neutralbouyancy

Neutral Buoyancy

Introduction

Float a ball of oil in a mixture of alcohol and water.

Material

A clear container, a 200 mL beaker or a clear plastic water cup
water
isopropyl alcohol (Rubbing alcohol 70% or 90%)
cooking oil
disposable pipette, between 2 and 10 mL

To Do and Notice

Fill a clear container 1/2 full of water

Carefully add a layer of isopropyl alcohol to bring the container to 3/4 full

Use a disposable pipette to add oil to the center of the top of the container.

Notice that the oil sinks through the alcohol and floats on the water.

Add more oil.

Notice that a ball of oil forms in the solution.

What's Going On?

The oil is less dense than the water and so floats on the water, the oil is less dense than the isopropanol and so sinks in the isopropanol.

With time the isopropanol diffuses into the water making a mixture of alcohol and water so that there is a density gradient changing from pure alcohol on the top to pure water at the bottom.