Unless you happen to have a specialized ice machine in your home, it is practically guaranteed that the ice your freezer makes is of the cloudy variety. A popular and well known trick to making clear(er) ice is to either boil the water first or use distilled water. However, even doing this won’t guarantee the kind of perfectly clear ice you’d find in a high-end bar or restaurant. This is because how clear ice is, is only partially dependent on the purity of the water you use, meaning even if you managed to procure the tears of a Saint and pass them through the world’s best water filter, you’d still likely end up with ice that was a little cloudy.
So what’s going on here?
For those of you have the time, go to any nearby sink and pour yourself a glass of water. Unless you live in an area with bad plumbing or poured the water into a dirty glass or something, we’re going to guess that the water is perfectly clear. Sure, you may see a few stray nondescript particles floating in the glass, but not enough to explain why it looks like someone crammed half a cloud into your freezer when you try to turn that same water into ice.
The answer to this mystery lies in the temperature of the water. You see, at room temperature there are a lot of impurities that are dissolved in regular old tap water. And as you may recall from high-school chemistry, the warmer water is, the more of a given substance it is possible to dissolve in it. For example, sugar has very weak molecular bonds that require only a small amount of energy to break. Thus, as you supply water with more energy by heating it, the amount of sugar you can dissolve within it increases and vice versa. You perhaps have noticed this phenomenon when sweetening hot tea vs. cold, or after letting a sugared cup of coffee get cold, with the sugar dissolving fine when it’s hot, but showing up at the bottom of your cup when the coffee gets cold.
This is essentially what happens with ice. As you cool the water, all of the impurities that were happily dissolved in it at room temperature separate themselves from the liquid and become visible.
You may now be wondering why these impurities tend to congregate towards the center of the ice cube, rather than being evenly distributed. This is because as water freezes, it crystallizes. This crystallization process generally rejects, or forms more efficiently, without most of the types of impurities found in tap water. This results in the impurities getting pushed into the unfrozen water towards the center as the ice freezes from the outside in. Eventually, of course, the final remaining bit of water in the center will freeze with the impurities having nowhere to go. (This fact can actually be used as something of a way to purify water, freezing water, then melting it and keeping only the outer bits.)
This is also why partially formed ice cubes, where most of the center is still liquid, are usually very clear- the water containing these still dissolved sediments and impurities hasn’t cooled enough yet for them to show up.
So what exactly are these impurities and where do they come from? Well the most common culprits are lime (colloquially known as limescale) followed by fluoride, calcium and a whole host of other organic materials that are practically unavoidable in water (with many being good for you like calcium and magnesium). Heating, “softening,” or filtering the water is one way of removing some, if not most of these impurities and is partially the reason that boiled and filtered water often yields somewhat clearer ice cubes.
However, as mentioned, you’re likely still not going to get a completely clear cube even if you filter off the impurities. So what else is going on here? As the water crystallizes, tiny bubbles of air form. These bubbles get trapped within the ice like any other impurity. Specialized ice makers avoid this by freezing water in layers to stop most air bubbles from forming in the first place. They also tend to freeze water extremely slowly (setting the temperature much higher than common household freezers) in order to form larger crystal structures and to allow time for any air bubbles that do form to escape.
