How long does it take for a cup of ice to freeze?

Whether you’re hosting a party or just trying to chill after a long week of work, there’s almost nothing more disappointing than opening the freezer to find empty ice cube trays. Now what? That glass of premium Scotch or iced coffee isn’t going to cool itself, and you’re left with the prospect of heading to the nearest gas station for a bag of ice or filling up the trays and waiting.

But how long will it take for those ice cubes to freeze?

Ice-making is a surprisingly complex subject, even though you’ve no doubt been doing it your whole life. Still, there are a remarkable number of factors that affect the speed at which your ice freezes and the quality of the ice cubes you get when the process is complete. Here’s what you need to know about the speed at which ice freezes.

In most situations, ice made in a standard ice tray — those plastic models with space for a dozen tapered cubes — takes about three to four hours to freeze in your home freezer. Water freezes when it reaches 32 degrees Fahrenheit (0 degree Celsius), but the time it takes to do so depends on several factors that may be different in your freezer than in your neighbor’s.

First, the size of the ice cubes matters. If you have an unusual ice tray designed to make very small ice cubes, these will freeze faster than large blocks of ice. The surface area of the ice will also affect freezing time, since ice begins to freeze from the outside in. This means that an ice tray that has air space between each cube will freeze faster than one that merely has dividers.

Second, the air temperature of your freezer matters. Most home freezers are set at 0 degrees Fahrenheit, which is optimal. However, if you leave your freezer open or fill it with room-temperature food at the same time you make your ice, you’ll unwittingly raise the air temperature in the freezer, which will slow down your ice making process.

For starters, make sure that you keep your freezer at the recommended temperature and keep it closed as much as possible — that means no checking on it for at least three hours.

If you’re really in a rush, you can try temporarily lowering the temperature of your freezer, which will help chill the water molecules faster.

You can also try using a stainless steel ice cube tray instead of a plastic or silicone one. Metal is a very poor insulator, so the water will cool down more quickly.

Finally, you might also try the world’s most counterintuitive trick: Use hot water to fill your ice cube trays. This method relies on the Mpemba effect to get your ice to freeze faster than it would if you used cold water in your ice trays.

The Mpemba effect is a bit of a paradox. Though Aristotle and other philosophers long claimed that hot water freezes faster than cold, it wasn’t until the 1960s that a Tanzanian high school student named Erasto Mpemba began experimenting with the phenomenon in earnest. Mpemba noticed that when he made ice cream with hot milk, it froze faster than chilled milk. He asked his teacher about it, but his teacher didn’t believe him. Mpemba then tried experiments using warm water to make ice, and scientists replicated them. They discovered that, in many cases, warmer water does freeze faster than cool water. This fact is now called the Mpemba effect after a high school kid who was determined to prove his teacher wrong.

There are several possibilities for why the Mpemba effect works in any given situation. Ice is complicated, and it’s hard to know exactly which reason — or combination of reasons — is responsible for the Mpemba effect in any given situation. Here are some of the theories scientists have come up with to explain why “hot ice” freezes faster:

• • Evaporation: Warm water evaporates more quickly than cool water, which leaves behind less water to freeze. Since the resulting ice cubes are slightly smaller, they freeze more quickly.
• • Released Gases: When tap water is boiled, gases and minerals in it are released into the air, which could slightly raise the freezing point and allow the ice to freeze solid more quickly than unboiled water. By this theory, distilled water may also freeze slightly faster than tap.
• • Convection: As the hot water cools, the difference in temperature between the cold surface and the hot bottom of the ice cube tray will cause a faster rate of cooling due to convection (the movement of heat through the fluid).
• • Covalent Bonds: A study at Nanyang Technological University in Singapore looked at this issue at a molecular level and discovered that heating shortens the bond between the hydrogen atoms and oxygen atoms in individual water molecules, which lets heat escape the water at a faster rate. This means that hot water will freeze faster than cold water.

Maybe. Most of these freeze water that’s pumped into a freezer tray in the freezer, which isn’t really any faster than putting water in your plastic freezer trays yourself — it’s just more convenient because the machine remembers to refill the trays for you. On the other hand, some built-in ice makers are designed with refrigerant coils that directly chill the ice trays, freezing water much faster than relying solely on cold air. This type of ice maker is usually located in a side-by-side refrigerator/freezer, on the fridge side of the appliance.

Yes. Countertop ice makers function more like large commercial ice machines than built-in fridge or freezer ice makers. This is because the metal ice tray is attached to compressor coils that pump refrigerant directly to the ice tray. This is similar to the direct-freeze type of ice makers described above. However, the ice trays in dedicated ice makers aren’t filled with water, but rather have water run over the super-cold metal. Ice crystals form instantly on the trays and build up as the water cascades over them. This is much faster than having water sit in a cup and waiting for it to freeze, because the center of an ice cube is warm and takes longer to freeze.

As a bonus, the cascading water that slowly builds up to make ice is a great way to get crystal clear ice for cocktails and other cold drinks. This is because standard ice cubes freeze from the outside in, which pushes minerals and gases toward the center, where they are trapped in an unattractive white cloud. Because the cascading water doesn’t allow for impurities to be trapped, you get purer, prettier ice.

When you’re looking for a dedicated ice maker for your home, you definitely don’t have to install a large commercial model! NewAir’s portable countertop ice maker is just 16 inches tall, which is small enough to slide under upper cabinets in a corner of your kitchen workspace. Despite its small size, this ice maker is powerful enough to make up to 50 pounds of ice each day. That means it churns out ice at a much faster clip than any in-refrigerator ice maker can, and you only need to fill up the reservoir once or twice rather than dealing with ice cube trays all day long. If you need to serve drinks to a crowd, this is definitely the way to go to make sure you stay fully stocked.

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The Last Word

For reliably fast and plentiful ice, a dedicated ice maker is the way to go. Countertop models are less expensive than you think, and they’re a great investment in convenience if you love to entertain or need a large store of ice for projects like making homemade ice cream or packing a cooler for camping or fishing.

If you plan to stick with your typical freezer ice cube trays but need ice fast, turn down the temperature on your freezer and try filling your trays with boiling water to take advantage of the Mpemba effect. For a more long-term solution for making ice cubes fast, invest in metal ice cube trays or trays designed to make smaller ice cubes or water bottle ice rods with lots of surface area. Both of these types of ice should freeze up faster than standard cubes.

Now that you know some of the science behind how ice freezes and the factors that can speed it up or slow it down, you can make an informed decision about the best way to get ice quickly when you need it most.

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