Ask pupils if turbid water means that water is polluted? (Not necessarily. Turbidity may come from suspended sediments or plankton)
Show some samples of turbid water from a river to the pupils and have a discussion:
Why can water from a river be so turbid? (Water turbidity depends on the quantity of insoluble inorganic and organic particles. Among organic particles, microorganisms and plankton might be present. During high discharge and floods, the rivers typically carry a lot of suspended solids eroded from riparian land)
How can we reduce the turbidity of water in a sample? (Filtering it, let it settle.)
How can we know that filtering really works? (Water turbidity can be measured. Light is absorbed with increasing depth in water and will disappear even more quickly if there is a suspended load of sediment. To see if the filtering really works, we should measure transparency before and after filtering the samples, and then compare the results.)
Measuring water transparency:
Make the mark on the paper like on the picture below.
Stick the marked white paper on the bottom of the measuring cylinder or tall bottle.
Prepare the lighting for the bottom part of the measuring cylinder (to make sure that the light conditions are always the same).
On the measuring cylinder (plastic bottle) mark a centimeter scale.
Pour the water slowly into the measuring cylinder and observe the picture on the bottom through the water column.
Write down the height of the water column for each water sample. You should still be able to see the picture with the cross and circles at the bottom of the measuring cylinder.