We've all come across those incredible soda commercials on television and in magazines, where the ice cubes are perfectly and completely clear in a pristine glass of ice-cold fizzy pop, but at home, it seems next to impossible to make perfectly clear ice.
Here's chemistry's answer to why your ice usually freezes cloudy, not clear.
What's the Problem with Cloudy Ice?
Cloudy ice cubes simply aren't as aesthetically pleasing, but looks aside, cocktail and beverage aficionados seek out clear ice cubes because they don't want even the smallest impurity to come between them and their drink. Cloudy ice is a sign of impurities: as ice cubes form, impurities and gases are trapped within, leading to cloudy appearance.
How Water Freezes into Ice Cubes
- Crystallization usually starts with an impurity, like a tiny speck of dust, something for crystals to grow from and build upon (also known as nucleation).
- Ice freezes from the outside in because the tray and surface of the water are exposed to colder temperatures.
- Crystallization pushes away impurities (such as dissolved minerals and gases), therefore in the case of ice cubes, these impurities are pushed to the center of the ice cubes.
Myths About Making Clear Ice Cubes at Home
There are a lot of how-tos on making clear ice at home when you scan through search engines, but few actually work.
- Myth #1: Start with boiling water. Hot water is better at dissolving impurities (including gases) than cold water, so while boiling water might lead to a certain level of degassing, overall, you end up with dissolved gas in the end and cloudy ice cubes.
- Myth #2: Use distilled water. Distilled water might have less dissolved minerals and solid impurities, but it still will have dissolved gases (like oxygen, which readily dissolves in water), so freezing distilled water will still lead to cloudy ice cubes at home.
How Nature Does It: Freezing from the Top Down
When lakes freeze, the outermost surface freezes first, exposed to cold air, and the denser cold water sinks below. As the water crystallizes and freezes, dissolved gases and minerals are also pushed out and down into the water. The ice above is therefore clear.
Methods That Can Work (Emphasis on the Can)
Sure, there are ways of achieving clear ice at home, but they aren't foolproof and they require special setups or dedicated fridges/freezers.
- Directional freezing: The idea here is to mimic how lakes freeze. Insulate the bottom of the ice cube tray so it stays warmer than the top of the tray, forcing ice to slowly form from top to bottom. This way, dissolved impurities are pushed downward to the bottom of the tray as the ice cube forms, leaving a cloudy layer at the bottom that can later be melted off. Of course, in practice, it's very hard to achieve a setup in a home freezer where the base of the ice cube tray is insulated enough to not chill, while the top of the tray chills.
- Slow freezing: For this, you'd need a dedicated freezer or fridge that you can cool to a temperature that is just below 0ºC (32ºF), say, to –1ºC (30ºF). This way, water freezes very slowly, promoting more movement within the freezing water, pushing out the impurities and gases into the water layer as the ice crystals form and grow ever-so-slowly.
I'd love to hear the steps you take to combat the looked-down-upon cloudy ice cube in the comments below! Do you have any tricks for making clear ice cubes at home?