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A YouTube channel called "kreosan" (made by Russian-speaking people who I believe live in Ukraine) posted a video showing rockets made from bottles of Coca-Cola.

Here's what I've figured out from the video.

  • First step: pour out some Coca-Cola to make room in the bottle to add gas.

  • Second step: add butane (looks like a canister for refueling butane lighters)

  • Third step: this is unclear. He adds a gas from a large container that appears to say "GAS-1". The YouTube description says "Coca Cola + propane = turbo ROCKET" so I guess it's propane.

Unknown: Does the rocket require both butane and propane? I think the second step was just to show that you don't get very much gas out of a little butane canister, and all you really need is the propane from the last step.

  • Final step: invert the bottle, and some reaction pushes Coca-Cola out the mouth of the bottle with enough force that the whole bottle flies quickly as a rocket.

What is going on here? How does inverting the bottle trigger the reaction? Is this like the "Mentos and Diet Coke" thing where the trick was to get the carbon dioxide dissolved in the drink to come out quickly?

My understanding is that the surface of Mentos candy provides a bunch of nucleation sites for bubbles, causing a lot of carbon dioxide to come out of solution quickly.

So, does the "kreosan" rocket fly on just carbon dioxide or is there some other reaction going on here?

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  • $\begingroup$ You should ask your question in PSE. Obviously its also chemistry but I don't think chemistry will get you the entire answer. :) $\endgroup$ – Hexacoordinate-C Sep 6 '15 at 22:23
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    $\begingroup$ The physics is rather obvious: gas expands at high speed, bottle flyes. What's a lot more interesting is the chemical reaction causing such a high increase in pressure... $\endgroup$ – Massimo Sep 9 '15 at 21:17
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Here's a paper written about propane forming small ice crystals when sprayed into a cloud of water vapor: James R. Hicks and Gabor Vali, J. Appl. Meteor., 1973, 12, 1025–1034.

I'm no expert, but I'm guessing the propane forms small ice crystals as it passes through the Coca-Cola, and when flipped, the Coca-Cola warms up the liquid propane, which causes it to expand as it travels upwards. This provides an immense surface of nucleating sites, similar to a Mentos reaction, but with millions of microscopic Mentos (this means larger surface area). The pressure waves caused by the violent nucleation further drives the reaction, as the liquid propane expands further and freezes more ice crystals and so on. This all happens quite fast, and with one easy way out, everything shoots out the mouth of the bottle.

Of course this was written by a Materials Engineering undergrad, so don't take this explanation too seriously.

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  • $\begingroup$ Seems plausible. Also in the video there is some white semi-solid layer visible above the Coca-cola surface. ... and probably only brave Russians would launch rockets from hand ;) $\endgroup$ – ssavec Sep 10 '15 at 6:58
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This behavior has little to do with using a carbonated beverage. Try search terms butane water rocket to see plenty of other examples of this.

The phenomenon that is happening is the large volume difference between liquidus and gaseous butane (with maybe some $\ce{CO2}$ to boot) forces the dense liquid water out of the vessel at a high pressure, and Newton's First Law takes care of the motion of the plastic bottle relative to its liquid contents.

The Steps:

Adding butane

When the butane is introduced into the container, it is an expanding vapor, which loses some energy from the expansion. Butane condenses into a liquid at about $0°\!\mathrm{C}$, and so when you add butane from the compressed can some of it condenses into the liquid from the heat loss due to expansion, aided by contact with the presumably cold (but still $>0°\!\mathrm{C}$) surface of the cola. Butane's density is about $0.6\,\mathrm{g}\,\mathrm{cm}^{-3}$ and so it floats on top of the sugar solution.

Releasing the container

When the vessel containing the two liquids (cola on bottom, butane nearer the mouth) is inverted, the (relatively) warm water (or cola, or otherwise) comes in intimate contact with the liquid butane. The high surface area of the interaction causes the temperature of the butane to be immediately raised above $0°\!\mathrm{C}$, and is given plenty of energy to enter the gas phase. In the case of cola, I suppose it is possible that during all of this, droplets of butane are providing nucleation sites for $\ce{CO2}$ as well, but as the whole effect can be shown to happen with water, it is rather beside the point.

The change in volume

As mentioned, the density of liquid $\ce{C4H10}$ is about $0.6\,\mathrm{g}\,\mathrm{cm}^{-3}$, so with a molecular weight of about $58\,\mathrm{g}\,\mathrm{mol}^{-1}$ and an ideal gas volume of $22.4\times10^3\,\mathrm{cm}^3\,\mathrm{mol}^{-1}$. Each $\mathrm{mL}$ of butane contains about $0.01\,\mathrm{mol}$ of $\ce{C4H10}$, and the liquid butane will expand to $230$ times the volume of the liquid that is added initially (using ideal gas approximation). If the liquid is about $1\,\mathrm{cm}$ deep and the diameter of the bottle is about $7\,\mathrm{cm}$, that is about $40\,\mathrm{cm}^{3}$ liquid, or about $10\,\mathrm{L}$ of gas.

If we take a look at the answer to another question about soda, we see that there is about $0.14\,\mathrm{mol}\text{ of carbon dioxide per }\mathrm{liter}\text{ of soda}$, and so if all of the $\ce{CO2}$ is released simultaneously (like the memetic Mentos example), it adds about $3\,\mathrm{L}$ of gas to our expansion.

The upshot:

The gas is aching to get out of the cola bottle, however now the liquid in the inverted bottle is standing between the gas and its equilibrium (atmospheric) pressure. The liquid, being a fluid, is forced out of the mouth of the bottle. Presumably the user had a loose grip on the vessel, which is then propelled in the opposite direction from the liquid plug with the equal and opposite momentum. For reference, since conservation of momentum states $\mathrm{\mathbf{p}} = 0 = m_1\cdot\mathrm{\mathbf{v_1}} + m_2\cdot\mathrm{\mathbf{v_2}}$ a $10\,\mathrm{g}$ soda bottle would travel fifty times faster than $500\,\mathrm{g}$ of liquid it is being forced to release, as in a partially empty bottle of cola.

Referring to the Wikipedia article on Water rockets, pressures from bike pumps ($75\,\mathrm{psi}$) and air compressors ($\mathop{>}200\,\mathrm{psi}$, said to be dangerous) are used to launch this type of rocket. Since we are starting from a non-enclosed vessel, we wouldn't really have static pressure but the $13\,\mathrm{L}$ of gas that we are generating (at final pressure $1\,\mathrm{atm}\approx 14.7\,\mathrm{psi}$) would have some equivalent. If confined to say, a $250\,\mathrm{cm}^3$ headspace, this would correspond to something like $750\,\mathrm{psi}$ to $1000\,\mathrm{psi}$!

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I have no professional knowledge about chemistry, so what's following are just my personal thoughts on the matter.

My first reaction to the video was "that's nonsense" but I have since seen other people repeating this, so I'm not so sure about that anymore.

Propane, Butane and Ethane have boiling points around or below $\pu{0 ^{\circ}C}$, so if there was any liquid (or solid) lighter fluid in the bottle before the reaction takes place it should be boiling. I haven't seen anything of that sort. Furthermore these compounds are nearly insoluble in water especially in the arrangement they used with a very small contact surface between the Cola and the gas. That's why I doubt that the explanation for the reaction is any kind of chemical reaction between the lighter fluid and the Cola.

If this is actually real, my best guess would be, that there is some kind of reaction taking place between the bottle wall and the gas (thermal shock or chemical reaction?) which then gives a similar reaction to the Mentos - Cola reaction just with a much bigger contact surface. I doubt however that you would get one liter of $\ce{CO2}$ from one liter of Cola (under normal conditions) and I'm not sure whether that would be enough to produce such a violent reaction, if you can't even replace all the liquid with gas.

It would be interesting (and dangerous, I suppose) to see this repeated with different kinds of containers (glass, aluminium can, etc.).

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  • $\begingroup$ Propane’s boiling point is around $-40~\mathrm{^\circ C}$, ethane’s around $-80~\mathrm{^\circ C}$. $\endgroup$ – Jan Oct 18 '15 at 16:58

protected by Community Dec 2 '15 at 2:07

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