Why Does Dry Ice in a Plastic Bottle Make It Burst?
― One small chunk turns into about 850 times its volume of gas
Dry ice doesn't melt into water the way ordinary ice does. It goes straight from solid to an invisible gas, and its volume swells to about 850 times. In a sealed container, that gas has nowhere to go. Even a fingertip-sized chunk can generate enough force to burst a plastic bottle.
You buy ice cream and bring it home. There's dry ice in the bag, giving off clouds of white smoke. The kids are delighted.
"Let's keep this smoke!" says a child, dropping a piece into an empty plastic bottle and screwing on the cap. After a while, the bottle starts to swell tight.
This is a very dangerous situation. The bottle may soon split open with a loud bang. People have repeatedly been hurt by flying caps and fragments. So what is going on inside?
There are two main dangers
Dry ice is carbon dioxide cooled until it freezes solid. When it turns back into gas, its volume grows about 850 times. In a closed container, that becomes pressure that keeps pushing on the walls.
The carbon dioxide that escapes is colourless and odourless. It is heavier than air, so it collects near the floor and in enclosed spaces. At high concentrations it can cause headaches and loss of consciousness.
The first is the danger of bursting, the second is the danger of suffocation. Both come from the same property: dry ice doesn't melt, it turns into gas. Let's take them in turn.
Why can one small chunk burst a bottle?
Ordinary ice melts at 0°C into water, and its volume barely changes. Dry ice, however, can't become a liquid at normal air pressure. At about −79°C it changes directly from solid to gas. This change is called "sublimation."
Carbon dioxide gas needs far more room than the solid. One gram of dry ice is smaller than half a sugar cube. As gas, it fills about 545 cubic centimetres, which is roughly one 500-millilitre plastic bottle.
With the cap on, the gas can't get out. Instead, the gas particles hit the bottle walls harder and harder. That is pressure. Look at Figure 1. The more dry ice you add, the more steadily the pressure inside the bottle rises.
Put 10 grams into a 500-millilitre bottle, and the calculation gives a pressure about 11 atmospheres higher than outside. Bottles for fizzy drinks are built tough, but they are still said to burst at around 10 atmospheres. Ten grams is only a few of the small chunks that come in an ice cream bag.
Glass jars and metal flasks are no safer. Glass shatters into sharp fragments. Containers like flasks and spray cans can have their caps or bases blown off. The basic rule: never put dry ice in any container with a lid, whatever it's made of.
Where does the invisible gas go?
If you don't put a lid on, nothing bursts. But the carbon dioxide itself is another danger.
Carbon dioxide is about 1.5 times heavier than air. So it doesn't mix with the surrounding air right away. It flows down and collects near the floor and in low spots. It has no colour or smell, so you can't tell it's there.
Figure 2 is an estimate for a passenger car in which 1 kilogram of dry ice turns completely into gas. The air inside a car is roughly 3 cubic metres. With the windows shut, the carbon dioxide share works out to as much as 18%. Normal air holds only about 0.04%, so that is several hundred times higher.
For the body, headaches and breathlessness are said to start at around 3–4%. At 7–10%, people may lose consciousness within minutes. In reality the gas leaks out little by little, so it won't reach 18% right away. Even so, carrying dry ice for a long time in a closed car is dangerous. The same thing can happen in freezer work rooms, basements and other places with no airflow.
The white smoke from dry ice is the surrounding air being suddenly chilled, so that water vapour in it turns into tiny droplets and ice crystals. It works the same way as your white breath in winter (the white breath article). Carbon dioxide itself is transparent, so even after the smoke disappears, the invisible gas is still there. "No smoke, so it's fine" is not a safe conclusion.
So what should we do?
- Never put it in a container with a lidPlastic bottles, glass jars, flasks and airtight food containers can all burst.
- Don't touch it bare-handed or put it in your mouthAt −79°C, it can give you frostbite in seconds. Use work gloves or tongs.
- Use it and dispose of it in a well-ventilated placeIn a car, open a window or bring in outside air. Leave any leftovers outdoors in the shade to turn into gas naturally.
If you get a headache, dizziness or shortness of breath in a room or car where dry ice has been placed, open the windows at once to ventilate, leave the spot and breathe fresh air. If someone has collapsed, put ventilation first so you don't breathe the gas yourself, and call emergency services (119 in Japan). Carbon dioxide poisoning is frightening because you may not notice it yourself. If you find a container that is swelling, don't try to open the lid. Keep your face away and move away from it.
Summary
Dry ice is a lump of carbon dioxide that turns straight into gas without melting. Its volume swells about 850 times. In a capped container that becomes pressure, and even around 10 grams reaches the point where a plastic bottle bursts. Without a cap, the invisible heavy gas instead collects in low places.
Dry ice is not a "little piece of ice."
It is a "lump that swells into 850 times its volume of gas."
What happens when pressure builds up can also be seen from other angles in the pressure cooker article and the article on how volcanoes erupt.
- Put one small chunk of dry ice in a small cup with no lid, handling it with work gloves.
- Stretch an uninflated rubber balloon over the mouth of the cup (a balloon is not a lid: it stretches and gives the gas a way out). Watch it slowly inflate.
- When the balloon is about the size of a fist, take it off right away and release the gas outdoors. Do this in a well-ventilated place.
Seeing how far one small chunk inflates the balloon lets you feel how the volume grows hundreds of times. Follow two rules: don't inflate the balloon too much, and never do this with a bottle or any airtight container.
For those who want more ― terms, formulas and links to textbooksEach part is labelled with its level, from middle-school science to university courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school chemistry or physics
- High school+Advanced high-school material, or textbook sidebars
- UniversityUniversity-level topics (physical chemistry, thermodynamics) not taught in high school
- ResearchTopics researchers are still investigating, not yet taught as settled fact even at university
Middle schoolTerms: this phenomenon has a name
- Sublimation: a change in which a solid turns directly into a gas without passing through a liquid. Dry ice is one example, and so is old ice in a freezer shrinking.
- Change of state: a substance changing between solid, liquid and gas. The number of particles stays the same; only the spacing between them changes.
- Pressure: how strongly something pushes on a surface. The pressure of a gas is the total force of many particles hitting the wall.
Middle schoolHigh schoolChecking with a formula: what pressure does 10 grams create in the bottle?
Consider 10 grams of dry ice sealed in a 500-millilitre plastic bottle. Assume a temperature of 20°C and that all the dry ice turns to gas. We ignore the air already inside and find only the added pressure.
| In symbols | P ＝ n × R × T ÷ V （n ＝ m ÷ M） |
| In words | Pressure ＝ amount of gas (moles) × gas constant × temperature ÷ container volume |
| Where it comes from | The ideal gas equation. At the same temperature, more particles or a smaller container means more hits on the walls and so higher pressure. |
| Symbol | Meaning and unit |
|---|---|
| P | Pressure inside the container (atm) |
| n | Amount of gas (moles) |
| m | Mass of dry ice (grams) |
| M | Mass of 1 mole of carbon dioxide: 44 grams |
| R × T | At 20°C, equal to the volume 1 mole of gas takes up at 1 atm: 24.0 litres |
| V | Container volume (litres) |
| Mass of dry ice | 10 grams |
| Mass of 1 mole of carbon dioxide | 44 grams |
| Volume of 1 mole of gas at 20°C and 1 atm | 24.0 litres |
| Volume of the plastic bottle | 0.5 litres |
| Density of dry ice | about 1.56 grams per cubic centimetre |
| Amount of gas (moles) | 10 ÷ 44 ≒ 0.227 |
| Volume when spread out at 1 atm (litres) | 0.227 × 24.0 ≒ 5.45 |
| Pressure when squeezed into 0.5 litres (atm) | 5.45 ÷ 0.5 ≒ 10.9 |
| Volume of 1 gram of solid (cubic centimetres) | 1 ÷ 1.56 ≒ 0.641 |
| Volume of 1 gram of gas (cubic centimetres) | 24000 ÷ 44 ≒ 545 |
| Expansion factor | 545 ÷ 0.641 ≒ 850 |
With 10 grams, the pressure inside the bottle is about 11 atm higher than outside. That is above the roughly 10 atm at which a fizzy-drink bottle is said to burst. For the car, the same formula applies: 1 kilogram becomes 545 litres at 1 atm, and in a 3000-litre car interior, 545 ÷ 3000 ≒ 0.18, or about 18%.
High schoolHigh school+Why it doesn't "melt," and what a lid changes
High schoolWhether a substance is solid, liquid or gas depends on the combination of temperature and pressure. A single chart that summarises this is called a phase diagram. In the phase diagram of carbon dioxide, the 1 atm line doesn't pass through the liquid region. So at ordinary pressure, it goes straight from solid to gas.
High school+Carbon dioxide can be a liquid only above about 5.1 atm. So in a strong sealed container, once pressure passes 5 atm, dry ice can turn into liquid carbon dioxide. In experiments with a strong transparent tube, the liquid can actually be seen. But the pressure keeps rising, so trying this with everyday containers is dangerous.
UniversityThe triple point, and departures from the ideal gas
The temperature and pressure at which solid, liquid and gas coexist is called the triple point. For carbon dioxide it is about −57°C and about 5.1 atm. This contrasts with water, whose triple point is a very low 0.006 atm. The calculation in this article used the ideal gas equation, but at high pressure the attraction between molecules and their size start to matter. More accurate results come from corrections such as the van der Waals equation. The relation between sublimation pressure and temperature is given by the Clausius–Clapeyron equation.
📖 Derivations and further reading: Ideal gas (Wikipedia) ／ Triple point (Wikipedia)
ResearchWhat is still not well understood
- The spread in the pressure at which containers split. The bursting pressure of a plastic bottle varies a lot with shape, thickness, scratches and temperature. How plastic that has become cold and hard breaks is a difficult problem even in materials research.
- How gas pooled in low places clears. How long a heavy gas takes to mix and thin out depends greatly on the shape of the room, temperature differences and slight air currents. Predicting this for each site is still said to be hard.
- Effects of long exposure to low concentrations. The short-term effects of a few percent are well studied, but studies disagree on how drowsiness and concentration are affected by long stays at much lower concentrations.
In other words, this article too is "an explanation of what is known so far." Treat the numbers as guides, and always handle dry ice with a margin on the safe side.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science year 1 "Changes of state," year 2 "Properties of gases" | Sublimation, why volume increases |
| High school | Basic Chemistry / Chemistry "Gas laws," "Phase diagrams" | Calculating pressure with the equation of state |
| High school+ | Chemistry, advanced "Phase diagrams" | Becoming liquid above 5.1 atm |
| University | Physical chemistry, thermodynamics | Triple point, corrections for real gases |
| Research | Material strength, ventilation engineering, occupational health | Spread in bursting pressure, how gas pools |
| ― | Links to daily life | Use as a coolant, carrying it in a car, disposal |
- Wikipedia, "Dry ice" (ドライアイス, Japanese edition)
- Wikipedia, "Triple point" (三重点, Japanese edition)
- National Astronomical Observatory of Japan (国立天文台), ed., Rika Nenpyo (理科年表, Chronological Scientific Tables), Maruzen Publishing (physical properties of carbon dioxide)
- Consumer Affairs Agency (消費者庁) and National Consumer Affairs Center of Japan (国民生活センター), "Safety warning on handling dry ice" (ドライアイスの取り扱いに関する注意喚起; cases of container bursts and poisoning in cars)
- Ministry of Health, Labour and Welfare (厚生労働省), "Anzen Site for Workplaces" (職場のあんぜんサイト; accident cases of oxygen deficiency and carbon dioxide)
*This article is a general-audience science explainer. The figures given are rough guides to help you understand how things work. For handling dry ice, follow the warnings from the product or store and instructions from fire services and local authorities. If you feel unwell, don't hesitate to consult a medical institution or call emergency services (119 in Japan).