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cpbayern24 t1_j37vbwz wrote

If you're talking about individual water molecules, they should be moving at very high speeds (average speed should be around 500 m/s at 20C) while breaking and reforming H bonds with other water molecules. However they do stay in their relative position when water freezes.

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Verneke t1_j37w2al wrote

Do you have a source referencing the local velocities? I'm curious to learn more detail

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[deleted] t1_j37xy5g wrote

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Movpasd t1_j3bx7ym wrote

> How the temperature relates to other state variables is different (and difficult!) in liquids but that doesn't apply to kinetic energy

Is there a simple explanation for why this is the case? Given the presence of intermolecular potentials (which are not quadratic terms), I wouldn't expect equipartition to hold. Is the argument that this effect is negligible, and if so, how does one argue that it is?

Furthermore, does your calculation account for vibrational and rotational modes?

If you could point me to sources that cover these questions, I'd be very grateful. Thanks.

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ForeverInQuicksand t1_j386kr9 wrote

What if you took a 2ft pipe, that can be capped at both ends, and placed a valve you could open and close in the middle. Then you could place a small valve that allows a drop at a time to fall on both ends.

If you filled the left side with pure deuterated water, and the right side with pure water made with only normal hydrogen-1, and then opened the valve in the middle, while simultaneously collecting and isolating single drop samples of the water at each end of the tube over time.

By testing the samples in a mass spectrometer, wouldn’t it be possible to measure the deuterated water composition of each drop to see how long it would take both sides of the tube to release drops of the same d2O/H2O composition.

If the water molecules are distributing at a rate of 500m/s, there would be near instantaneous mixing of the two water types, as soon as the two samples touched.

I don’t think that would be the case.

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LiveNeverIdle t1_j388gxw wrote

The individual molecules travel that fast, but soon bump into other molecules. Those other molecules get bumped and speed off into still other molecules. So the molecules move very fast but don't travel very far. Eventually all of the water would mix though, which we call diffusion.

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EPIKGUTS24 t1_j39wqup wrote

It'd also mix via convection as deuterated (heavy) water is, well, heavier than regular water.

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ELDOR-King t1_j395hq5 wrote

the individual molecule speed is not necessarily the same as diffusion, as the movement is disordered. you can very easily measure this diffusion with NMR. (provided you have an NMR with pulsed field gradients. No need for valves or deuterium labelling etc.)

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abat6294 t1_j3archb wrote

Fun fact. The speed of sound through a substance is dictated by the average speed of the molecules within that substance. Speed of sound in air is about 750mph, so the average speed of each air molecule at any given moment is 750mph.

But they only go extremely short distances before bouncing of another molecule and going another direction.

Edit: The average speed of air molecules is actually closer to 1000mph at room temperature.

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fastspinecho t1_j3axbm2 wrote

Almost, but not quite. The average speed of air molecules at room temperature is 1000mph.

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slashdave t1_j39rebm wrote

Speed doesn't tell you very much, since they aren't traveling in a straight line.

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TurboTurtle- t1_j3a9s4w wrote

Yeah. Is this the speed of vibration or larger scale movement?

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badatmetroid t1_j3c1xpq wrote

They move (on average) at 500m/s but they are bouncing back and forth randomly. On average a molecule moves away from it's starting location at the square root of time. So if it takes 1 us to move 1 um then it takes 4 us to move 2 um, 1e6 us to move 1e3 um and 1e12 us to move 1e6 um (or 1 million seconds to move 1 meter).

I pulled those numbers out of my ass but if you know the root mean velocity (your 500m/s number) and the root mean path (average distance until collision) you can use the two numbers to derive the diffusion coefficient from first principles.

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pzerr t1_j3azuh0 wrote

How do they not create a great deal of friction/heat when moving around?

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KingSupernova t1_j3i3zac wrote

Friction against what?

Heat is the average movement speed of the particles. The average isn't going to spontaneously increase without external energy being put into the system.

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[deleted] t1_j38nfo2 wrote

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cdstephens t1_j396e66 wrote

For a high temperature gas, it would eventually instead look like a Maxwell-Jüttner distribution.

https://en.wikipedia.org/wiki/Maxwell%E2%80%93J%C3%BCttner_distribution

This happens when kT ~ mc^2 , so when the temperature is close to the rest mass energy of the particle.

However, there are effects this new distribution don’t take into account, so it has limited applicability.

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