the inverse square law