Pour a one-inch layer of cooking oil into a tall, clear glass.
Fill the glass to near the rim with water. Watch as the oil and water separate into layers.
Then add a few drops of food coloring to the top of the oil. Watch what happens over the next few minutes.
Why do the oil and water separate? And why don’t the droplets of food coloring spread and color the oil as they do in the water? Think of your own best answer. Then read our explanation below.
How It Works
Water is denser, or heavier, than oil, so water sinks to the bottom of the glass. Water molecules are “polar” (more positively charged at one end and more negatively charged at the other), but oil molecules are not. Polar molecules are attracted to other polar molecules, but they don’t mix well with non-polar molecules. So the water and oil separate.
Like water, food coloring is polar. (In fact, it is made of food coloring molecules dissolved in water.) So the dense, watery food coloring drops do not mix with the oil. Instead, they fall to the bottom of the oil layer. When they push through the oil-water boundary, they mix with the water below.
