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McREL Standards Activity

Layering Water

Purpose:As a result of this activity, students will understand how ocean water differs from fresh water in regards to density.
Related Standard & Benchmarks:
 Standard 1.Understands atmospheric processes and the water cycle
   Level III [Grade 6-8]
   Benchmark 7. Knows that most of Earth’s surface is covered by water, that most of that water is salt water in oceans, and that fresh water is found in rivers, lakes, underground sources, and glaciers
Student Product:record of observations
Material & Resources:3 medium-sized jars with lids (spaghetti sauce jars work well); 1 smaller jar or short drinking glass; 1 empty milk jug; tap water; 1 container of salt; food coloring; a pipette, long eyedropper, or even a turkey injector.
Teacher's Note:This can be done as a demonstration or in small groups. If done in groups, then more small jars and pipettes will be needed; the prepared salt solutions can be shared amongst the groups.
This activity can be introduced by asking if anyone in the class has ever been to the ocean. What was it like? How was it different than water you get out of the faucet? What does ocean water taste like? No doubt, the students will reply that the ocean tastes salty. But what else do they know about it? Discuss density with the students. What does "density" mean? What does it mean that salt water is "denser" than fresh water? Explain to the students that they are going to "layer" water. Is this possible? How does density come into play? The salt solutions can be prepared the day before (if not made ahead of time, make sure all of the water samples use the same temperature water): 1) Label jars #1-3. 2) Make a supersaturated salt solution in jar #1; shake well. 3) Pour about half of that solution into jar #2 and fill the rest with water. 4) Add food coloring to jars #1 and #2 (any two colors that are easy to see, such as red and blue). 5) Fill jar #3 with tap water and add a third color. 6) Fill the milk jug with tap water. The experiment: 1) Fill the small jars or drinking glasses with fresh water from the milk jug, but not all the way (make sure the jars are short enough so that it is not awkward to reach the pipette or eyedropper all the way to the bottom--the water must not have any turbulence or the layering will not work). 2) Using the pipette, add the colored water from jar #3 to the bottom of the fresh water. Beforehand, allow the students to predict what they think will happen (they could draw a diagram of their predictions). At this point, the students should understand that neither water sample has any salt in it, only that the colors are different. They may not yet understand that color does not affect density. 3) Because the density of the two different water samples are the same, there should be no layering affect. Have students discuss and record their observations. What has happened here? Did they expect the water to layer? Why did it not layer? 4) Dump out the water and refill with fresh water. 5) This time, carefully pipette the salt water from jar #1 to the bottom of the fresh water. Again, have the students first predict what will happen. They should understand that one sample is fresh water and the other is salt water. 6) The colored water will create a layer at the bottom of the fresh water. Have students discuss and record their observations. What has happened here? Why did layers form this time? 7) Ask the students what might happen if a second solution of salt water, one with less salt, is added. 8) Very carefully, pipette the salt water from jar #2 on top of the bottom layer (this must be done carefully and slowly for the full effect!). If done correctly, the second salt solution will layer on top of the first salt solution. Discuss and record observations. Extension: Have the students ever visited or do they know about the Great Salt Lake in Utah? There are many resources on the web about this lake that is several times saltier than the ocean. Discuss why it is so much easier to float on the Great Salt Lake than in a fresh water lake.