## Introduction: Oil and Ice Density Experiment

Oil and Water. Plaid and Argyle. Hulk Hogan and a movie acting career. Some things just don't mix. In science, we can use these things to experiment with things such as density, buoyancy, and other physical properties. Oil and Water have been played with for a long time, but amazing things happen when you add ice to the mix!

This experiment is a quickie and easy to make, but a great way to increase the complexity of the simple oil and water experiment with young scientists. Take this simple idea, and I'd love to see the different variations you come up with!

• What: Oil and Ice Density Experiment
• Why: Because if Hulk Hogan can get a movie deal, then we can be scientists
• Cost: ~\$0.25, mostly for oil
• Materials:
• Oil (vegetable works great and is cheap)
• Ice Cube tray + water
• Blue food coloring
• Cup
• Pipette or Dropper (optional)

Let's mix it up!

## Step 1: Make Blue Ice Cubes

If you like clear ice cubes, you're going to love blue ones! These are great to make the melting effects more visible. You'll see soon!

Add a hefty amount of blue food coloring to water in an ice cube tray, and set in the freezer to mix. Let them freeze for a while so they are solid all the way through. You'll find that the color collects toward the bottom of the ice cubes, but that's okay!

Blue ice cubes are useful for lots of experiments, including this other experiment dealing with temperature and density.

## Step 2: Add Oil and Cubes

When you're ready for the experiment, add vegetable oil to a cup, and then gently place an ice cube in the top. Do you think the ice cube will float or sink? The answer is: it depends.

To start the discussion, you can ask: what floats when you put oil and water together? What about when you put ice and water? So can we know what will happen when we put ice and oil together? We can't, so we'll have to experiment!

Some ice cubes have a lot air trapped inside of them. Others don't. The ones with air trapped inside will float because air is less dense with oil and works like a lifejacket for the frozen water in the ice cube.

Find an ice cube that floats for the remainder of the experiment.

## Step 3: Watch Your Experiment in Action!

The amazing part of this experiment happens as you let the ice cube melt. In water you normally can't watch this process happen, but it's beautiful in oil.

Liquid water begins to collect on the bottom of the ice cube, and as enough accumulates, gravity overtakes adhesive forces and a drop forms. It separates from the oil pulling some liquid water, which snaps back like a rubber band when the drop breaks off.

As the drop falls to the bottom of the jar, it forms into a sphere (the ideal shape for minimizing a surface area to volume ratio). Once it hits the bottom, many drops stay separate due to surface tensions on the oil/water boundary, but after a while, they join together.

It's overall an incredible process to witness all of these physical events together, and takes a while to fully digest. A couple of prompts I ask students include:

• Describe what happens to the ice cube over time
• How often does a water drop form on the ice cube?
• What shape are the drops in?
• What happens to water drops on the bottom of the jar?
• Why does the ice float?
• Why does the liquid water sink?

## Step 4: Going Further

You can take this experiment further, and I'm excited to see how you go about it!

A few starting options include:

• Floating Water: Suck up blue water from the bottom of the jar and carefully drip them on the surface. Does the water float or sink? What happens if you poke it? And why?
• Adding Substances: Do the same experiment with some salt added on top of the ice cube? What happens? What about with sand or sugar?
• Temperature: What happens if we warm the oil a bit before the experiment? What happens to the rate of melting? What about if we cool it down in a freezer?
• Other Liquids: What would happen if we tried this experiment in denser solutions? Like honey?

Have fun, and as always, keep exploring!

Let me know your thoughts, comments, or what you come up with below!