Men in the Boat – Discovery Activity
Opening Problem
Picture a wide, slow river emptying into the ocean. Outside the mouth of the river, in the ocean, is a small boat anchored to the bottom with two people in it. One of the people is a fisherman and the other is his friend, a scientist.
After finishing for a while, the fisherman catches a fish which is called a Cichlid (pronounced sick-lid). Now this fish is similar to a perch, and is a fresh-water fish that is not at all good at surviving in salt water. Time passes and another fish is caught, this time a Yellow Fin Tuna. The scientist knows that the Yellow Fin Tuna is strictly a salt water fish and cannot live in fresh water. This situation really puzzles the scientist since both fish were caught at the same place.
Question to Class: What would you do to help you understand how this could happen?
Activity:
Preparation
Take three, 1-liter containers with water and add six drops of food coloring to each container to make the following colors: Blue, Yellow, and Red.
Use Kosher (it dissolves more readily) salt to mix the following solutions:
Do not add salt to the Blue water.
To the Yellow water, add four teaspoons of salt.
To the Red water, add 12 teaspoons (4 tablespoons) of salt.
Note: The salty water may become cloudy, but this will not affect the experiment.
For each group of students:
1. Four Cups (clear is best, but paper will work) – Put some of each liquid into each of the cups, the fourth cup is the discard cup after each test. About 5 cm (2 inches) of liquid for each cup.
Note: Have students label each container with its salt content in teaspoons (i.e. 0-S for Blue, 4-S for yellow, and 12-S for Red)
2. Clear drinking straws as found at fast food restaurants.
The Investigation : (All directions to students are oral)
1. Have students use the straws as pipettes by placing the end of a straw ? way to the bottom of the blue liquid and then putting their thumbs on the end of a straw to seal it. They can then lift the straw out and the liquid will stay in the straw.
2. CAREFULLY put the straw almost to the bottom of the yellow liquid and slowly release your thumb to let the yellow liquid into the straw. Place your thumb back onto the end of the straw to seal it and then take the straw out of the liquid and observe the colored liquid in the straw. The liquids will have either mixed or formed layers. Write down your observation.
Teacher Note: You should observe that the liquids are layered, with the blue layer on top. There should be a very distinct boundary between the yellow and blue.
3. Drain the liquid into the discard cup that was empty at the start of this activity.
4. Make some kind of graphic organizer to show the relationships of the three liquids. There are six possibilities. Try two of the liquids at a time.
5. From the data, try to come up with a general pattern or law, something like:
“If I do this, then this will happen.” (It should be about the salt solutions comparison.)
Teachers, the laws will vary but should be something along the lines of:
Liquids that have more salt in them will be “heavier,”
Salty water is different in weight from fresh water and will sink in fresh water,
If a salty water is on top of a fresh water, the two will mix; if the salty water is on bottom, they will not mix.
6. Students, take that general law that you just came up with and do some deductive reasoning and answer the question:
How could the fishermen catch both types of fish in the same spot?
Make a model or diagram to help describe your hypothesis.
7. If you had this hypothesis that you just stated, how could you test it?
8. Is this the only possible answer as to why the two types of fish could be caught in the same place? Are there other alternatives, and what could they be?