Dry ice doesn’t melt in the traditional sense; it sublimates, turning directly from a solid into carbon dioxide gas without passing through a liquid phase. How fast that happens depends heavily on the size and shape of the piece, but as a rough guide, small pellets lose about 2.5 to 3 percent of their weight per hour at room temperature, while larger blocks lose closer to 1 to 1.6 percent per hour under similar conditions. A five-pound block left on a countertop will typically be gone within a day or so, while a bag of pellets can vanish in half that time. The speed is governed by a handful of interacting factors, and understanding them makes the difference between dry ice lasting a few hours and lasting through a long road trip.
Why Dry Ice Sublimates Instead of Melting
At normal atmospheric pressure, solid carbon dioxide sits at about −78.5 °C (−109.3 °F). When heat from the surrounding environment reaches the surface, COâ‚‚ molecules escape directly into the gas phase rather than forming a liquid puddle. Liquid COâ‚‚ does exist, but only under pressures well above what you encounter at sea level. That’s why dry ice seems to shrink and disappear without ever getting wet on its own.
This phase behavior matters practically because the sublimation happens entirely at the surface. Every square centimeter of exposed dry ice is actively losing mass. That makes surface area the single most important variable controlling how fast a given quantity disappears.
Size, Shape, and the Surface-Area Problem
A kilogram of dry ice shaped into a compact block sublimates far more slowly than the same kilogram broken into pellets. Research on dry ice geometry found that chunked blocks and pellets sublimated at roughly 2.5 to 2.8 percent per hour, while block forms with much lower surface-area-to-volume ratios sublimated at only about 1.0 to 1.6 percent per hour.1Engineering Reports. Dry ice sublimation performance as affected by binding agent, density, and age That’s nearly a twofold difference just from changing the shape.
The reason is straightforward. A single large block has far less surface exposed per unit of mass than a pile of small pellets. Since sublimation only occurs at the surface, less exposed area means slower total mass loss. If you’re buying dry ice for a cooler, choosing a solid slab over pellets can buy you meaningfully more time. Pellets are better when you need fast cooling, since all that extra surface area means they dump cold into their surroundings more aggressively.
Density also plays a role. Dry ice that is loosely compacted has more internal pore space, giving gas an easier escape path and slightly increasing the effective sublimation rate. Tightly compressed blocks hold together longer, both because of their lower porosity and because their geometry tends toward lower surface-area-to-volume ratios.
How Wind and Airflow Speed Things Up
Still air lets a thin blanket of cold carbon dioxide gas accumulate right above the dry ice surface. That blanket acts as a buffer, slowing down further sublimation because the immediate environment is already saturated with COâ‚‚ and is colder than the surrounding room. Any breeze disrupts this protective layer, exposing the surface to warmer, less COâ‚‚-rich air and accelerating mass loss.
Experimental work on this confirmed what you’d expect: higher wind speeds drive faster sublimation. The direction of airflow matters too, with upward-blowing air producing higher sublimation rates than side-blowing air.2Applied Thermal Engineering. A combined experimental-mathematical study on the kinetics of dry ice sublimation under different airflow velocities and blowing modes This makes sense because COâ‚‚ gas is heavier than air and naturally pools around the base of the dry ice. Blowing air upward lifts that cold gas layer away from the surface more effectively than hitting it from the side.
The practical takeaway: if you’re storing dry ice in a cooler, keep it out of drafts and don’t open the lid more than necessary. Every time you lift the lid, you’re letting warm air in and letting the protective COâ‚‚ blanket escape. Conversely, if you’re using dry ice for theatrical fog or to flash-freeze something, a small fan can noticeably increase how fast the ice gives off gas.
The COâ‚‚ Concentration Around the Surface
The composition of the air immediately surrounding dry ice has a surprisingly large effect on its behavior. At standard atmospheric pressure, when the surrounding gas is pure CO₂, the sublimation temperature sits at the familiar −78.5 °C. But as the CO₂ concentration in the surrounding air drops, the dry ice actually gets colder. In experiments that systematically varied the CO₂ concentration, researchers found the sublimation temperature dropped to roughly −97.3 °C when the surrounding atmosphere contained zero percent CO₂, about 19 °C colder than the commonly cited value.3International Communications in Heat and Mass Transfer. Experimental and theoretical investigation of the dry ice sublimation temperature for varying far-field pressure and CO2 concentration
This happens because of sublimative cooling. When the surrounding gas has very little COâ‚‚, there’s a steeper concentration gradient at the surface, which drives faster sublimation. That faster sublimation absorbs more heat, pulling the surface temperature down further. The dry ice reaches a steady-state “wet-bulb” temperature that can be dramatically lower than −78.5 °C.
In everyday terms, this means dry ice in a well-ventilated space will sublimate faster and run colder than dry ice sealed in a container where COâ‚‚ has built up. The common refrain that dry ice is “always −78.5 °C” is only true when it’s surrounded by COâ‚‚-saturated gas. In a typical room with normal air, the surface is actually several degrees colder and sublimating more rapidly than that number implies.
Altitude and Air Pressure
Lower atmospheric pressure accelerates sublimation, which is a real concern for anyone shipping dry ice by air. The Federal Aviation Administration funded a study specifically to measure this effect. Researchers placed dry ice packages in an altitude chamber, depressurized it to simulate 8,000 feet (the cabin altitude of most pressurized commercial aircraft), and held it there for six hours. The average sublimation rate came out to about 2.0 percent per hour, with a margin of roughly ±0.3 percent per hour.4ROSA P. The Sublimation Rate of Dry Ice Packaged in Commonly Used Quantities by the Air Cargo Industry
That rate is consistent with what you’d see from pellets or chunked blocks at ground level, which suggests the altitude effect compounds with the geometry effect. For cargo shipped in unpressurized holds at even higher effective altitudes, the loss rate would be steeper. Airlines and shippers use this data to calculate how much dry ice to include so that temperature-sensitive shipments stay cold for the full journey. If you’re packing a cooler for a mountain camping trip at high elevation, you should expect your dry ice to disappear somewhat faster than it would at sea level.
Ambient Temperature
This one is intuitive but worth quantifying in your head. The rate of heat transfer into the dry ice depends on the temperature difference between the ice and its surroundings. At room temperature (around 20 °C), that gap is roughly 100 °C. In a freezer set to −18 °C, the gap shrinks to about 60 °C. In a walk-in freezer at −30 °C, it’s smaller still.
That’s why keeping dry ice in a standard home freezer can extend its life somewhat, though a chest freezer won’t stop sublimation entirely because the temperature difference is still large. You’ll slow the rate, but the dry ice will still be gone within a few days. There’s a common misconception that a home freezer can “preserve” dry ice indefinitely. It can’t. Even at −18 °C, the temperature gap is too wide. The sublimation slows but never stops at any temperature above −78.5 °C.
On the hotter end, leaving dry ice in a car trunk on a summer day when interior temperatures can exceed 50 °C means the temperature difference exceeds 125 °C, and sublimation accelerates accordingly. Direct sunlight on an uninsulated container makes things even worse because radiant heating adds to the convective heat transfer.
Insulation and Container Choice
Since you can’t change the physics of sublimation, the most practical thing you can do is manage the insulation. A good polystyrene (Styrofoam) cooler is the standard for a reason: it’s cheap, lightweight, and has low thermal conductivity. Thicker walls help, and sealing the lid as tightly as possible retains the cold COâ‚‚ blanket discussed earlier.
A few practical tips that follow from the science:
- Fill dead space: Crumpled newspaper or additional Styrofoam blocks reduce the volume of air that the dry ice has to cool, meaning less convective loss.
- Don’t use airtight containers: Sublimating dry ice produces a large volume of COâ‚‚ gas. A sealed container can build dangerous pressure and eventually rupture. A cooler with a slightly loose lid is fine; a sealed thermos or screw-top container is not.
- Choose blocks over pellets: As covered above, the lower surface-area-to-volume ratio of a solid block means slower sublimation. If you need pellets for their faster cooling ability, pack them tightly together so they behave more like a single mass.
- Layer strategically: Dry ice on top of your food items works better than on the bottom because cold air sinks. The cold COâ‚‚ gas will settle downward over the items you want to keep frozen.
High-end shipping containers use vacuum-insulated panels or multi-layer insulation that can extend dry ice life considerably beyond what a Styrofoam box achieves. For most consumer uses, though, a decent cooler and some common sense about lid opening and fill material will get you most of the way there.
What Happens When You Drop Dry Ice in Water
Putting dry ice in water produces the dramatic billowing fog that makes it a staple of Halloween parties and stage effects. It also makes the dry ice disappear much faster than it would in air. Water is far more effective at transferring heat than air, so the sublimation rate jumps dramatically. A chunk that would last hours in a cooler can be gone in minutes submerged in warm water.
The fog itself is widely misunderstood. A common explanation, even among science educators, is that the cold COâ‚‚ gas rising from the water causes atmospheric water vapor to condense. Research has shown this isn’t quite right. The fog forms through a more complex process involving tiny water droplets that are carried aloft with the escaping COâ‚‚ gas, not simply from atmospheric humidity condensing onto cold gas.5ACS Publications. A Molecular Explanation of How the Fog Is Produced when Dry Ice Is Placed in Water The vigorous bubbling at the surface launches microscopic water droplets into the gas plume, and those droplets are what you see as “fog.”
Water temperature affects the visual output and the sublimation rate in opposite directions. Hot water produces more dramatic fog initially because the large temperature gap drives violent sublimation, but the dry ice disappears quickly. Cold water produces less fog but makes the dry ice last longer. For sustained fog effects, adding small amounts of dry ice to warm water periodically tends to work better than dumping a large quantity into hot water all at once.
Dry ice-solvent mixtures are used in laboratory settings to create cold baths at specific temperatures. A common combination is dry ice in ethanol or isopropanol, which can reach temperatures near −78 °C. The heat transfer rate in these baths depends on the size of the object being cooled; smaller-diameter items experience higher heat transfer coefficients, meaning they cool faster per unit of surface area.6PubMed. Experimental determination of surface heat transfer coefficient in a dry ice-ethanol cooling bath using a numerical approach
Rough Timelines for Common Quantities
Exact numbers depend on all the factors above, but here are reasonable ballpark expectations for dry ice stored in a standard polystyrene cooler at room temperature with the lid kept closed:
- 5 pounds (about 2.3 kg) of pellets: roughly 12 to 18 hours.
- 5 pounds as a single block: roughly 18 to 24 hours.
- 10 pounds (about 4.5 kg) as a block: roughly 24 to 36 hours, because the larger mass has proportionally less surface area.
- 20 pounds or more: can last 36 to 48 hours or longer in a well-insulated container.
These estimates assume you aren’t opening the cooler frequently. Each time you open the lid, you vent the cold COâ‚‚ atmosphere and replace it with warm room air, which resets the protective blanket and exposes the dry ice to a fresh burst of heat. In practice, a cooler opened every 30 minutes will lose its dry ice significantly faster than one left sealed.
For shipping, the FAA study’s finding of about 2 percent per hour at simulated cruising altitude gives planners a useful planning number. A 10-pound shipment in a standard insulated box at altitude would lose roughly a fifth of a pound every hour, which means it can survive a six-hour domestic flight with several pounds to spare, assuming the box is properly packed.
Dry Ice on Mars
Carbon dioxide ice isn’t just a laboratory curiosity on Earth. It’s a major geological actor on Mars, where the atmosphere is roughly 95 percent COâ‚‚ and surface pressures are far below Earth’s sea level. Seasonal COâ‚‚ frost deposits grow and retreat with the Martian seasons, and the sublimation of this ice shapes the landscape in ways that have puzzled planetary scientists for decades.
Laboratory experiments simulating Martian conditions have shown that sublimating COâ‚‚ blocks can carve pits, levees, and furrows into loose granular surfaces, closely resembling features photographed on Mars by orbital cameras. The morphology of these features depends on grain size: finer substrates produce deeper pits and taller levees.7Scientific Reports. Experiments On Sublimating Carbon Dioxide Ice And Implications For Contemporary Surface Processes On Mars Some of the linear gullies on Martian slopes may have been carved not by flowing water but by blocks of COâ‚‚ ice sliding downhill and sublimating as they go, leaving behind furrow-like tracks in the sand.
The Martian environment pushes all the variables discussed in this article to extremes. Surface pressures on Mars are less than 1 percent of Earth’s sea level pressure, which shifts the sublimation equilibrium dramatically. The atmosphere is nearly pure COâ‚‚, which works in the opposite direction by reducing the concentration gradient at the ice surface. These competing effects mean that COâ‚‚ ice on Mars behaves quite differently from the dry ice blocks you’d buy from a grocery store, but the same underlying physics governs both. The factors that matter on your kitchen counter, surface area, pressure, airflow, and surrounding gas composition, are the same ones shaping Martian terrain millions of miles away.