Does Dry Ice Melt?

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Dry ice is a great addition to any party or theatre performance and can change a lame retelling of Macbeth into an ominous, mystical reimagining.

At themed parties like Halloween parties, it can really help to set the mood.

You might think it would be dangerous to have large blocks of something that sits at -108 °F around excitable performers or slightly loose willed party goers but it’s actually surprisingly safe, you still shouldn’t touch it with bare hands but relatively speaking it’s quite safe. 

Dry ice is so cold that when it touches a solid it actually doesn’t fully make contact without a large amount of force. This is because of something called the Leidenfrost effect.

Before we go into that however we’ll have to answer the question: does dry ice melt?

There are a few things we need to look at to answer this question but in short: dry ice does not melt it sublimates.

Now for the explanation as to why and what this means. Sublimation is a type of transfer of a material between states that skips one state; in this case from solid to gas.

Dry ice skips the liquid stage during its heating process but this doesn’t mean it can’t be liquid. 

Most of us will know that dry ice is actually just solid Carbon Dioxide (CO2), for those of us that didn’t know that, now you do.

Carbon dioxide can only exist in 2 states of matter at normal atmospheric pressure. It will sublimate at any temperature warmer than 108.4 °F. This is because of the instability of the bonds in the CO2 molecules.

For CO2 to be liquified it needs to be pressurized. The state of matter of a material transfers due to changes in temperature and pressure. A change in temperature can make up for a change in pressure and vice versa.

This is because pressure forces the particles of a  material closer than they would be at normal atmospheric pressure, when forced together bonds start to form and when there is a substantial amount of bonds made a chain reaction occurs and those particles are then stuck together; either loosely (liquid) or tightly (solid).

Lowering the temperature does something similar but instead of forcing the atoms together it reduces the amount of energy in the atoms and therefore lowers their erratic movements so that they can organize themselves into either a liquid or solid. 

Sometimes however a change in temperature isn’t enough to change the state of matter linearly, meaning that a liquid wouldn’t become solid or a gas wouldn't become liquid.

This is the case with CO2, for the dry ice to become liquid it needs to be rapidly depressurized. The depressurization causes the stronger bonds of the solid CO2 to be released, the resultant liquid is colorless and has the same density of water but it quickly evaporates away to gaseous CO2. 

Dry ice is created by liquifying gaseous CO2. There are 2 ways for this to be done, for the first method the gas needs to be put under pressures around 69 bar or 1000.76 psi. It then needs to be cooled under pressure to around 64.4 °F.

It is then stored in high pressure steel tanks awaiting solidification. The second method is done at lower pressures but also at lower temperatures. This method requires the pressure to be a steady 21 bar or 304.58 psi and the temperature to be -0.4 °F.

The storage containers for this ‘low pressure’ liquid CO2 are kept under constant refrigeration and are fitted with multiple alarm systems for changes in temperature or pressure.

This liquid CO2 must then be solidified, for this to happen the pressure chambers that it is stored in quickly has its pressure released. 

This release in pressure forces some of the liquid to immediately turn to solid (the rapid increase in pressure forces some of the particles together) and some of the liquid to instantly vaporize into a gas again (the instant increase in temperature boils off some of the liquid).

The resultant CO2 snow can then be compressed using hydraulic or mechanical action, into large blocks or pellets depending on its intended use. Any remaining gas can be recaptured and put through the process endlessly to change it into dry ice.

Because of its sublimation, dry ice actually works extremely well as a temperature control system for transport and storage.

Unlike other methods of transporting cold/frozen produce, dry ice will only sublimate at a rate of 3%-4% a day meaning that you can store something at below 0° for up to a week without extra refrigeration.

Back to the safety of dry ice, you might be wondering why we say it’s relatively safe. Direct contact with solid CO2 can be extremely harmful and cause severe frostbite but if you use tongs or thick insulated gloves to handle the blocks or bags you will be completely fine.

The key point is that it is only harmful through direct contact (you also shouldn’t breathe in the vapors as they are almost completely CO2).

The dry ice cannot melt and saturate anything with sub zero temperature liquids, it will always sublimate and disperse throughout the air.

The dry ice is also so cold that it is impacted by the Leidenfrost effect. The Leidenfrost effect essentially is when a liquid or solid comes into contact with something that is significantly hotter than its boiling point or sublimation point, it produces an insulating vapor later keeping it from boiling or sublimating rapidly.

This is why if you’ve ever handled dry ice with tongs you’ll have heard the loud and extremely shrill screech that it lets out.

The insulating vapor layer is being compressed and escapes through the path of least resistance (which is usually the small creases in the tongs or the small gaps between the uneven dry ice and the smooth metal) and causes a whistling sound. 

In conclusion, no dry ice does not melt; it forgoes the standard solid to liquid state transfer and goes through a process known as sublimation.