Layered Liquids and Density

Purpose:
You will learn about density and layers of liquids with different densities.
On their trip to the North Magnetic Pole, explorers will encounter salt and fresh water, and ice formed from these two liquids. 
 

Definition of terms:
Density ù the weight of something in relationship to its volume.
Diffusion ù to spread out

What you will need:
2 large clear glasses
2 cups of cold water
common table salt
red food coloring
blue food coloring

Before you begin:
Ask an adult to help you with this activity. 

What to do:

1. Put one cup of cold water in a glass. 
2. Add common table salt until the water is saturated with salt.  This is done by adding small amounts of salt and swirling the solution in the glass.  When the salt you add no longer dissolves, then the water is ôsaturated with salt.ö
3. Add three drops of red food coloring to the salt solution. Swirl the solution to dissolve the food coloring. 
4. Put one cup of cold water in the other glass.
5. Add three drops of blue food coloring to the water.  Swirl the solution to dissolve the food coloring. 
6. Tilt the glass containing the red salt solution a little to the side.
7. Gently pour the blue water down the inside of the glass and into the red salt solution.
8. Note that the two solutions donÆt mix.  Gently tilt the glass with the solution back and forth. You will notice that there are two layers.

What is going on here?
Why are the liquids layered?  Why isnÆt the blue liquid mixing with the red liquid? Will the layers eventually mix?  If so, why?

The liquids are layered because of the density difference of the two liquids.  The salt dissolved in the water and made the red solution denser because the small salt particles occupy spaces between the water molecules that were previously empty.  Eventually the two layers will mix as the salt particles, or ions, diffuse upward.

Other examples:
Think of an empty box.  Now think of the same box with a ball inside.  The space the box occupies is the same, but the box is more dense with the ball inside, meaning it weighs more for the same space.  The ball occupies the space that was empty and thus increases the density of the box. 

Another example is a milk jug.  The jug is much heavier when it is filled with milk than when it is "empty" (filled with air).

Field Guide to the Universe