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I saw in the YouTube video Don't Mix Coke with Liquid Nitrogen! that when you fill a bottle part way with water then pour liquid nitrogen into it then turn it upside down, it blasts off at high speed. Why does that happen? How is it possible?

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I cannot figure out the answer for sure but here's my theory. According to the article Liquid Nitrogen Is Beautiful When It's Dancing Across Gasoline, liquid nitrogen is less dense than water. I'm guessing that means at first, the liquid nitrogen floats on the water. When it's turned upside down, the liquid nitrogen starts mixing with the water but surface tension limits its rate of mixing. However, the mixing still occurs to some extent which boils some of the liquid nitrogen creating thrust. Once it starts accelerating, the acceleration speeds up the mixing which in turn creates an even stronger thrust.

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    $\begingroup$ Suggest you add a line to say when the nitrogen boils its volume increases 1000-fold so creating large pressure on the water, and the momentum of the squirting water is consequently enough to cause a big rocket effect. $\endgroup$ Jul 20, 2019 at 8:28
  • $\begingroup$ @AndrewSteane Once the bottle starts accelerting so quickly, some of the liquid mixture might be under extreme pressure and it probably doesn't expand by that large a factor anymore under extreme pressure. The higher pressure probably also raises the boiling point of the liquid nitrogen. It's probably actually the vapour pressure of the nitrogen at the temperature it's at that determines how much force it will exert, not the amount it expands. I do also realize that evaporation absorbs heat. I think the acceleration makes the surface tension become a smaller factor so the liquid nitrogen forms $\endgroup$
    – Timothy
    Jul 20, 2019 at 16:38
  • $\begingroup$ very tiny droplets but the acceleration also speeds up the mixing so the liquid nitrogen ends up absorbing energy from the water very fast. $\endgroup$
    – Timothy
    Jul 20, 2019 at 16:39
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When the bottle is turned upside down, the liquid nitrogen floats to the top and absorbs heat from the water. It therefore boils, and creates pressure in the air space at what is now the top of the bottle. This pressure then blasts the water out of the narrow end at high speed, and the bottle is compelled to obey Newton's 3rd Law of Motion (i.e. for every action there is an equal but opposite reaction).

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  • $\begingroup$ That's basically so simple and reasonable.. these are the best answers. $\endgroup$
    – Ben
    Sep 14, 2020 at 5:16

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