Ever wondered why ice cubes made out of clear water turn out white? While queries like these almost always come when you have too much time on your hands, we do believe that no question is stupid, so we are going to answer this one for you!
The white coloration of the ice cubes depict the minuscule air bubbles trapped inside the water. Almost all natural water has some oxygen molecules, which is the reason fish can breathe in it. In scientific terms, the amount of dissolved oxygen determines the health of the water and the environment, which means your tap water should also be pretty well oxygenated.
As the water freezes it forms regular crystalline structure, meaning impurities like oxygen and other dissolved gasses are pushed inward from the crystallization front, which begins from outside the cube. These impurities were first dissolved and distributed throughout the liquid, but once the temperature drop they segregate from the liquid. Thus the center of the ice cube ends up with most of the impurities and the white color.
This can be further analyzed by looking at the part of the ice cubes at the bottom of the tray, which are milkier colored than the rest, as it was the last part to freeze. The ice cubes in the tray freeze from top to bottom, which pushes the oxygenated air downward until it is trapped at the bottom and freezes into the milky colored ice.
You can get a clear ice cube if you can deoxygenate it first. Oxygen solubility in water decreases with increasing temperature, so if you boil the water first, you are essentially deoxygenating it. And if you create ice cubes from this water, it will result in transparent ice cubes.
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