If place a can of pressurized liquid butane (such as a lighter refill canister) into a freezer to get it below its boiling point of $-1\ \mathrm{^\circ C}$, and then release it into a container kept at a freezing temperature, will it remain in the liquid state without boiling off (until it reaches its boiling point), or will it boil off immediately?

I had a cold metal container kept at a stable temperature of $-7\ \mathrm{^\circ C}$, but when I released room temperature butane into it, it boiled off instantly.

Why did that happen, and what will happen when it is released from the can at a temperature at or below $-1\ \mathrm{^\circ C}$?

  • 2
    $\begingroup$ You should never store flammable liquids in a normal household refrigerator or freezer, which have various ignition sources (lights, switches, compressor motors) that could ignite vapours in the unventilated cooling compartment. Note that the flash point of butane (the lowest temperature at which it can vaporize to form an ignitable mixture in air) is approximately −60 °C. $\endgroup$ – Loong Jul 26 '15 at 8:31

Since the room temperature butane didn't instantly cool when it hit the container, it was still above the boiling point.

If you had a large freezer that was roughly at equilibrium at a temperature below $-1^\circ C$, and you put both the pressurized container and the empty container in there and let them reach equilibrium, then when you opened the pressurized container it would not boil and you could conceivably pour it into the other container.

However, butane has a large vapor pressure (the enthalpy of vaporization is relatively small). This means that even at low temperatures, it will evaporate very quickly. This is especially true because the atmosphere does not normally have any butane in it - so even a small partial pressure combined with convective transport in the air will cause it to evaporate even faster. If you imagine pouring some rubbing alcohol out on a table and then putting a fan next to it, it is the same idea. Butane molecules randomly have enough energy to escape the liquid, and air currents carry them away. Since there is no butane vapor nearby to establish an equilibrium with the liquid, more butane evaporates to take its place.

The net result of this is that even if you did the experiment in a freezer and made sure everything was below $-1^\circ C$ (which includes you wearing well-insulated gloves and not breathing on it), it might still evaporate so fast that it would look as if it were boiling.

From wikipedia, here is a plot of vapor pressure vs T for butane:

The boiling point is the point where the vapor pressure line exceeds atmospheric pressure. Atmospheric pressure is usually around 760 mmHg. This is a log plot for the P axis, which means one large grid step is ten times larger than the previous one. From this graph, you can see that at $-30^\circ C$, the vapor pressure is still ~200 mmHg. This means that roughly 26% of the air at equilibrium would be butane. Unless you have a very tiny freezer, that is a lot. This means that you might have to wear a gas mask and get a 55 gallon drum of butane before you could really do any "pouring" in the traditional sense.

At $-100^\circ C$ you are down to 1 mmHg vapor pressure. At this point I would say that even a small amount (like what is in a lighter) would stay liquid long enough that you could pour it.

Log plot of vapor pressure of butane vs temperature

  • $\begingroup$ Thanks for the detailed explanation. You definitely cleared a lot up about vapor pressure. Also, if the air was saturated with 100% butane (if that's the right word for it), would it still evaporate readily? And yes, I did the experiment wearing some work groves right next to a fan by an open window ;), although the fan probably made the results worse though! $\endgroup$ – baharini Jul 12 '14 at 0:54
  • $\begingroup$ Yes, the fan will definitely make it evaporate faster. If the air was at 100% saturation of butane, it would no longer evaporate. You would probably need a self-contained breathing unit at that point - so please don't try to do this without the right equipment. $\endgroup$ – thomij Jul 12 '14 at 1:14
  • $\begingroup$ Okay. Right now it's pretty much hypothetical to me so I won't be doing any more Butane experiments. Also another Butane question-I have a butane mini blowtorch. When I heat up a metal surface (it's Aluminium and is large so it stays relatively cool for 20 secs or so), many droplets of some liquid start "condensing" (???) on the metal surface within seconds around the general area of where the jet is aimed. The droplets are odorless (I hope they're safe....) but it is mysterious to me. Remember this happens when the metal is still cold, I haven't looked if try evaporate when it starts heating. $\endgroup$ – baharini Jul 12 '14 at 1:25
  • $\begingroup$ I'll make you a deal - you ask this as a separate quesition, and I'll answer it for you. $\endgroup$ – thomij Jul 12 '14 at 1:26
  • $\begingroup$ Sure, I'll do so. $\endgroup$ – baharini Jul 12 '14 at 1:31

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