1. Why Does a Block of Dry Ice Turn Into Gas Without Melting?
Dry ice is solid carbon dioxide, the same gas that fizzes in soda and that you breathe out. It is extremely cold, about −109 °F (−79 °C) [1]. An ice cube warms up and melts into a puddle. Dry ice does not make a puddle. At ordinary air pressure it changes straight from a solid into a gas, a change called sublimation [1]. That is why it is handy for keeping things cold without leaving them wet. It also means the block is always producing gas. The gas that comes off is colorless and almost odorless [2]. So if the gas cannot be seen, what is the fog?
2. If Carbon Dioxide Is Invisible, What Is the White Fog?
The fog is made of water. The gas coming off dry ice is far colder than the room, and it chills the air around it. Air always holds some water vapor, an invisible gas. When that vapor is cooled enough, it turns into tiny droplets, and a crowd of droplets looks white. This is the same thing that happens when your breath fogs on a cold morning, or when a cold glass of lemonade gets wet on the outside. This "chilled air" explanation is the one usually taught for dry ice in open air, and a middle-school chemistry lesson from the American Association of Chemistry Teachers lists condensation of water vapor as one of the things students explore with dry ice [5]. I did not find a study that measured the fog in open air, so treat this as the standard classroom explanation rather than a measured result. You can also see why it is called "smoke" only by habit: smoke is made of burned bits, and this fog is closer to a cloud.
3. When Dry Ice Goes Into Water, Where Does the Fog's Water Come From?
Here the story takes a turn. Chemistry teacher Tom Kuntzleman and his coworkers wrote in the Journal of Chemical Education that the thick fog above dry ice in water does not mainly come from water vapor in the room, even though many educators assume it does [3]. Their explanation, as Kuntzleman summarized it, goes like this [4]:
- Dry ice in water turns into gas and wraps itself in a bubble of carbon dioxide that contains no water at all.
- Water from the cup evaporates into that dry bubble.
- The bubble is extremely cold because it touches the dry ice, so the new water vapor quickly condenses into tiny droplets.
- Removing vapor this way lets even more water evaporate, until the bubble fills with cloud.
Slow-motion video showed the bubbles already looking cloudy before they reached the surface, which is hard to explain if the fog were made from room air [4]. Kuntzleman calls this "one possible mechanism," so it is a well-supported explanation rather than a final word. It also does not overturn the open-air story above. It says that the cup of water is a special case, because the water is right there inside the bubbles.
4. Why Does the Fog Creep Along the Table Instead of Rising?
Dry ice fog tends to spill over the edge and flow along the table like a slow river. Safety sheets note that carbon dioxide gas is heavier than air and gathers in low places [2]. Cold air is also heavier than warm air. It is reasonable to think these two things together make the fog sink, but I did not find a source that tested this directly, so this is a likely explanation, not a proven one. The same fact matters for safety: the gas collects in low or closed spaces too.
5. Are the Fog Particles Droplets, Ice Crystals, or Both?
The sources I read describe the fog as tiny droplets of water. The cloud is extremely cold, so some of it might freeze into ice crystals, but I found no measurement that settles it. Fog in the air seems to be a simple story, yet even a classroom demonstration still has details that teachers argue about. That is a good sign that there is something left for curious people to check.
6. How Do You Handle Dry Ice Safely, and What Can You Try Instead?
Dry ice is not a toy. At about −109 °F (−79 °C), skin contact can cause severe frostbite very quickly, so adults should use tongs and loose, insulated gloves, never bare hands [1]. Never seal dry ice in an airtight container or bottle: one kilogram turns into hundreds of liters of gas, and a safety alert from the Australian state of Victoria describes a sealed container that exploded [2]. Because the gas is heavier than air, use it only where there is plenty of fresh air, and never in a closed room or car; too much carbon dioxide can cause dizziness, headaches, and loss of consciousness [2]. Let leftovers vanish in a well-ventilated spot.
If you do not have an adult and dry ice at hand, you can test the "cooling makes water appear" idea with no risk. Fill two glasses with the same amount of water, one with ice and one with room-temperature water. Leave both in the same room and check which glass gets wet on the outside first and how much. The water comes from the air, which is the same idea behind the fog above a cold block, though the cold here is only mild.