Tap water typically contains benign impurities such as lime, calcium, fluoride, nitrates, magnesium, and other organic elements. When water freezes, it hardens from the outside in, and those impurities are pushed into the center of the cube, causing the ice to have a cloudy appearance. However, boiling the water first will remove most of the impurities and provide clearer ice.
Ice, ice, baby:
- Using bottled water that has been purified using a reverse osmosis process, or another type of distillation, will also improve ice cube clarity.
- Cloudy ice is also related to how quickly the water is chilled. Most home freezers cool very rapidly, and tiny air bubbles (actually, dissolved air) get trapped before they can dissipate.
- High-end restaurants use systems that freeze pure water slowly, in layers, so that air bubbles have plenty of time to escape.