That's Heavy! main content.

That's Heavy!

Part of Hall of Ocean Life.

Milstein Family Hall of Ocean Life Educator's Guide: Activity

This "heavy" experiment allows students to discover that salt water sinks in fresh water.

What you need:

  • measuring spoon
  • 1 tablespoon of salt
  • glass of cold water
  • glass baking dish
  • food coloring

What you do:

  1. Add the salt to the glass of water.
  2. Stir in the food coloring.
  3. Pour fresh water into the baking dish, but not to the top.
  4. Slowly add the salty water from the glass to the fresh water in the baking dish.
  5. Watch what happens!

What's the meaning of this? 
Salty water sinks to the bottom of the glass baking dish because salt water is denser than fresh water. In salt water, salt ions attach to water molecules, which means the molecules are closer together and more dense.

Why is this descending water important to us? One reason is that, the motion of cold sinking saltwater creates thermohaline convection currents in the ocean.

It is also a feature important to estuaries. An estuary is a dynamic body of water where freshwater and saltwater meet. Estuarine areas are found around the world along the ocean's edge. Along with tides, winds, and waves, the sinking of saltwater is one of the factors in the mixing of salt and fresh water in an estuary.

Because of its higher density, saltwater enters the estuary and flows landward at the bottom of a river, and fresh river water flows seaward at or near the surface of the ocean. Some mixing can occur in this process.

Adapted from: Nye, Bill. Bill Nye the Science Guy's Big Blue Ocean. New York: Hyperion Books for Children: 1999.

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