When a solid transforms directly into a gas without first melting into a liquid, the process is called sublimation. It is one of the basic phase transitions in chemistry and physics, sitting alongside the more familiar melting and evaporation. Although it sounds exotic, sublimation is surprisingly common in everyday life, from the shrinking of ice cubes in your freezer to the disappearing fog of a dry-ice display.
How Sublimation Works
Every solid has molecules that vibrate in place. When a solid is heated, some of those surface molecules gain enough energy to break free and escape directly into the surrounding air as gas. In most situations you encounter, a solid warms up, melts into a liquid, and then evaporates. But sublimation skips the liquid stage entirely. This happens when the surrounding pressure is low enough, or the substance’s chemistry is such, that the liquid phase is unstable under those conditions.
Carbon dioxide is the textbook case. At normal atmospheric pressure, solid CO₂ (dry ice) cannot exist as a liquid. When it warms, it goes straight to gas. Under standard conditions, dry ice sublimes at about −78.5 °C. But that familiar number assumes the surrounding air is saturated with CO₂. Researchers who varied the concentration of CO₂ around a dry-ice sphere found that when the ambient air contained no CO₂ at all, the sublimation temperature dropped to roughly −97 °C, about 19 °C colder than the commonly reported value.1International Communications in Heat and Mass Transfer. Experimental and theoretical investigation of the dry ice sublimation temperature for varying far-field pressure and CO2 concentration In other words, the environment around a sublimating solid matters just as much as the substance itself.
Sublimation You Have Already Seen
Dry ice is the most dramatic household example. Drop a chunk into warm water and the thick white fog that billows out is water vapor condensing around the escaping CO₂ gas. But there are subtler examples all around you.
Freezer burn is sublimation at work in your kitchen. When frozen food is poorly wrapped, ice crystals on the surface of the food sublimate into the dry air inside the freezer. This leaves behind dehydrated patches with that telltale tough, discolored texture. The food is still safe to eat, but its quality suffers because moisture has literally escaped from the surface as vapor.2Journal of Food Science Education. How Does the Freezer Burn Our Food? The same process explains why ice cubes left uncovered in the freezer for weeks become smaller and develop an odd, pitted surface.
Mothballs are another familiar example. Traditional mothballs are made of naphthalene or paradichlorobenzene, both of which sublimate at room temperature. That pungent smell you notice when you open a closet full of them is the solid slowly turning to gas. Over weeks or months, the ball shrinks and eventually vanishes entirely, all without ever becoming liquid. Research on moth repellent products has found that the chemical vapor from these sublimating solids permeates fabrics stored nearby. A regular cotton shirt kept in a cabinet with moth repellent products can absorb milligrams of these chemicals even without direct contact, and the contamination persists after an hour of airing out.3PubMed. Domestic sampling: exposure assessment to moth repellent products using ultrasonic extraction and capillary GC-MS
Mothballs and Indoor Air Quality
Because mothball chemicals sublimate continuously, they can become a meaningful indoor air pollutant. Naphthalene, one of the classic mothball ingredients, is found at some level in most homes. But concentrations spike dramatically when people use it as a pest repellent or deodorant. A review of naphthalene sources and exposure found that the most important indoor source is its deliberate use as a repellent or deodorant, with excessive use driving the highest concentrations. Secondary sources include attached garages, cigarette smoke, and outdoor pollution.4PubMed Central. Sources, concentrations, and risks of naphthalene in indoor and outdoor air This is a case where understanding the process of sublimation has direct health relevance: if you know the solid is constantly releasing gas, you can make better choices about ventilation and storage.
Sublimation in Nature
Snow and ice sublimate in nature on a massive scale, especially in cold, dry, windy environments where the air pulls moisture away from frozen surfaces faster than melting can occur. This matters most in arid mountain regions, where a significant fraction of the snowpack can simply vanish into the air without ever producing meltwater.
In the semiarid Andes of Chile, researchers found that snow surface sublimation is the dominant way the snowpack loses mass. Sublimation accounted for roughly 71 to 90 percent of total snow loss from a studied catchment, reaching values between 100 and 250 millimeters of water equivalent per year.5Hydrology and Earth System Sciences. Spatial distribution and controls of snowmelt runoff in a sublimation-dominated environment in the semiarid Andes of Chile In regions like that, sublimation is not a minor curiosity; it is the primary mechanism by which snow disappears. For communities that depend on snowmelt for their water supply, this means far less of the winter snowpack actually makes it to streams and reservoirs.
A similar story plays out on glaciers in Asia’s high-altitude drylands. On a glacier in China’s Qilian Mountains, measurements showed that annual sublimation removed about 115 millimeters of ice, while condensation and deposition returned a negligible amount. Sublimation was strongest in autumn and spring, when dry winds were most persistent.6Journal of Geophysical Research: Atmospheres. Surface Sublimation/Evaporation and Condensation/Deposition and Their Links to Westerlies During 2020 on the August‐One Glacier, the Semi‐Arid Qilian Mountains of Northeast Tibetan Plateau The takeaway for climate science is that in arid, high-altitude places, warming air temperatures do not just melt glaciers; they accelerate sublimation too, draining ice reserves in a way that produces no runoff at all.
Sublimation Beyond Earth
Sublimation is a defining process on comets. A comet is essentially a body of frozen gases, dust, and rock orbiting the Sun. As a comet approaches the inner solar system and absorbs more solar radiation, ices on and near its surface sublimate, releasing gas and dust that form the glowing coma and long tail visible from Earth.
Different ices sublimate at different distances from the Sun. Carbon dioxide and carbon monoxide ices can sublimate at great distances, making some comets active far from the Sun. Water ice begins to play a major role in comet activity closer in. Observations of comet C/2017 K2 as it approached a distance of about 2.5 astronomical units from the Sun suggested that water ice-containing dust chunks ejected from the nucleus were sublimating in the coma, contributing to the comet’s activity near the boundary where water ice sublimation becomes significant.7Astronomy & Astrophysics. Coma environment of comet C/2017 K2 around the water ice sublimation boundary observed with VLT/MUSE Spectroscopic studies of another comet, C/2014 Q2 (Lovejoy), found that the spatial distribution of water vapor in its coma was distinctly different from other volatile species, likely because a sizable portion of the water came from icy grains sublimating as they drifted away from the nucleus.8The Planetary Science Journal. Volatile Abundances, Extended Coma Sources, and Nucleus Ice Associations in Comet C/2014 Q2 (Lovejoy)
Mars is another place where sublimation shapes the landscape. The planet’s polar ice caps are made partly of frozen CO₂, and as Martian seasons change, these caps grow and shrink as carbon dioxide deposits and then sublimes back into the thin atmosphere. There is no liquid phase involved because Mars’s atmospheric pressure is far too low to support liquid CO₂ at the surface.
Deposition, the Reverse of Sublimation
If sublimation is a solid becoming a gas, the reverse process, a gas becoming a solid without passing through the liquid phase, is called deposition (sometimes called desublimation in older or more technical texts). Frost forming on a cold window is the classic example: water vapor in the air deposits directly as ice crystals on the glass without first condensing into liquid water.
Research on frost formation has identified several distinct mechanisms depending on the temperature of the cold surface. When a surface drops below about −50 °C, more than one frost formation mechanism tends to operate simultaneously. Frost crystals that grow by deposition alone tend to form slowly and take on cluster-like or fluffy shapes. Frost that grows when the dominant process is desublimation, meaning gas molecules crystallize rapidly onto existing ice, grows fastest and tends to form feathery or pine-like structures.9Applied Thermal Engineering. Mechanisms of frost formation on cold surfaces from Ordinary-Low to cryogenic temperatures under natural convection conditions This is more than academic trivia; understanding frost growth patterns matters for designing heat exchangers, cryogenic equipment, and aircraft de-icing systems.
Freeze-Drying and the Food and Pharmaceutical Industries
Freeze-drying, also known as lyophilization, is one of the most commercially important applications of sublimation. The idea is straightforward: freeze a product, then lower the surrounding pressure and gently warm it so that the frozen water inside sublimes away as vapor. What remains is a dry, lightweight, shelf-stable product that retains much of its original structure and can be rehydrated later.
Freeze-drying is used extensively in food production and pharmaceuticals. In the food industry, it preserves fruits, coffee, and ready-to-eat meals with better flavor and texture retention than conventional drying methods.10PubMed Central. The Freeze-Drying of Foods-The Characteristic of the Process Course and the Effect of Its Parameters on the Physical Properties of Food Materials In pharmaceuticals, it stabilizes vaccines, biologics, and other sensitive drugs that would degrade in liquid form over time. The primary drying phase, where most of the ice sublimes, is the most energy-intensive and time-consuming step, which is why a great deal of research goes into optimizing it.11AAPS Open. Optimization and transfer of robust primary drying protocols for biopharmaceuticals through lyophilization process modeling
Early research into the physics of freeze-drying established that water vapor escapes through open channels left behind by prior sublimation of ice crystals. Factors like freezing rate, the thickness of the already-dried product layer, temperature, pressure, and the concentration of dissolved solutes all influence how quickly sublimation proceeds.12PubMed. Physical chemistry of freeze-drying: measurement of sublimation rates for frozen aqueous solutions by a microbalance technique A product that was flash-frozen with small ice crystals dries differently than one that was slowly frozen with large crystals, because the channel structure is different. Getting these details right determines whether you end up with a high-quality product or a collapsed, unevenly dried mess.
Sublimation Printing
If you have ever seen a custom-printed mug, phone case, or polyester jersey with photo-realistic colors that do not crack or peel, you have likely seen the result of sublimation printing. The process uses special dyes printed onto a transfer paper. When heat is applied (typically around 180–210 °C), the dye sublimates from the paper, turning directly from a solid into a gas. The vapor penetrates into the fibers or coating of the target material, and as the material cools, the dye is locked inside the structure rather than sitting on top.13Scientific Reports. Exploration on ability of printable modified papers for the application in heat sublimation transfer printing of polyester fabric
This is fundamentally different from screen printing or inkjet printing, where pigment sits in a layer on the surface and can eventually crack or fade with wear. Because the sublimated dye is embedded within the material itself, the result is more durable and the colors tend to be more vibrant. The limitation is that sublimation printing works best on polyester fabrics and polymer-coated hard surfaces. Natural fibers like cotton do not have the right molecular structure to absorb and trap the dye vapor, which is why you rarely see sublimation printing on pure cotton garments.
Growing Semiconductor Crystals
One of the more advanced industrial uses of sublimation is in manufacturing silicon carbide (SiC) crystals for the semiconductor industry. SiC is prized for power electronics and radio-frequency devices because it handles high voltages and temperatures far better than ordinary silicon. The dominant method for growing large SiC single crystals is called physical vapor transport, or PVT. In this process, a SiC powder source is heated to extreme temperatures so that it sublimes, and the resulting vapor travels to a slightly cooler seed crystal where it deposits and grows into a large, high-purity single crystal.14Crystal Research and Technology. Numerical Simulation of the Transport of Gas Species in the PVT Growth of Single‐Crystal SiC
Getting this right is remarkably difficult. The temperature gradient between the source and the seed has to be tightly controlled, and even small impurities in the vapor can introduce defects into the growing crystal. Researchers have experimented with using crushed blocks of high-purity SiC produced by chemical vapor deposition as the source material, aiming to improve the productivity and quality of the PVT process.15PubMed Central. Growths of SiC Single Crystals Using the Physical Vapor Transport Method with Crushed CVD-SiC Blocks Under High Vertical Temperature Gradients The SiC crystals produced by sublimation-based growth are sliced into the thin wafers that end up inside electric vehicle inverters, solar power systems, and 5G base stations. It is a striking example of how a simple phase transition, a solid becoming a gas, underpins some of the most advanced technology in the world.
Why Some Substances Sublimate Easily and Others Do Not
Whether a substance sublimes readily comes down to how strongly its molecules hold onto each other and the surrounding conditions of temperature and pressure. Substances with relatively weak intermolecular forces, like naphthalene (mothballs), iodine, and dry ice, sublimate at or near room conditions because their molecules can escape the solid lattice without needing a huge energy input. Substances with strong bonds between molecules, like table salt or iron, essentially never sublimate under everyday conditions because the energy needed to free a molecule from the solid is enormous.
Every substance has what chemists call a triple point, the specific combination of temperature and pressure where solid, liquid, and gas can all coexist. Below that pressure, no liquid phase is possible. Carbon dioxide’s triple point is at about 5.1 atmospheres of pressure, well above normal atmospheric pressure, which is why dry ice sublimes in open air instead of melting. Water’s triple point, by contrast, is at a very low pressure (about 0.006 atmospheres), so at normal atmospheric pressure, ice usually melts before it can sublimate. But even water ice sublimes slowly whenever the surrounding air is dry enough, which is why wet laundry hung outside on a freezing dry day will eventually dry without ever thawing.
Pharmacological research has explored how sublimation behavior can be modified. For instance, a crystalline drug compound called 2-(2-nitrovinyl) furan (known as G-0) sublimes at temperatures as low as 35 °C, well below its melting point. Researchers used thermogravimetric analysis to characterize its sublimation rate and found that the compound follows a predictable pattern that can be slowed by complexing the drug with cyclodextrins, ring-shaped sugar molecules that essentially cage the drug and reduce its tendency to escape into the gas phase.16MDPI / Molecules. Characterization of the Sublimation and Vapor Pressure of 2-(2-Nitrovinyl) Furan (G-0) Using Thermogravimetric Analysis: Effects of Complexation with Cyclodextrins This kind of work matters because a drug that sublimes too readily can lose potency during storage or processing, and understanding the thermodynamics of sublimation helps formulators design more stable products.