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Clear Ice

This experiment is the result of my forgetfulness. I took a tray of ice cubes out of the freezer, used half the cubes and then forgot to put the tray back in the freezer. When I remembered, the cubes were half melted. Only the centers of the cubes were left, and that ice was very cloudy.

To find out what happened, you will need:

  • ice cube trays
  • a freezer
  • a water filter or distilled water
  • a pot for boiling the water
  • water

If you already have a tray of ice cubes in the freezer, get one of the ice cubes and look at it carefully. We want to see the inside of the cube, so it may help if you get the cube wet first. What do you notice? The outside of the cube is made of clear ice, but the center of the cube is white. That white center is what I had left when I left the ice out. Why is it white?

To find out, fill one ice cube tray with distilled water or water that has been through a good water filter. Fill another tray with distilled water that has been heated to a full boil. Be very careful not to burn yourself with the boiling water. Mark both trays so you can tell which is which and put them in the freezer. If your freezer is like ours, then you may have to finish off a container of ice cream to make room for the extra ice trays.

One thing that can make the center of the ice cube cloudy is mineral content. Most water contains some dissolved minerals. As the water freezes, the water molecules fit together neatly into a pattern that does not leave room for the minerals. Since the ice begins freezing at the top, this ice will be clear, with the minerals forced out of it. As more and more of the water freezes, the minerals are concentrated in the remaining liquid at the center of the cube. Finally, the minerals are left as tiny pockets in the ice. These pockets of minerals give the ice a cloudy appearance. The ice cubes that were made with distilled water should have a thicker clear layer around the outside and a smaller, cloudy center.

Another thing which causes the cloudy appearance in the center of the cube is air. Most water contains dissolved air. As the water freezes, the same thing happens with the dissolved air that happened with the dissolved minerals. You wind up with many small bubbles and often one large bubble trapped in the center of the ice.

As we have seen in the past, boiling water will remove most of the gas dissolved in the water. The ice cubes made with boiled, distilled water should be even more clear, with an even smaller cloudy center.

The next time you are in a restaurant, look at the ice cubes. You will probably find that they do not have cloudy centers. How do they do that? They use special ice machines which freeze the ice in layers, starting from the inside out. This is very similar to the way that icicles form. An icicle start with a hanging drop which freezes. Water flowing over the outside of the icicle freezes layer on top of layer. Any dissolved minerals are carried away by the water that drips from the icicle. Dissolved gases are free to go directly into the air, so the icicle is nice and clear. The fancy ice machines in restaurants work the same way, giving them crystal clear ice for your iced tea.