Dry ice is the solid form of carbon dioxide (CO₂), the same gas you exhale with every breath. It earns the name “dry” because it never melts into a liquid under normal conditions. Instead, it transforms directly from a solid into a gas, a process called sublimation. This unusual behavior, along with its extreme cold, makes dry ice one of the most practically useful substances in both chemistry labs and everyday industry.
Chemical Identity and Structure
Carbon dioxide has the molecular formula CO₂: one carbon atom bonded to two oxygen atoms in a straight line. At room temperature and normal atmospheric pressure, CO₂ is a colorless, odorless gas that makes up roughly 0.03% of the air around you. Cool it down far enough, though, and those gas molecules slow down and lock into a crystalline solid. That solid is dry ice.
Unlike water ice, which forms a loosely packed crystal lattice with lots of open space, solid CO₂ packs its molecules tightly in a cubic arrangement. This gives dry ice a density of about 1.5 times that of water ice, which is part of why a small block feels surprisingly heavy.
Why Dry Ice Skips the Liquid Phase
The key to understanding dry ice lies in the phase diagram of carbon dioxide. Every substance has a “triple point,” the specific temperature and pressure where solid, liquid, and gas can all exist at once. For CO₂, the triple point sits at −56.6 °C and 5.1 atmospheres of pressure. That pressure requirement is critical: normal atmospheric pressure is just 1 atmosphere, well below the 5.1 needed for liquid CO₂ to exist.
Because everyday air pressure is far below the triple point pressure, solid CO₂ can never melt into a puddle the way water ice does. Raising the temperature simply pushes it straight into the gas phase. This sublimation happens at around −78.5 °C (−109.3 °F) under standard conditions, which is why dry ice is so cold to the touch and why it “steams” as it sits on a countertop. That white fog, by the way, isn’t CO₂ gas itself (which is invisible). It’s tiny water droplets from the surrounding air condensing in the extreme cold.
Interestingly, the commonly cited −78.5 °C figure assumes the surrounding air is already saturated with CO₂ vapor. In open air with little ambient CO₂, experiments have measured the actual surface temperature of sublimating dry ice as low as −97.3 °C, roughly 19 degrees colder than the textbook number. For most practical purposes, though, −78.5 °C is the reference chemists use.
How Dry Ice Is Made
Manufacturing dry ice starts with liquid CO₂ stored under high pressure in a tank. When that pressurized liquid is released through a valve into normal atmospheric pressure, it expands rapidly and cools so dramatically that it instantly forms a fine white snow. This snow is then mechanically compressed and forced through a mold (called an extruder plate) to produce pellets, blocks, or slabs in whatever size is needed. The machines that do this are called pelletizers, and they can produce everything from rice-sized pellets to large bricks weighing several kilograms.
Uses in the Chemistry Lab
Dry ice is a workhorse in chemistry, primarily as a way to reach very low temperatures without specialized equipment. The most common technique is the cooling bath: you drop chunks of dry ice into a solvent to create a stable, ultra-cold liquid. Different solvents give you different target temperatures. A dry ice and acetone bath holds steady at −78 °C, which is cold enough to slow or halt most chemical reactions, trap volatile gases, or condense vapors. Swapping acetone for acetonitrile gives you a warmer bath at −40 °C, useful when you need moderate cooling without going to extremes.
These baths are popular because they’re cheap, easy to prepare, and self-regulating. As long as solid dry ice remains in the solvent, the temperature stays locked at the sublimation point. Researchers use them to preserve biological samples, control sensitive reactions that would run out of control at room temperature, and cold-trap solvents during vacuum distillation.
Industrial and Commercial Applications
Outside the lab, dry ice shows up in shipping, food preservation, and an especially clever cleaning method called dry ice blasting. In this process, small pellets of dry ice are fired at a surface using compressed air, similar to sandblasting but without any abrasive grit left behind.
The cleaning works through three mechanisms at once. First, the pellets hit the surface at high speed, physically knocking contaminants loose. Second, the extreme cold (around −79 °C) makes dirt, grease, and coatings brittle and easier to crack away from the underlying material. Third, and most importantly, when the pellets strike a warm surface they sublimate instantly. That phase change expands their volume by roughly 700 times in a fraction of a second, creating a micro-explosion under the layer of grime that lifts it cleanly off. Because the CO₂ simply becomes gas, there’s no water, no solvent residue, and no secondary waste to clean up. This makes dry ice blasting particularly valuable for cleaning electrical equipment, food processing machinery, and delicate industrial molds.
Handling Risks and Safety
Dry ice poses two distinct hazards: cold injury and oxygen displacement.
At −78.5 °C, dry ice is cold enough to cause severe frostbite on contact with bare skin in just a few seconds. The injury looks and feels similar to a burn, with redness, blistering, and potential tissue damage. Insulated gloves, tongs, or towels should always be between your hands and the surface of dry ice. Even brief skin contact can damage tissue, so the “grab it quick” approach is not safe.
The second hazard is less obvious but potentially more dangerous. As dry ice sublimes, it floods the surrounding space with CO₂ gas. Normal air contains about 300 parts per million (ppm) of CO₂. Workplace safety limits cap prolonged exposure at 5,000 ppm, and concentrations above 40,000 ppm are considered immediately dangerous to life. In a small, poorly ventilated room, a large quantity of sublimating dry ice can push CO₂ levels high enough to cause headaches, dizziness, difficulty breathing, and in extreme cases, loss of consciousness. Symptoms can come on without much warning because CO₂ is odorless.
For the same reason, dry ice should never be sealed in an airtight container. As it sublimes, the expanding gas builds pressure rapidly. A sealed bottle, cooler, or thermos can rupture violently. Always store dry ice in insulated but vented containers, and always use it in spaces with good airflow.