Student Portal:


Investigation 2 – Lab









This Investigation is designed to:

  • investigate how the abiotic factors of water velocity, sedimentation, and turbidity affect a watershed,
  • investigate how water velocity affects sedimentation and turbidity in a watershed.


Student Preparation for the Investigation includes having students gather the following materials. This preparation takes place on lab day after student lab groups have settled at their assigned lab tables.

Note: The materials are listed in students’ SDRs. They are also listed below for your reference.

  • (1) triple beam balance
  • (1) spectrophotometer
  • (1) cuvette
  • (1) hotplate/stirrer
  • (1) stir bar
  • (2) weigh dishes
  • (1) beaker, 400ml
  • (5) centrifuge tubes, 15 ml
  • (6) plastic droppers
  • (1) wooden test tube rack
  • marker
  • (1) glass stir rod
  • 1.0g flour
  • 300ml distilled water
  • 30ml 50% glycerin
  • (1) stopwatch
  • (1) clear metric ruler
  • (1) box Kim-wipes
  • (1) water bottle with water
  • masking tape


    • Be sure to read the steps before Trial 1 and the steps included in each Trial before you begin the experiment. If necessary, review the directions as a class.
    • In Trial 1, they should not stir the mixture. As you begin stirring in Trial 2, you may need to use a stir rod to move flour away from the stir bar so that it can begin stirring freely on the bottom of the beaker. Once the stir bar begins stirring, you should not shut the stir plate off until Trial 4 is completed.
    • As you make your observations in Trials 1-4, pay close attention to the cloudiness of the water and the sediment along the bottom and walls of the beaker. You should record or draw your observations in Table B in your SDR.
    •  Spectrophotometers are laboratory instruments designed to measure the relative amount of light of a particular wavelength that is transmitted or absorbed by a liquid. In this Investigation, you will be looking at the transmittance of light or the amount of light that passes through a substance. When measuring the transmittance of light, the spectrophotometer will report the amount of light transmitted as a percent from 0% to 100%. A value of 100% indicates that all of the light that is released from the light source is transmitted through the liquid. A percentage less than 100% indicates that some light is absorbed by the liquid.


    • Read the Background preceding the steps for Trials 5-8 of the SDR. Turbidity is the cloudiness of the water. The turbidity of water can be measured analytically through the use of a spectrophotometer. If water is very turbid, less light will pass through the water and it will have a low % transmittance.
    • You should gently mix the glycerol and sediment solutions before placing them in the spectrophotometer. The glycerol keeps the flour in suspension long enough to get an accurate reading of percent transmittance. Remember that the “blank” used for the spectrophotometer should be prepared by combining 5 ml of the 50% glycerol solution and 1 ml of water.
    • Set the wavelength of the spectrophotometer to 600 nm.
    • To ensure the accuracy of your results, you should use the same cuvette for the blank and each sample and place the cuvette in the sample compartment the same way each time. This will account for any variations in the glass. You should also be careful to avoid getting fingerprints on the cuvettes, wiping with a Kimwipe before placing the cuvette in the sample compartment.
    • Once your data is collected, plot velocity versus percent transmittance and draw a Best Fit Line.  Note: Your teacher may assign this as homework postpone it to PostLab.


Be sure to clean up your lab bench after finishing the experiments. Spilled glycerol, although sticky, comes off with water.