I am sure there are those times in your lives when you are just so bored that anything and everything interests you. A question to occupy you the next time you face such a period is, 'What makes the center of ice cubes white?'
Run and take an ice cube out of your freezer and have a good look. It might look something like this.
Now I am sure you are wondering why is it that clear water when it freezes forms a cloudy ice.
Worry no more. You will find the answer to this mystery today.
The white stuff in your ice cubes is actually very tiny air bubbles. Virtually all natural water you deal with is oxygenated to some extent. It's why fish can breathe in it. Scientists measure dissolved oxygen in streams to determine how healthy the environment is. And when the water flows from your tap, it tends to be pretty well oxygenated also.
As the water freezes, it wants to form a regular crystalline structure (ice). That means impurities like oxygen and other dissolved gasses are pushed away from the crystallization front into the remaining liquid. This means that the last parts of the ice cube to crystallize end up with most of the impurities.
If you'll take a look at your photo of the ice cubes, you'll note that the parts of the ice cube that were at the bottom of the tray are the milky colored bits. That's because those were the last parts to freeze. The ice cubes in your tray froze from the top downward, which pushed the contaminant air downward until it was trapped at the bottom and had nowhere else to go.
So, the milky part of the ice cube is nothing but last minute air bubbles that club together to give a milky white appearance. You can, however, get bubble-free ice by first deoxygenating it. Oxygen solubility in water decreases with increasing temperature. If you boil the water first, you remove the oxygen from it. Next, cool it and then pour it into your ice cube trays to freeze. It should be clearer.
Although microscopic bubbles clearly contribute to cloudy ice, merely using boiled water does not result in clear ice cubes. Try it yourself. Similarly, as boiling water causes dissolved minerals to precipitate out of the water, dissolved minerals clearly are not the sole cause of cloudy ice.
Camper English has performed numerous experiments, and his conclusion is that the center of ice cubes are white because the ice in a freezer freezes at the same rate from all directions. You get clear, perfectly structured ice on the outside but the irregularities are trapped in the center. By using insulation to direct the freezing process, he has a repeatable process for producing clear ice that's used commercially. But the composition of the irregularities in the ice structure are still up for debate.
To understand this concept better let's take the example of snow and icicles. Snow appears white but icicles are clear. What is the reason behind this?
This has to do with the "texture" of the object in question, or really how smooth and consistent the material is.
With a smooth surface, you get specular reflection, where all the light comes in and bounces off at the same angle. With a bumpy surface, you get a diffused reflection, with all the light bouncing off at random angles. The specular reflection looks like a mirror; while the diffuse reflection looks more like light bouncing off of paper or a white wall.
The same thing applies to the internal makeup of an object. If the material is transparent, and consistent in density and composition throughout, then light rays can pass straight through the material, and you will be able to see all the way through it. However, if it isn't consistent the light rays will scatter randomly, and you won't be able to see through it, although you will be able to tell, for instance, that a light is turned on the other side.
With the white areas in ice cubes, you're seeing a mix of air bubbles and possibly other contaminants that are scattering light rays as they travel through the ice. In the clear areas of the ice cube, the water is pure and consistent so light rays travel in straight lines and don't get dispersed.
Finally, there's a really easy test for this theory: take some smooth, clear plastic, like on a soda bottle. Scuff it up with sandpaper, and you'll see that it is no longer clear in those areas.
You've diffused the light rays traveling through and bouncing off the plastic, so it now looks white.
So, this solves the mystery of ice cubes being white in the center. It depends on the impurities in the water and the texture of the ice.