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Klutz-Proof Density Column

Gravity doesn’t care if you trip.

Density columns have many layers of liquids that sit on top of each other, but if you accidentally mix them up, most will settle into just two layers. You can shake this one all day long—it will always return to its original three layers.

Grade Bands:

Subject:

Keywords:

density column

salt

solubility

NGSS and EP&Cs:

Structure and Function

Stability and Change

Tools and Materials

Water
Vegetable oil
Isopropyl alcohol (70% or 91%)
Non-iodized or kosher salt
Empty, clear bottle with cap or lid—smooth-walled bottles work best
Food coloring

Assembly

Pour equal parts of water, vegetable oil, and alcohol into the bottle. If you’re using 70% isopropyl alcohol, pour in a little more alcohol and a little less water.

To Do and Notice

Once you’ve poured in all your liquids, put the cap on the bottle and let the mixture sit. How many layers do you see once the mixture settles?

Add salt and mix until it dissolves. Keep adding and mixing until there is some salt that does not dissolve into the mixture. Replace the cap and let everything settle. You may have to wait a few minutes. How many layers are there now?

Add a drop or two of food coloring to make it easier to differentiate the layers. Replace the cap. Shake the bottle so that everything mixes together. Where does the color go?

Optional: To get rid of any excess salt on the bottom of the bottle, you can decant the liquid into another container, wash out the salt from the bottle, and then pour the mixture back into the bottle.

What’s Going On?

Density columns are made by layering liquids of different densities. If neighboring layers are insoluble with each other, the column will stay in separate layers unless disturbed.

The problem arises when the layers are mixed and exposed to other layers with which they are soluble. If this happens, the soluble layers will combine and usually end up settling into two layers: a hydrophilic (polar) watery layer and a hydrophobic (nonpolar) oily layer.

The Klutz-Proof Density Column uses a phenomenon called “salting out” to preserve its layers. When salt is added to the mixture, the ions bond with the water molecules and exclude the slightly less polar alcohol molecules from forming hydrogen bonds with water, making it no longer soluble with water. The alcohol stays separate and ends up as its own layer on top of the column because it has a lower density than oil.

Since the salt causes all three liquids to be insoluble with each other, you can mix the column as much as you like—it will always settle into three layers. This makes it the perfect density column for klutzes.