School House

Do fish drink the water?

Objectives
To understand how osmosis works on fish in saltwater and freshwater.

Activities
Investigate the effects of saltwater and freshwater on raisins in order to understand what fish must do to survive.

Background
In saltwater solutions, salt will diffuse from areas of high concentration to areas of low concentration. Conversely, if a membrane blocks the salt form moving (but not the water) the water will diffuse from the low salt areas to the high salt areas. This is osmosis.

All creatures living in water are adapted to their surroundings. This includes adapting to the effects of osmosis. Seawater has a higher salt concentration than the body fluids of fish. Therefore, fish in saltwater constantly lose to the sea. These fish have to drink continually, urinate very little, and push the salt out through their gills.

Freshwater has a lower salt concentration than the body fluids of fish. Fish living in freshwater never drink. Freshwater tends to flow into their bodies. If freshwater fish drank even a little, they would swell up.

Fish in freshwater (water flows in) Fish in saltwater (water flows out)

Materials

2 glass jars
raisins
salt
teaspoon
measuring cup,
label (paper and tape), pencil
knife (optional)

Procedure

1. Fill two glass jars with water and label one saltwater and the other one freshwater

2. Add salt to one jar and stir until dissolved - a teaspoonful of salt to 250 ml of water.

3. Add a handful of raisins to each jar. Keep a handful of raisins dry for comparison.

4. Let stand for an hour.

5. Observe the raisins

Questions

1. a) What happens to the raisins in the freshwater?
   b) What would happen to freshwater fish if they drank water?
2. a) What happens to the raisins in the saltwater?
   b) What would happen to fish in saltwater that did not drink a lot?
3. What would happen to you if you drank saltwater?

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