Island Explorers Curriculum Home


Unit 1

Lesson Plan 1

Lesson Plan 2

Lesson Plan 3

Lesson Plan 4

Lesson Plan 5

Concepts/Objectives | Vocabulary/Background | Activity | Extension


Lesson Plan 5





Make sure that the following concepts are clear to students before the activity begins:

  1. The colored solutions are models for demonstrating how river water, brackish water, and ocean water mix or form layers. Water from natural sources has a greater range of dissolved substances than the solutions in this activity.
  2. When layers form in the ocean, the water containing the most dissolved salt tends to form the bottom layer. The water having the least amount of salt is usually on top, because it is the least dense.
  3. As more solids are dissolved in water, the water becomes more dense.
  4. The density difference between two volumes of water can prevent them from easily mixing. Less dense water will remain on top of more dense water.


Information important to the activity:
The pickling salt recommended for the activity produces very clear salt solutions. Other types of salt may produce somewhat cloudy solutions because of various substances added to them. Clear solutions make it easier to determine whether different samples are mixing or forming layers.

If you use a different kind of salt, test the proportions of salt and water before class to be sure that they give clear separations of the layers. As a guideline, about the same volume of table salt can be substituted for the pickling salt. If you use coarser salt (such as kosher salt or ice cream salt), you may get better layers if you increase the amount of salt in the "Ocean Water" to about 120 ml in 500 ml of water, and in "Brackish Water" to about 40 ml in 500 ml water:

Prepare the colored solutions as follows:

 "Ocean Water"  "Brackish Water"  "River Water"
 500 ml water  500 ml water  500 ml water
 90 ml salt  30 ml salt  no salt
 20 drops blue food coloring  20 drops red food coloring  20 drops green food coloring

Stir the "Ocean Water "and the "Brackish Water "until the salt is totally dissolved. Place each solution in a jar beaker with no labeling to indicate the contents.


  1. Divide students into pairs or groups of three and have a member from each group pick up a tray of materials.
  2. Stick the plastic straw into the slice of clay at a 45° (as shown in FIGURE 1) Be careful not to stick the straw all the way through the clay. If the straw comes out through the bottom of the slice of clay, remove the straw and stick in into a different place on the slice.
  3. Test the straw for leaks by filling it with tap water. If the water leaks out of the bottom of the straw, remove the straw and stick in into a different spot on the clay. Test for leaks again if you stick the straw into a different spot.
  4. When you are sure that the straw is not leaking, empty the water into your waste container by picking up the entire assembly (the block of clay and the straw) and tipping it so the water drains out of the straw. You may need to shake the straw several times to get all of the water out, but do not remove the straw from the clay.
  5. You know only these two things about the three solutions on your tray:
    1. The only difference between the three solutions is their color and the amount of salt that they contain.
    2. If you add small amounts of each solution to the straw in the correct order, you will produce three distinct layers that do not mix.

Your goal is to determine which color corresponds to salt water, which is brackish water, and which is fresh water by adding a bit of each solution to the straw as described in step 6. But first write a hypothesis in the data table about the order in which the liquids should be added and explain your hypothesis.

  1. Test your hypothesis by adding a small amount of each solution to the straw in the order you described in your guess. FIGURE 2 illustrates how to do this using a pipette or medicine dropper. Fill about one third of the straw with each solution. Place a white piece of paper behind the straw and observe the colored solutions inside. Record your observations in the data table.
  2. If you do not see three distinct layers, revise your hypothesis.
  3. Empty the contents of the straw and test your revised hypothesis using the same procedure as before. Record your observations.
  4. If you did not see three distinct layers, continue revising your hypothesis and testing it. Record new hypotheses and observations in the data table.


  1. What was the order of adding solutions that resulted in three distinct layers?
    [Bottom to top: blue (ocean or salt water); red (brackish water); green (river or fresh water)]
  2. How can you explain this result?
    (The blue water has the highest density, the green water has the lowest density, and the density of the red water was between the other two. Liquids of different densities will form layers based on their respective densities.)
  3. Suppose you are at the seacoast in an area where a river runs into a somewhat salty bay before it reaches the ocean. Where do you predict that you would find the saltiest water: near the surface of the bay or near its bottom. Explain why.
    (Salt water has a higher density than fresh water and would therefore be found near the bottom of the bay. The fresh water from the river would be found near the surface of the bay. Liquids of different densities will form layers based n their respective densities.)


  • As individuals or in groups, students can describe their hypothesis, methods, and results.
  • Review students data table or hypothesis and observation



Concepts/Objectives | Vocabulary/Background | Activity | Extension