How liquid Nitrogen fuels a hand-launched bottle rocket

If you have ever handled liquid nitrogen (well, you have certainly not touched it without painful freezing damage to your skin), you have worked with one of the coldest chemicals known to man. To keep it cold, you probably carried it in a dewar, which is the name used by scientists for a vacuum flask, or thermos. The dewar consists of two flasks, placed one within the other and joined at the neck. The gap between the two flasks is partially evacuated of air, creating a near-vacuum which prevents heat transfer by conduction or convection.

The dewar is named after its inventor:

Dewar, Sir James. 1842-1923, Scottish chemist and physicist. He worked on the liquefaction of gases and the properties of matter at low temperature, invented the vacuum flask, and (with Sir Frederick Abel) was the first to prepare cordite

Source: Sir James Dewar

Liquid nitrogen is used in cryogenics, i.e., to freeze things. Nitrogen makes up 78% of earth’s atmosphere. As it is plentiful, it is used for cryogenics instead of other elements which are rarer. Is Anything Colder than Liquide Nitrogen?

As a word of caution, if spilled in a closed space, liquid nitrogen can cause suffocation without a person being aware of anything unusual occurring. For this reason, it is emphasized by Thompson that liquid nitrogen should be used out of doors.

Another warning: Although it is used to cool food, without problems as it evaporates quickly, if liquid nitrogen is actually ingested before it evaporates into gas, it can cause serious injury.

Use of liquid nitrogen in making ice cream is described here: Follow Instructions when Making Ice Cream Cold Using Liquid Nitrogen.

As to its use as a fuel for the bottle rocket, the theory of bottle rockets is well explained in this article: Bottle Rocket Science.

So, you have seen that Grant Thompson, in his video, “5 Crazy Science Stunts You Won’t See at School,” has placed some liquid nitrogen to float on water. The liquid nitrogen evaporates into its gaseous form, since it boils and becomes vapor unless kept very cold. If he plugs a bottle filled with water and liquid nitrogen and turns it upside down, the liquid nitrogen still floats on the water, so it goes to the bottom of the upside-down bottle.

The liquid nitrogen becomes a gas, due to exposure to warmer substances. As it occupies a small space, it becomes more and more compressed as it heats and expands due to heat, so that it will push out the water in the bottle, if not held in by a plug.

When Thompson removes his thumb from the mouth of the upside-down bottle, that act hand-launches the bottle as a bottle-rocket, as the compressed gas expands rapidly, pushing out the water.

According to Wikipedia, it is not the nitrogen gas that propels the bottle rocket. The nitrogen gas becomes very compressed, and stores the potential thrust energy. Wikipedia clarifies that it is the water (which does not compress), expelled by the force of the compressed nitrogen gas, that actually pushes the bottle upwards, in accordance with Isaac Newton’s “Third Law of Motion,” which states: To every action there is always opposed an equal reaction. . . . Third Law of Motion.