Why do ice-cubes tend to have a globular white center?

Question

If it is air, why does the air collect in the center of the cube?

Answer 1

When Ice freezes it makes crystals only with water. Anything that is not water will be poshed out of the way. That will usually include dissolved gasses, but might contain salt, alcohol, or anything else that was dissolved in the water.

A fun experiment is to fill the ice trays with deaerated distilled water. Without gas or other things in the water the ice will be nice and clear

An Island off the coast of Nova Scotia, used this trick to make fresh water for the island. They would freeze huge mounds of salt water but let the salty part drain away. Come spring they would have large ponds of fresh water.

Answer 2

The other guys are correct; it is air and it collects in the center because, in a home freezer the water isn't moving; the cubes typically freeze from the exterior sides first, trapping the air in the core, and gradually solidifying towards the center. You've probably seen if you pull an ice tray out too soon the centers of the cubes are still liquid.

More icecubeology:
If you would like ice without cloudy centers try boing the water first to drive out the air, fill it in the trays as soon as its cool enough that it won't melt the plastic and freeze.

A far as why restaurant ice is usually clear, where I work we used to have a restaurant ice machine, I was the one who kept it running, so I know how it worked. It froze ice really fast, it would dump a slab of cubes every few minutes. The reason the ice came out clear was because of the manner in which it was frozen. The operating cycle went like this: (I'm trying to keep it simple) water was pumped over a cold (below 0 F) heat exchanger plate. It was shaped like an icecube tray, but sideways, and made out of chrome-plated copper. The water would be continuously fed to the top of this unit and run in a even stream down the face of the cold metal "ice cube tray" the water would gradually build up as ice on the bottom and sides of each cavity; this way any air could escape, once the cavities were full of ice; the machine would go into the "harvest cycle" where the metal heat exchanger was warmed just enough to let the ice fall out into the collection bin. Then it would start all over.

Answer 3

Yes it is trapped air. This occurs because most household freezers will cause the water to freeze too quickly, where the exterior of the cube freezes before the interior and trapping the gases. The ice you find in restaurants is usually clear because the machines freeze the ice slowly.

Answer 4

Yes it is air, it forms that way because ice freezes from the outside in.
The air that is in the still liquid water is trapped as the outside edges freeze.