The Chemistry of Clear Ice and How to Make It
Have you ever noticed that ice served in nice drinks is often crystal clear, but the cubes from your freezer are cloudy and white? Have you ever wondered why?
As it turns out, the perks of clear ice are not just aesthetic. Cloudy ice is not only weaker than clear ice, it melts more rapidly than its “pure” counterpart. The reasons? Ice becomes opaque when 1) dissolved gases get trapped and forced into micro-bubbles, or when 2) the ice freezes in such a way that large crystal lattice structures are unable to form properly.
Why Does Ice Freeze Cloudy?
Cloudy ice is most often caused by dissolved gases–mainly nitrogen and oxygen– that come out of water during the freezing process. Tap water also contains dissolved minerals–generally calcium and magnesium–which can be present in the form of bicarbonates and/or as calcium and magnesium sulphate. We’ll make sure to cover water hardness soon – but the short of it is that the sum of magnesium and calcium concentrations in your water is a measure of how “hard” your water is. Hard water can lead to cloudy ice cubes.
Cloudy ice can also form due to hindered structure formation. As water freezes, a thin layer of ice begins to form on its outermost edges and grows inward–forming a crystal lattice structure.
However, other molecules in tap water (such as the impurities and gases we just mentioned) do not fit in the lattice structure. So, if the ice forms faster than the gas molecules can diffuse away, the gases get trapped. Micro-bubbles form as the concentration of gases and impurities in the freezing water becomes supersaturated. These small bubbles get caught in the ice, and voila–you’ve got cloudy ice.
How to Make Clear Ice: Boiling Water + Directional Freezing
There are two quick and easy ways to make crystal clear ice yourself: boiling water and directional freezing.
The solubility of gases in water depends on temperature and is directly proportional to the partial pressure of the gas (for those of you who like this kind of stuff, we’re talking about the Ideal Gas Law). Boiling water helps remove dissolved gases and minerals. If boiled for a sufficient amount time, the water vapor will remove most of the gas and should result in clear ice if other impurities are also removed. Dissolved magnesium and calcium will turn into limescale (those white spots you see on pots and pans) as the water boils. Pour your finished boiled water into another bowl before transferring to the freezer so the minerals that precipitated out during the boil don’t follow your water into the ice tray.
Directional freezing is another way to create crystal clear ice, as the process forces the ice to freeze from the top down–rather from all sides of the ice cube inwards. By controlling the direction that water freezes (i.e. from top to bottom), you can control the movement of impurities and gases within the water and subsequently, where cloudiness occurs.
The easiest way to control the direction of freezing is to fill an insulated mini-picnic cooler with water and allow it freeze with the top off inside your freezer. The top ~75% of the ice cube frozen using this method will be crystal clear. The bottom ~25% will be very cloudy. This is because all of the impurities are pushed to the bottom of the cooler, because ice prefers to freeze in the crystal lattice structure mentioned above. When the water is about three-quarters frozen, the trapped impurities have nowhere to go, and the resulting portion of the ice at the bottom of the cooler is opaque and cracked. You can either cut off the cloudy ice from the rest of the cube or you can simply remove the cooler from the freezer once ~75% of the water has frozen.
There you have it, two tips to help you make the perfect, clear ice cube. So, what are you waiting for? Get freezing!