Physics question

[cjolley]

When I was a teacher I taught expansion due to raising tem by having the kids cram into a corner of the room then drew a chalk mark on the floor marking the edge of their feet.
Then I had them try to stay in place but do jumping jacks. Needless to say the group expanded.
Seemed to get the idea across.

It's just a run of the mill bad question. Cube root of the volume change is as good an answer as any, I guess.
Might be a good question on an IQ test. As a subject matter question it just sucks right out loud.

[UtwigMU]

When is the 1st grade of hi school. I don't have my chemistry books handy but we took gas laws, Brownian motion at physics and chemistry in 1st or 2nd year of hi school which is 15-19.

[Umfriend]

@cjolley: Oh, that kind of enlightening teaching is very rare here, but that was not really that different 30 years ago I think. How large were the classes you taught and what age-range?

@UtwigMU: Over here, you have 8 years of basic school from 4 to 12 yo, then you go to high-school, of which there are, basically, three types, (say, lower, mid, higher). My kid is in the first grade of the bridge-class where they'll decide on mid or higher.

Edit: Correction, they actually did something similar. Instead of jumping they had to flay their arms but not hit each other.

[Brian Ellis]

The question is totally invalid. For a 12-y.o. or thereabouts, the gas laws cannot apply to liquids. Liquid gases at their boiling point, assuming no air in the vessel, are in an equilibrium between the liquid and the gaseous phases. If the question applied to the gaseous phase above the liquid at the BP, assuming a standard pressure of 1,013.25 hPa at the unspecified room temperature of, say, 20 C, then we are talking about (to the nearest degree) of 293 K to 77 K. Assuming Boyle and company are correct over this range without any anomalies, I would guess the volume of an initial 170 l of gas would drop to 49.93 l. To assume that ~50 l in gaseous phase would drop to 1 l in liquid phase is specious. The density of liquid N2 is 0.807 g/ml at 77.35 K while the vapour density at 77.35 K is 4.622 g/l, so 49.93 l in vapour phase would have a mass of 230.8 g and this mass in liquid phase would have a volume of about 286 ml or 0.286 l.

Ergo, the premisses of the question are scientifically wrong (assuming that I have not made an error - always possible - and that my assumptions of the unknown data are reasonable and correct).

[Umfriend]

So, you are saying that 170 l shrinks to 0.286 l and therefore, obviously, the molecules' distance shrinks by a factor of 170 / 0.286 = 594.4 times approximately?

Just kidding

[dZeus]

So, you are saying that 170 l shrinks to 0.286 l and therefore, obviously, the molecules' distance shrinks by a factor of 170 / 0.286 = 594.4 times approximately?

Just kidding

That'd be assuming that volume change and change in distance between molecules are equivalent, which was also the original answer by the teacher

As you said yourself, my answer obviously is not correct either as it only takes into account average distance between every 2 molecules that are in at least two for the same planes (X, Y and Z plane).

Edit: did not see white highlighted text; explains a few things

[Brian Ellis]

So, you are saying that 170 l shrinks to 0.286 l and therefore, obviously, the molecules' distance shrinks by a factor of 170 / 0.286 = 594.4 times approximately?

Not necessarily. We assume that a molecule in gaseous phase is diatomic (N2) but is it at all temperatures? This implies there is a very strong covalent bond between the two atoms. However, is liquid nitrogen diatomic, as has been currently thought since I was a young engineer? The nitrogen atom is peculiar and there may be reason to think that in liquid phase some of the molecules may be triatomic (N3). In any case, the question presupposes that the covalent bond length is constant at all temperatures and phases; I don't know whether this true, but I pose the question on the grounds that, at high temperatures, the bond splits apart.

[cjolley]

I'm not at all sure the phase change or diatomic-triatomic factors matter as the distance between the atoms is averaged for this problem.
You are given the volume change and told the count of the atoms is held constant. Because it is a ratio the absolute distances will cancel out and give a dimensionless number.
I think

How large were the classes you taught and what age-range?

They were 12 to 20 students and 3ed year of High School, so around 16.

[Umfriend]

Not necessarily.

How about "Necessarily not."? My point was that apparantly a teacher of science is of the opinion that volume, a 3-dimensional concept, and distance, a 1-dimensional concept, are interchangeable. That the relationship between the two is linear.

[cjolley]

My point was that apparantly a teacher of science is of the opinion that volume, a 3-dimensional concept, and distance, a 1-dimensional concept, are interchangeable. That the relationship between the two is linear.

I sure hope you are wrong. That would just be sad.

[Brian Ellis]

@Umf

It would be amusing to see your kid's teacher's reaction if he were to read this thread!!!

[Dr Mordrid]

Liquid nitrogen is diatomic (LN2), which it retains in the solid state.

[Brian Ellis]

Liquid nitrogen is diatomic (LN2), which it retains in the solid state.

There is ample evidence that triatomic nitrogen exists in small proportions in the solid phase of nitrogen. It has not been proven in the liquid or gaseous phases, but neither has it been ruled out.