Joanne Beals
Saint Anselm School
Philadelphia, PA

Albert Rieck
Rohm and Haas Company
Spring House, PA

Grade Level:

5 through 8

Disciplines:

Earth Science, Engineering, Mathematics

Goals:

Upon completion of this lesson, the students will

1. understand that the sun in the driving force for the water cycle.
2. understand evaporation, condensation, and precipitation.
3. identify, test, and measure the solids in fresh and salt water.
4. learn how to desalinate water using a simple still.
5. investigate the different densities of salt and fresh water.
6. recognize the importance of El NiŅo and its affects on ocean currents.

Summary:

This unit on salt water is divided into three investigations related to the water cycle.

1. Evaporation - Water is constantly evaporating from the oceans. The salt, however, remains behind during this process. The students will perform a simple windowsill evaporation test on distilled, tap and salt water. They will examine the solids and determine what percentage of each water sample was solid.
2. Condensation - Using a still, students will simulate the evaporation of ocean water, have it condense on the sides of a container and allow it to precipitate back into the container as fresh water, demonstrating how precipitation is fresh water from the ocean.
3. Precipitation - El Nino and other weather patterns induced by ocean currents will be simulated in an investigation in the differing densities of fresh and salt water.

Investigation 1: How much salt is in distilled, fresh, and ocean water?

Objectives:

Upon completion of this lesson, the students will

1. measure and weigh water samples and aluminum weigh pans.
2. use a scale correctly.
3. determine percentage of salt in the samples.

Materials:

Salty water - 35 grams of salt per liter of water
Tap water
Distilled water
Pipettes
Balance - capable of measuring to 0.01 gram
Aluminum weigh pans
Oven for faster evaporation - 300⁰F for 20 minutes (optional)

Vocabulary:

Distilled water

Procedure:

1. Weigh six numbered aluminum weigh pans. Record the weight of each pan.
2. Fill two of the pans with a pipette of tap water, two with distilled water, and two with simulated ocean water. Weigh and record the weight of each pan. This is the wet weight.
3. Place the pans on the windowsill and let the water evaporate. In a few days, depending on the temperature, humidity, and time of year, the pans will be dry. Weigh the pans again. This is the dry weight.
4. Calculate the percentage of solid in each sample.
1. wet weight - pan weight = solution weight
2. dry weight - pan weight = solids weight
3. solid weight divided by solution weight times 100% = % of solids in solution

Sample Data Chart:

Critical Thinking Questions:

1. Why does seawater have the largest amount of solids?
2. Why does the distilled water have the least amount of solids?
3. Why do we test two samples of each type of solution?

Investigation 2: Why is rainwater fresh if it evaporates from a salty ocean?

Objectives:

Upon completion of this lesson, the students will:

1. build a still from the materials provided.
2. simulate ocean water by mixing salt and tap water.
3. test condensed water for saltiness.
4. understand that only fresh water evaporates from the ocean.

Materials:

2 liter beaker tape
400 ml beaker plastic wrap
100 ml beaker straws
salty water - 18 grams of salt per 500 ml

Vocabulary:

Desalination, distillation, purification

Procedure:

1. Make seawater by mixing 18 grams of salt in 500 ml of water.
2. Measure out 50 ml of salt water into a 100-ml beaker.
3. Place the 400-ml beaker upside down inside the 2-liter beaker.
4. Place the 100-ml beaker of salty water on top of the 400-ml beaker.
5. Build a teepee of straws and tape them to the inside rim of the 2-liter beaker.
6. Place plastic wrap around the straws and the outside edge of the 2-liter beaker. Make sure that it is airtight.
7. Place the container in bright sunlight and check for condensation.
8. After considerable condensation has occurred, unwrap the plastic and taste the water that has condensed on the sides of the container.

Diagram:

Straws - Plastic wrap

2 L beaker

100 mL beaker

400 ml beaker

Discussion Questions:

1. Why does the water condense on the plastic wrap?
2. Why does the container need to be placed in the sun?
3. What does the plastic wrap do?
4. What part of this experiment corresponds to precipitation?
5. Where are the salt particles if the condensed water is fresh water?

Investigation 3: What type of water is denser - Fresh or Sea Water?

Objectives:

Upon completion of this lesson, the students will:

1. simulate ocean water.
2. demonstrate that fresh water floats on top of salt water because it is less dense.
3. recognize that wind helps create surface currents.
4. understand that deep cool ocean currents regulate surface temperature.
5. see that cold deep water currents can rise above warmer currents because of wind action.
6. learn that El Nino is characterized by lack of winds on the ocean's surface.
7. see that weather patterns are the result of ocean temperatures.

Materials:

Clear Pyrex baking dish
ice
Red and blue food dye
beaker
Salty water - 145 grams of salt in 1 liter of water

Vocabulary:

density

Procedure:

1. Fill a glass baking dish half full with warm tap water (20-25⁰C).
2. Add a few drops of red food dye and mix.
3. In a separate beaker mix 145 grams of salt with 1 liter of tap water and dye the water blue.
4. Add ice to the salt water until it is very cold (5-10⁰C).
5. Slowly pour the icy salty water into the red warm water.
6. Observe the results.
7. Blow gently on the surface of the water and observe the results.

Diagram:

Notes:

The icy blue water will flow along the bottom of the pan. It does not mix with the red surface water. The students will expect to see purple. When they blow on the surface they can see the icy water begin swirling to the surface. When there is an El Nino the ocean winds do not blow, so the surface water continues to heat up making it more difficult for the cool deep water to surface because it is denser that the warm surface water. During El Nino, there is widespread ocean warming which has the effect of changing weather patterns, such as those experienced in 1998.

Discussion Questions:

1. Why isn't the water purple from the side view?
2. Why are there two distinct layers?
3. Why did the blue water sink and flow along the bottom?
4. Why does the wind cause the cold water to rise to the surface?

Data Chart: