This Week's Experiment - #318 Turning Ice

ice cubes
a bowl
warm water
food coloring

Before we get into the things that sidetracked me, lets do the experiment that I started to do. Fill the bowl with warm water. Place an ice cube into the water. Now watch it as it melts. Don't get bored and give up yet. Watch for a minute and something should happen. The ice cube will tip over on its side. A short while later, it will turn again, and then again. It will keep turning over every minute or so until there is no ice left.

Why did it turn over? The warm water was melting the ice that was under the surface, but the top of the ice cube melted more slowly. This caused the ice cube to get top-heavy. It became unstable and tipped over. Then the same thing happened again and again.

One problem that I ran into was being able to tell whether the ice cube was turning over or just rocking a bit. To make it easier, I made some special ice cubes. Of course that meant clearing some space in the freezer, and the ice cream was taking up a lot of space. After my snack, I put a few drops of red food coloring into some water and then filled the ice cube tray half-full with the red water. I put that in the freezer and let it freeze solid. Then I added some very cold, plain water. The plain water must be very cold to start with (I stirred it with some ice) to keep it from melting the red ice. Quickly putting this back into the freezer, I wound up with ice cubes that were half-red and half clear.

Make some red and clear ice cubes and try the experiment again. It is much easier to see when the ice cube tips, but you will also see other interesting things. The red ice melts, and you get streamers of red water flowing downwards from the ice cube. Though the ice is less dense than the water (which causes it to float), the very cold water of the newly melted ice is more dense than the surrounding water, so it sinks. It is not the coloring that makes it denser. You can see this even without the food coloring, but the color makes it much easier to see. Very cold water sinks, but when it gets even colder, it suddenly expands, rises and freezes into ice which floats. This is just one of the many things that make water one of the strangest chemicals we know of.

Have a good week.

From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com

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