This week's LabZone activity
April 2, 2008
Can Water Float on Water?
The goal of this project is to investigate what happens to layers of water with different densities. You will investigate density differences caused by both temperature and salinity.
Water covers 70 percent of Earth's surface. Seen from space, the blue of the oceans and the white of clouds are the dominant visual features. The water of the oceans is not uniform. Climatic processes create large-scale differences in ocean water temperature and salinity, illustrated in the first two maps, below. As you might expect, ocean waters near the equator tend to be warmer than those at higher latitudes. The first map shows sea surface temperature, coded in color (see legend).
Color-coded map of sea surface temperature.
The second map shows global differences in ocean surface salinity (dissolved salt concentration). At the surface, in general salinity is higher in equatorial regions and lower at the high latitudes.
Color-coded map of sea surface salinity in parts per thousand (ppt or 0/00).
What goes on at the ocean surface does not tell the whole story. The ocean has depth, too. In the deep ocean, huge masses of water circulate around the globe, driven by differences in temperature and salinity. This is called the thermohaline circulation, sometimes also known as the global conveyor belt. Differences in temperature and salinity cause differences in ocean water density. As water warms, it expands, decreasing density. As salt concentration rises, density increases, because the salt molecules can occupy spaces between the water molecules. Denser water sinks beneath water that is less dense. As denser water sinks, water must rise somewhere to replace it. As you are doing your background research for this project, you should read up on how the thermohaline circulation works.
In this project, you will do experiments to see what happens when layers of water at different densities are brought together. You'll create your two "layers" in plastic or glass bottles, coloring them with different food colors to tell them apart. Then, you'll bring the two layers together by flipping one bottle over on top of the other (we'll tell you how to do it without spilling half the bottle!). You can see for yourself if water can float on water.
Experimental Procedure and Additional Information
Used with permission. Copyright © 2002-2008 Kenneth Lafferty Hess Family Charitable Foundation. All rights reserved.
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