Liquid Layers

An Experiment With Density, Bouyancy and Polar Solvents

What you need: 1) A Small Jar

2) Water

3) A Small Spoon

4) Cooking Oil

5) Food Coloring

What to do:

1) Fill the jar 1/3 with water

2) Stir in a few drops of food coloring to color the water

3) Slowly, fill another third with oil

4) Tightly close the jar and shake

5) Wait and watch for about three minutes

What Happens

When you pour the oil slowly onto the water, it forms a layer on top of the water, because it is less dense than water – that is, the mass of one liter of oil is less than the mass of one liter of water. Thus, the buoyant force on the oil due to the water is greater than its weight and, by Archimedes’s principle, the oil floats – just like a solid object would.

When you shake together the oil and water, they separate quickly, because they are “immiscible” (literally non-mixable). This is because of the differences between the water and oil molecules: Water is a “polar” molecule – meaning its positive and negative charged parts are separated onto opposite ends of the molecule and oil is non-polar. This means that the oil molecules don’t interact with the water molecules and there is no attraction to drive the two fluids to mix.

Generally, non-polar fluids do not mix well with other fluids (even other non-polars), because without the attraction there is no reason for the fluids to mix (or stay mixed)