Feb. 25, 2011

Water and Ice

by Lynn Brunelle

Video by Lynn Brunelle

Tip of the Iceberg

Icebergs are huge floating islands of ice that float in the ocean. When you see an iceberg you are only seeing about 10% of the whole thing. The rest of the iceberg is floating below the surface of the water. How is that possible?

Water is weird. Unlike most liquids, it expands when it freezes. Because of this expansion, ice floats in water. You can see the expansion with this experiment.

What you need
• A plastic pop bottle
• Water
• A sharpie pen
• A freezer

What you do
1. Fill the pop bottle about ¾ full of tap water and mark the level on the outside of the bottle with the pen.
2. Stick it in the freezer and let it sit for a few hours.
3. Pull out the bottle. Check out where the level of the ice is in comparison to where the water was.

What’s going on?
To do & notice
The ice expanded when it froze. Most liquids shrink when they freeze. As the temperature drops, the molecules slow down and pull tighter together. This makes the substance more dense. Denser objects sink in less dense fluids. Solid lead sinks in liquid lead and solid steel sinks in liquid steel. If this were true of ice and water, ice would be denser than water and would sink rather than float. But it doesn’t.

With water, the solid form, ice, is less dense than the liquid form, water. So it floats. How can this be? As water is cooled, it does become denser and denser, but only until it reaches a temperature just before freezing. At this point, the molecules line up with spaces in between. It expands to form crystals that are in fact less dense than liquid water. So ice floats.

Oil and Ice

Oil and water don’t mix. Oil will float on top of water. It’s less dense. But what about oil and ice?

What Do You Need?
• A 2 liter pop bottle with the top part removed
• Cooking oil
• An ice cube

What Do You Do?
• Fill the bottle about half full with cooking oil.
• Drop the ice cube in, and watch what happens as the cube melts.

What’s going on?
An ice cube looks more solid than oil, but its molecules are a lot less densely packed together. When the ice melts and turns to liquid water, the molecules become denser. You can see that happen when the melted drops fall in perfect little spheres down through the oil, coming to rest at the bottom of the bottle.

Want to Get Technical?
The next time your friends or parents have iced water, you could give a fancy explanation for floating ice. Casually mention that ice floats because it has a density of 0.8 grams per cubic centimeter while water has a greater density of 1 gram per cubic centimeter. They’ll be amazed!

About Lynn Brunelle

Lynn Brunelle is a four-time Emmy Award-winning writer for the television series Bill Nye the Science Guy. An editor, illustrator, and award-winning author, Lynn has created, developed, and written projects for PBS, NPR, and Disney, among others.

The views expressed are those of the author and are not necessarily those of Science Friday.

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