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Q & A: Why liquid nitrogen is cold

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Most recent answer: 10/22/2007
Q:
why is liquid nitrogen cold? does it stay cold longer than water? if so why?
- nate (age 17)
chandler high school, v
A:
Well, an alternative way to phrase that question is: why does nitrogen have to be cold to become liquid? A related question is: why does nitrogen become liquid when itís cold? Nitrogen, below about 77 degrees Kelvin, at one atmosphere of pressure, will be a liquid. If the N2 molecules are moving slowly enough, then the attractive interactions between the molecules become important.

If two N2 molecules collide at high speed, they will bounce off of each other. During a high-speed collision, the molecules get close to each other. The force is repulsive because the electrons in orbit in the N2 molecule must occupy different orbits of higher energy when thereís another N2 molecule nearby (the electron orbits get pushed out of shape a little bit).

If the collision is much more gentle, then the colliding molecules will spend some time at a distance at which the force between N2 molecules is attractive. The forces between many different kinds of neutral atoms and molecules shares this feature. At large distances it is an attractive force, and at short distances it is repulsive.

As an analogy, even the Earth is a bit like this -- for objects farther from the center of the earth than the radius of the earth, the main force is an attractive gravitational force. Stand on the earthís surface, though, and you will feel a repulsive force thatís electromagnetic in nature, which pushes out. If youíre standing on something springy, and most things are a springy to an extent, pushing down makes you go down a little bit but only increases the strength of the upward force resisting your attempt to go down, pushing you back up. So there you stay, at a nice equilibrium radius, at the surface of the Earth.

At temperatures above 77 K, most collisions between N2 molecules involve so much kinetic energy that the molecules just bounce off and never settle in to the equilibrium radius. When N2 forms a liquid, the molecules donít always fly off after a collision but spend some time attracted to each other. If N2 forms a solid (lower temperature still), the molecules arrange themselves in an orderly pattern in three-dimensional space, all at equilibrium spacings from each other.

N2 has a lower specific heat than water, and the heat of vaporization is also lower than waterís, so it takes less heat energy to vaporize a gram of N2 than to vaporize a gram of water.

The rate at which something heats up depends on the rate at which heat energy is transferred in to the material. This has a lot to do with the outside temperature and whether thereís any insulation in between. Water melts and boils at such higher temperatures than liquid N2, that almost always the "outside" temperature is closer to the water temperature than the liquid N2 temperature. Newton found that the rate of heat flow is proportional to the temperature difference, and so heat usually flows in faster from hot objects to cold objects than from lukewarm objects to cool objects, but of course insulation is important.

Of course if your "outside temperature" is very low, then both the nitrogen and the water will stay cold indefinitely. Both water and nitrogen on the surface of Pluto will stay cold.

Tom

(published on 10/22/2007)

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