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Density: Ice, Oil, and Water

This time we are going to explore the world of density. This is a very useful science, important for many things from boats to hot air balloons.

To try this yourself, you will need:

  • a tall, clear drinking glass
  • cooking oil
  • ice

Fill the glass almost full of cooking oil. I found that cheap, vegetable oil worked very well. Canola oil did not work at all, as it was not dense enough. Place the glass of oil on a flat surface and then gently add an ice cube. The ice should float. If it does not, try using a different kind of oil.

Now comes the interesting part. Oil floats on top of water and ice floats on top of oil. What will happen when the ice begins to melt? Watch a minute or two and you will see.

What happens? As the ice begins to melt, you will see a drop of water hanging from the bottom of the ice cube. As the drop grows, the ice cube will float lower, as it is being weighted down by the denser water. Finally, the drop gets large enough to pull free of the ice and it slowly sinks to the bottom of the glass.

Water is a strange chemical. Most liquids get smaller when they freeze, which means the solid form is denser. When water freezes, it gets larger. It still weighs the same, but it takes up more space, which means it is less dense. That is why ice floats in water, and in the oil. If something is denser than water (or in this case, oil), it sinks. If it is less dense than the liquid, it floats. As the ice melts, the water takes up less space, becoming denser, and the denser drops of water sink to the bottom of the glass.

After the ice had all melted, I tried reversing the process by putting the glass of oil and water in the freezer. I was hoping that I would find a lump of ice floating on top of the oil. Instead, the lump of ice was at the bottom. As the water began to freeze, the surface tension of the water was strong enough to keep the ice from rising up through the oil. Soon, it froze to the side of the glass and then was firmly trapped at the bottom. I did manage to gently warm the glass and get the ice to come free and float to the top. If you try that, be sure to warm the glass slowly, so it does not crack. Why would it crack? For that, you will either have to do some research or wait until next week's experiment.