# How would one go about making a cheap freeze spray?

I'm interested in making a liquid freeze spray, for a fun science project. My only experience with this comes in spraying inverting canned air dusters, which spray a -60F mist/liquid (liquid if you spray while the can is inverted). I'd like to make a spray that I can store and use. And if it can't be stored more than a few hours, then hopefully something easy to make on the spot.

My chemistry understanding is high-school level basic, so I do not have a working knowledge of chemistry. On one end of the spectrum, I know there is liquid nitrogen, but it's expensive and complex to store correctly (high-pressure canisters).

I found this list of cooling bath configurations. Could someone point out which configuration can be stored long term (months), stored cheaply (e.g. a plastic or thin metal thermos; not a heavy high-pressure fire-extinguisher container), and be a liquid when "sprayed" (assuming keeping conditions in our pressure and room temperature)? I'd like a spray temperature of less than -60F, so ice and water won't cut it.

I'm thinking of going with dry ice and > 90% concentration ethanol. I have two questions regarding this approach and a random question:

1. If you have a solution of half dry ice and half 99% ethanol, how can you keep it at a low temperature? Won't the solution slowly return to room temperature? My canned air dusters always spray out at the -60F; I'd like something like that if possible.

2. Can this solution be stored in a cheap plastic or thin metal container? Will there be some pressure buildup from the dry ice that will eventually crack or explode the container? Assuming a 10 oz container with a solution of half dry ice and half > 90% ethanol.

3. How are canned air dusters always able to spray their liquid solution (if inverted while sprayed) at -60F, even if they are stored for months at room temperature? Is it because the contents of the can are under pressure and that this pressure makes the liquid cold?

• It's worth noting, for anyone else looking for this sort of thing and not set on making it themselves, that sprays designed to rapidly cool small things are commercially available for electronics and plumbing work. For example: maplin.co.uk/p/maplin-freezer-spray-400ml-n68an – Aesin Jun 2 '15 at 16:27
• SAFETY - Putting dry ice in a sealed container is DANGEROUS. As the dry ice warms it sublimes to gas. The gas will build up pressure inside the container. If the container cannot hold the pressure it will EXPLODE. – MaxW Mar 4 '17 at 17:17

The reason the 'canned air' dusters and other aerosol-based things like anti-perspirant cans produce a cryogenic effect is from the decompression involved. They are stored at room temperature, but the can is pressurised.

There are two effects that happen when you spray out some aerosol.

Firstly, by the ideal gas law, $PV=nRT$, when you release the pressure (i.e. lower $P$), the temperature ($T$) also drops. (The increase in volume $V$ mostly compensates for this, though.)

Secondly, the contents of the can are stored under enough pressure to keep at least one of the parts of the mixture in a liquid state. When you release the pressure by spraying it out, the liquid boils and evaporates, and takes energy in the form of heat to do so. You'll notice that the contents of a canned air duster, if you hold them upside-down when you spray, include more than just air -- there's also a liquid in there which is designed to be mostly inert to electronics and to quickly evaporate just as described above when the pressure inside the can drops, maintaining the pressure needed to produce a gas jet. If you spray for an extended period of time, you'll normally notice a boiling sound coming from inside the can, and the can getting cold, as that liquid evaporates. (As a safety note, these substances are often flammable, so don't use these air dusters near naked flames or ignition sources unless you want fire.)

Most people don't have the equipment needed to produce this sort of thing at home.

None of the cooling baths in that list are designed to be left as-is for months at a time -- they're designed to be made up as and when needed, with occasional addition of liquid nitrogen or dry ice to keep them cool. While you could produce a cold pressurised spray on the spot using dry ice and ethanol, there's a high probability that any sort of robust switched-release mechanism could make the vessel you're using explode under the pressure of sublimating carbon dioxide. Also, unless you relied on a pressure source other than the carbon dioxide itself, your spray probably wouldn't have time to get very cold.

So, I don't recommend it.

• Instead of a cooling bath which can't be left as-is for months at a time, can I use the same concept of the aerosol spray: find a compound such that it can be pressurized into a liquid at room temperature, and when sprayed, will change state (hopefully to a gas), expand, and therefore cool significantly? – Jason Sep 17 '13 at 20:52
• Yes, but given that you'd most likely be pressurising flammable gases, possibly using improvised equipment, I'd think really carefully about whether you want to be trying that, and how much skin you plan to have in later life. Having done that, your best bet is probably dimethyl ether. – Aesin Sep 17 '13 at 22:53