Why does ice from the freezer turn cloudy only in the middle?
― Air dissolved in the water gets cornered in the last place to freeze
Look closely at a cube from an ice tray. The outside is clear, but the middle is clouded white. That whiteness is not dirt. It is invisible air that was dissolved in the water. Ice grows from the outside in and refuses to take the air with it. With nowhere to go, the air gathers in the center and gets locked in as tiny bubbles.
You drop ice from the freezer into a glass of cold barley tea. Inside the cube, you can see a white core floating.
But the bagged rock ice from a shop, or the big blocks used in coffee shops, are clear right through to the center. They are just frozen water too. So what is different?
Even your home ice is clear for the outer few millimeters. The cloudiness is always only in the middle.
There are just two reasons the middle turns cloudy
Water holds a little dissolved air. But when water turns to ice, the neatly arranged ice crystal has no room for air. The air is pushed out into the water that has not frozen yet.
An ice tray is surrounded by cold air on top, bottom and sides. The ice grows from the edges, and the last water to remain is in the middle. The air that was pushed out is cornered there.
There is a limit to how much air water can hold. As less water remains, the air gets more concentrated, and eventually it can no longer stay dissolved. The excess becomes tiny bubbles that freeze in place. That is the white cloudiness.
The more the ice grows, the more concentrated the air in the leftover water
One liter of tap water is said to hold about 20 milliliters of dissolved air. You can't see any bubbles, but air molecules are tucked in among the water molecules.
When water starts to freeze, the ice crystal is built in an orderly way from water molecules alone. Air molecules are pushed out into the neighboring water. The amount of air stays the same while the water shrinks. So the air in the remaining water becomes more and more concentrated.
Look at Figure 1. When you change the slider for "how much has frozen," the water left in the center gets smaller and the bar on the right grows. Once the bar passes the dotted line (the dissolving limit), the air has no choice but to come out as bubbles.
The calculation shows that when tap water freezes, the remaining water can no longer hold all its air once about one third has frozen (we check the numbers in the fold-out at the end). From there on, the more it freezes, the more bubbles appear. The outer few millimeters are clear because they formed early in the freezing, before the air had become concentrated.
Besides air, the water's "minerals" also gather in the white core
Tap water also holds small amounts of minerals such as calcium. Like air, they are not let into the ice crystal. They become more concentrated toward the center, which freezes last, and can remain as whitish, powdery streaks.
Another factor is freezing speed. A home freezer is very cold, so the ice grows fast. When the growing ice surface advances quickly, it catches up with the pushed-out air before the air can escape far, and the bubbles are trapped on the spot. This is why you can sometimes see thin streaks of bubbles running outward from the center like rays.
Ice makers are said to freeze water in big cans slowly, over about two days. Some also stir the water as it freezes, and are said to draw off the last water, where air and minerals have become concentrated. The air is washed away before it reaches the ice and is driven out before it can be trapped.
How to make clear ice at home? Change the direction of freezing
Once you understand the reason, you can try a trick at home. Move the place where the air collects from the "middle" to the "bottom." As on the right of Figure 2, put a container of water into a polystyrene foam box, leave the top open, and place it in the freezer. The sides and bottom are insulated, so the water freezes only from the top surface downward.
Take the ice out before it freezes all the way to the bottom and cut off the cloudy lower part, and you are left with clear ice only. A commonly suggested method is to use water that has been boiled and cooled. It does help a little because it contains less air, but air dissolves back in while the water cools, so this alone does not always give ice that is clear to the core. What matters more than the type of water is the "direction of freezing" and the "speed of freezing."
Summary
Water contains invisible dissolved air. Ice grows from the outside in inside the freezer, without letting that air into its crystal. The pushed-out air builds up in the remaining water, and whatever can no longer stay dissolved becomes bubbles trapped in the middle, which freezes last. If you make the water freeze from one direction only, you can change where the air ends up.
The white core is not dirt in the water but "air with nowhere to go."
As it freezes, ice cleans the water.
The mechanism behind the "crackle" you hear when you drop freezer ice into a drink is explained in Why does ice from the freezer crack when you put it in a drink? And why even clear ice looks white when crushed is covered in Ice is clear, so why does snow look white?
- Pour the same tap water into an ice tray and a small plastic cup. Put the cup in a polystyrene foam box (or a box wrapped in several layers of towel), with only the top left open.
- Put both in the freezer and leave them for half a day to a day. Check the cup now and then, and take it out before it freezes all the way to the bottom.
- Hold the two pieces of ice up to the light and compare where the white cloudiness is. It should be in the middle for the tray and toward the bottom for the cup.
If you float tray ice in water and let it melt slowly, you can also see small bubbles coming out of the white core. You can confirm with your own eyes that what was trapped was air.
For those who want to know more ― terms, formulas and links to textbooksEach part is labeled with its level, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school chemistry
- High school+Advanced high school material, or treated as a textbook sidebar
- UniversityNot taught in high school; university specialist courses (materials engineering, crystal growth)
- ResearchNot yet settled textbook knowledge; researchers are still investigating it
Junior highTerms: this phenomenon has names
- Gas solubility: the limit on how much gas can dissolve in water. The colder the water, the more it can hold.
- Saturation: the state where no more can dissolve. Anything beyond saturation comes out as bubbles or crystals.
- Solidification: a liquid turning into a solid. For water, this means freezing.
Junior highHigh schoolChecking with a formula: when does the air spill out of one ice cube?
If ice takes up no air at all, the amount of air stays the same while only the water decreases. From this simple idea we estimate when bubbles start to appear and how much air ends up cornered at the end.
| In symbols | C ＝ C₀ ÷ (1 − f) (bubbles appear when C ＞ Climit) |
| In words | Air concentration in the remaining water ＝ starting concentration ÷ fraction not yet frozen |
| Where it comes from | Conservation of the amount of air. Ice takes up no air, so the amount of air stays the same while the amount of water shrinks to (1 − f) times. The dissolving limit Climit is set by Henry's law (the colder, the larger). |
| Symbol | Meaning and unit |
| C₀ | Concentration of air dissolved in the water before freezing (milliliters per liter of water) |
| f | Fraction frozen (0 to 1; 0.6 means 60% is ice) |
| C | Concentration of air dissolved in the not-yet-frozen water (same unit as C₀) |
| Climit | Concentration of air that 0°C water can dissolve (same unit as C₀) |
| Air dissolved in 1 liter of 20°C water (C₀) | about 19 mL |
| Air that 1 liter of 0°C water can dissolve (Climit) | about 29 mL |
| Water in one ice-tray cube | about 30 mL (0.03 L) |
| Diameter of one bubble in the white cloudiness (rough guide) | about 0.1 mm, volume roughly 0.00052 cubic millimeters |
| Air dissolved in the water of one cube | 19 × 0.03 ＝ 0.57 mL |
| Concentration at which bubbles start (multiple of the start) | 29 ÷ 19 ≒ 1.53 times |
| "Fraction not yet frozen" at that point | 1 ÷ 1.53 ≒ 0.65 |
| Fraction frozen when bubbles start | 1 − 0.65 ＝ 0.35 |
| Concentration at 90% frozen (remaining fraction 0.1) | 1 ÷ 0.1 ＝ 10 times |
| Air the last 10% (3 mL) can dissolve | 29 × 0.003 ≒ 0.087 mL |
| Air that cannot stay dissolved and becomes bubbles | 0.57 − 0.087 ≒ 0.48 mL |
| Convert 0.48 mL to cubic millimeters | 0.48 × 1000 ＝ 480 |
| How many bubbles of 0.1 mm diameter? | 480 ÷ 0.00052 ≒ 920000 bubbles |
In this simple estimate, once about one third of the ice has frozen (frozen fraction 0.35), the remaining water can no longer hold all its air. By the time 90% has frozen, the air concentration is 10 times the starting value. The air cornered in the middle of one cube is about 0.5 milliliters, which as fine bubbles would number from several hundred thousand to nearly a million. That many bubbles scatter light in every direction, which is why the core looks white. In reality some air escapes from the surface, so this is closer to an upper limit.
High schoolHigh school+How easily gases dissolve, and the "sorting" that happens in freezing
High schoolIn high school chemistry, Henry's law says that at a fixed temperature, the amount of gas that dissolves in water is proportional to the gas's pressure (partial pressure). The constant of proportionality changes with temperature, and colder water can dissolve more. This is why no bubbles appear at first when 20°C tap water is cooled to 0°C. Because there is room to spare below the limit, the air stays dissolved until about one third has frozen.
High school+The property of dissolved substances being unable to enter ice is not limited to air. Salt and minerals behave the same way, and sea ice, formed when seawater freezes, is far less salty than the seawater it came from. The concentrated brine pushed out of the frozen part drains downward through gaps in the ice. Freezing also acts as a kind of natural filtration.
UniversitySegregation, the Scheil equation, and zone melting
When something solidifies, dissolved material is pushed toward the liquid side. In materials engineering this is called segregation. The ratio of the concentration entering the solid to that in the liquid is called the equilibrium partition coefficient, and for air and ice it is nearly zero. When the liquid is well mixed, the concentration in the remaining liquid is given by the Scheil equation. Setting the partition coefficient to zero gives the "concentration ＝ start ÷ remaining fraction" rule used above. Turning this property to advantage, zone melting (zone refining) moves a molten band again and again from one end of a rod to the other, collecting impurities at one side. It has been used to purify high-purity materials for semiconductors. Whether the advancing ice surface "pushes away" or "swallows" a bubble is said to depend on the speed of the surface, and it is studied as the trapping of bubbles at the solidification front.
📖 Derivations and further reading: Henry's law (Wikipedia) ／ Zone melting (Wikipedia)
ResearchWhat is still not well understood
- How is the pattern of bubbles decided? Bubbles in ice sometimes form regular patterns such as streaks or layers. How freezing speed, air concentration and the way bubbles are born combine to set the pattern is said to be not fully explained in the finer details.
- What triggers the first bubbles. Even when the calculated limit is passed, bubbles do not always form right away. How concentrated the air must become before bubbles begin, and how tiny scratches on the container wall affect this, vary greatly with conditions.
- Air trapped in Antarctic ice. The ice of the Antarctic ice sheet holds ancient atmosphere as bubbles, giving clues to past climates. These form by a different mechanism, through snow being compressed, but how the bubbles change inside the ice is still being studied.
In other words, this article too is "an explanation within what is known today." Even a small event in a home freezer still has parts that have not been pinned down.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science, Grade 7: "Aqueous solutions" and "Changes of state" | Air is dissolved in water, and dissolved material is pushed out when water freezes |
| High school | Chemistry: "Gas solubility and Henry's law" | Colder water dissolves more air; calculating when bubbles start |
| High school+ | Advanced chemistry: "Solidification and purity" | Sea ice has low salinity; sorting by freezing |
| University | Materials engineering, crystal growth | Segregation, equilibrium partition coefficient, Scheil equation, zone melting |
| Research | Bubble trapping at the solidification front | How bubble patterns are set, what triggers bubbles, bubbles in ice sheets |
| ― | Connection to daily life | Tricks for making clear ice at home (freeze from one direction, slowly) |
- The Chemical Society of Japan (日本化学会), ed., Kagaku Binran, Kiso-hen (化学便覧 基礎編; Handbook of Chemistry, Basic Edition), Maruzen Publishing (solubility of gases in water)
- Hobbs, P. V. "Ice Physics", Oxford University Press, 1974
- Carte, A. E. "Air bubbles in ice", Proceedings of the Physical Society, 77, 1961
- Wikipedia, "Henry's law" (ヘンリーの法則)
*This article is a general-audience science explainer. The figures given are rough guides and estimates meant to help you understand the mechanism. The amount of air dissolved in water varies with water temperature and tap water quality.