Science Questions: Beyond the Scope of the MCAT

[Nutmeg]

It's the BSM thread! This is the place where people can seek help with their homework or ask for help understanding questions where the answer isn't necessarily expected of people taking the MCAT. Since most premeds have similar sorts of undergraduate majors, hopefully if anyone needs help in their undergraduate homework or wants further clarification, then they can ask the question here and hope for some help. Of course, there's no guarantee that someone can answer any given question, but it's here now, and you can give it a try.

Acceptable topics include:

• Advanced science questions in bio, physics, chemistry, etc
  Mathematics
  Engineering
  Psychology
  Cognitive science

Unacceptable topics include:

• Anything actually required for the MCAT
  Non-science questions

My undergraduate majors were chemical engineering and molecular and cell biology (neuro emphasis). Hopefully some of the bright people on these boards will drop in regularly. Even if you can't help, you might learn something, and that's basically what this thread is all about.

Cheers!

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[QofQuimica]

I have a question. I'm taking a Genetics night class and we are studying chromosomal alterations, specifically uniparental disomy.

Why does non-disjunction in meiosis I create a gamete with two homologous non-identical chromosomes, while non-disjunction in meiosis II creates a gamete with identical ones? It seems like both would create a gamete with identical homologous chromosomes. What am I missing here about meiosis?

I understand that these scenarios both result in UPD, the first in heterodisomy and the second case in isodisomy, but I don't understand why the nonidentical/identical chromosome situations arise in the first place.

It has to do with how the chromosomes separate during each stage of meiosis. Remember that in meiosis I, the homologs are being separated from each other. If there is non-disjunction, the homologs, which are not identical, stay together in the same cell. This gives rise to a cell w/ two non-identical chromosomes (assuming that the sister chromatids of each homolog separate normally during meiosis II).

After meiosis I, the homologs are in separate cells, but the chromosomes are still paired with their sister chromatids. If these don't separate in meiosis II, then the cell will have two identical copies of the chromosome b/c both copies come from the same homolog. This gives rise to a cell w/ two identical chromosomes.

If this is difficult for you to conceptualize, look at a diagram of meiosis. Pay attention to when the non-identical homologs separate from one another (during meiosis I) versus when the identical sister chromatids separate from one another (during meiosis II).

 

[NapeSpikes]

Umm, okay I think I get it.

So the second part, meiosis II, I understand--the sister chromatids are identical and if there's nondisjunction, it results in two identical chromatids in the gamete.

In meiosis I, the homologs are different (from crossing over?), and if there's nondisjunction, we'll have two homologs in the same cell. Is that right?

Thanks, Q!

 

[QofQuimica]

Umm, okay I think I get it.

So the second part, meiosis II, I understand--the sister chromatids are identical and if there's nondisjunction, it results in two identical chromatids in the gamete.

In meiosis I, the homologs are different (from crossing over?), and if there's nondisjunction, we'll have two homologs in the same cell. Is that right?

Thanks, Q!

Even if you neglect crossing over (which I wasn't even considering in my earlier argument), the homologs will not be identical. Remember that each homolog comes from a different parent. So if we are talking about chromosome #3, for example, you have two copies of it: one from your mom, one from your dad. Since your mom and dad are not genetically identical (I hope! 😱 ), their chromosomes are also not identical. Hence, homologs are similar to each other, but are not identical. If the two homologs end up in the same gamete, they will be two similar but non-identical copies of the same chromosome.

 

[NapeSpikes]

Oh yeah! I think. Homologs are from different parents. So if I use myself as the example, if nondisjunction occurs in meiosis one, my gamete will have an extra say, chromosome 15. One from mum and one from dad, but normally, there should only be one chrom 15, from mom OR dad (but really a hybrid from crossing over). When this (#15 from my mom and dad) is passed on to my offspring, if the zygote, with three chrom 15s, gets rid of the wrong chrom 15 (the one from my wife), then we have uniparental nondisjunction, and more specifically paternal heterodisomy.

Do I have it?

 

[QofQuimica]

Oh yeah! I think. Homologs are from different parents. So if I use myself as the example, if nondisjunction occurs in meiosis one, my gamete will have an extra say, chromosome 15. One from mum and one from dad, but normally, there should only be one chrom 15, from mom OR dad (but really a hybrid from crossing over). When this (#15 from my mom and dad) is passed on to my offspring, if the zygote, with three chrom 15s, gets rid of the wrong chrom 15 (the one from my wife), then we have uniparental nondisjunction, and more specifically paternal heterodisomy.

Do I have it?

I follow you up to the point where you talk about the zygote "getting rid" of one of the chromosomes. 😕 It is the GAMETE that is disomic if it has an extra copy of the chromosome, not the zygote. If one gamete that fused to make that zygote had two copies of chromosome 15, you are going to wind up with a zygote having trisomy 15. That assumes, of course, that an embryo with trisomy 15 is viable. I have no idea whether it is.

 

[liverotcod]

That assumes, of course, that an embryo with trisomy 15 is viable. I have no idea whether it is.

Complete trisomy 15 is lethal.

 

[QofQuimica]

Complete trisomy 15 is lethal.

Thanks. I thought it might be.

 

[NapeSpikes]

I follow you up to the point where you talk about the zygote "getting rid" of one of the chromosomes. 😕 It is the GAMETE that is disomic if it has an extra copy of the chromosome, not the zygote. If one gamete that fused to make that zygote had two copies of chromosome 15, you are going to wind up with a zygote having trisomy 15. That assumes, of course, that an embryo with trisomy 15 is viable. I have no idea whether it is.

Thanks, Q. I think I got it. I was indeed talking about the zygote. The gamete is disomic, too, but after fertilization, the zygote is UNIparental disomic 15 in my example, trisomic 15 all together.

What I meant was, if the zygote were to try to "correct" itself (trisomic rescue) but instead got rid of the wrong set of chrom 15, it would create a UPD 15 offspring. I think.

Sorry, most of genetics is brand new to this former english major, so I may not have the best phrasing.

 

[Lindyhopper]

Ek's & TPR emphasize that it always requires energy to break a bond. This endogonic "breaking" of bonds pushes the reaction to the transition state. If the overall reaction is exogonic the subsequent forming of new bonds must release more energy than Ea.
I wonder about some possible implications of the breakdown of a compound into its standard state elements. Consider some compund with a positive delta G of formation. In the reserve reaction the compound will be broken down into standard state elements while freeing energy. I don't understand where this freed energy comes from as no new bonds are formed. Does the elements electron reconfiguration into its SS from the transition state free energy?

Thanks,

 

[QofQuimica]

Ek's & TPR emphasize that it always requires energy to break a bond. This endogonic "breaking" of bonds pushes the reaction to the transition state. If the overall reaction is exogonic the subsequent forming of new bonds must release more energy than Ea.
I wonder about some possible implications of the breakdown of a compound into its standard state elements. Consider some compund with a positive delta G of formation. In the reserve reaction the compound will be broken down into standard state elements while freeing energy. I don't understand where this freed energy comes from as no new bonds are formed. Does the elements electron reconfiguration into its SS from the transition state free energy?

Thanks,

I am going to move these posts to the Questions Beyond the Scope of the MCAT thread, because this is way beyond what you need to know for the MCAT.

I am not sure about the answer to your question, but my guess is that many of these compounds with positive free energies of formation are comprised of elements that come in pairs in their standard states. For example, if you dissociate water, you don't get individual hydrogen and oxygen ATOMS; you get diatomic hydrogen and oxygen MOLECULES. So you do then have a scenario where you would be forming bonds if you reverse the reaction and convert a compound like water to its standard state elements. Maybe Dr Chandy or nutmeg will have some other ideas.

 

[BoyGenius]

HI

I was wondering what the explanation was to a question posed a long time ago. Please explain in depth. A bubble is rising in a bucket of water or beer bottle or whatever. This tests two concepts, I'm not exactly sure what they were. One was Bernoullis equation (wheter its speed increases or decreases) ,since it is a moving fluid, and the second thing was pressure on it or something. If you know what I am referring to please explain everything. 😉

 

[DrChandy]

HI

I was wondering what the explanation was to a question posed a long time ago. Please explain in depth. A bubble is rising in a bucket of water or beer bottle or whatever. This tests two concepts, I'm not exactly sure what they were. One was Bernoullis equation (wheter its speed increases or decreases) ,since it is a moving fluid, and the second thing was pressure on it or something. If you know what I am referring to please explain everything. 😉

Speed of the bubble will increase
(Pressure and velocity of a fluid at a given point are inversely related according to bernoulli's principle)

Pressure on the bubble will decrease
(Pressure=dgh where d=density of water/beer, g=acceleration due to gravity, and h=depth of water/beer)

 

[BoyGenius]

Speed of the bubble will increase
(Pressure and velocity of a fluid at a given point are inversely related according to bernoulli's principle)

Pressure on the bubble will decrease
(Pressure=dgh where d=density of water/beer, g=acceleration due to gravity, and h=depth of water/beer)

Although the pressure explanation makes sense to me the speed one does not. As the bubble rises its "height" and therefore elevation head increase so from Bernoulli's eqn. does this mean that it does not "speed up as much as it should?" A little confused, please clarify. Also, does the bubble expand as it rises because pressure decreases?

 

[DrChandy]

Although the pressure explanation makes sense to me the speed one does not. As the bubble rises its "height" and therefore elevation head increase so from Bernoulli's eqn. does this mean that it does not "speed up as much as it should?" A little confused, please clarify. Also, does the bubble expand as it rises because pressure decreases?

Just know that for problems which test bernoulli's concept, or where pressure and velocity of a fluid are involved, if pressure on the fluid is high, then the velocity of the fluid will be low. If pressure on the fluid is low, then the velocity of the fluid will be high.

The bubble will expand as it rises due to the pressure decrease of the surrounding fluid.

 

[Caboose]

Hey there Smarties,

Today, my light-bulb went out.

I tried to stare at it to make it go back on, but it didn't work, so I went to replace it. "Say, what are these mysterioius numbers...?" If I have a 100Volt/36W resistor, (my burned out bulb), does that mean I=.36? If so, what will stop me from turning the light on for various increments of time, calculating the use of AC current in the wee hours of the morning?
I wanted to know if these equations are really applicable or if 0.36 is a bunch of pish-posh.

Keep on rockin' in the free world,
Caboose.

 

[QofQuimica]

Hey there Smarties,

Today, my light-bulb went out.

I tried to stare at it to make it go back on, but it didn't work, so I went to replace it. "Say, what are these mysterioius numbers...?" If I have a 100Volt/36W resistor, (my burned out bulb), does that mean I=.36? If so, what will stop me from turning the light on for various increments of time, calculating the use of AC current in the wee hours of the morning?
I wanted to know if these equations are really applicable or if 0.36 is a bunch of pish-posh.

Keep on rockin' in the free world,
Caboose.

I'm assuming that you used P=IV to come to that conclusion. The only thing I'm unsure about is whether you can use that equation for AC current. If your house were wired for DC, I'd feel more confident in saying that you're right. 😉 Ok, I'm sure about one thing: this will definitely NOT be on the MCAT. Moving to the Outside the Scope of the MCAT Thread. 🙂

 

[Lests55]

OK...Big Bang Theory and 2nd Law of Thermodynamics...At the beginning of the universe, the universe was the "system"...so it had a certain level of disorder at that point. How could the disorder of the universe increase (when in fact it is the system with no "universe" to create disorder in) when all the planets had to collide, evolution etc....

 

[Nutmeg]

OK...Big Bang Theory and 2nd Law of Thermodynamics...At the beginning of the universe, the universe was the "system"...so it had a certain level of disorder at that point. How could the disorder of the universe increase (when in fact it is the system with no "universe" to create disorder in) when all the planets had to collide, evolution etc....

Don't try to make sense of the Big Bang. All of our laws of the universe break down when you try to trace back to a singularity.

 

[Caboose]

I keep mysteriously trying to make the lightbulb go on... I've converted to the efficient use of flourescent bulbs.
When the lightbulb goes on before April 22nd and it flutters a little when you turn it on - does that have anything to do with the phosphor coating moving it's electrons to a higher energy level? I just wondered about the enchanting flickering that occurs in the wee morning hours. Anybody wanna take the Pepsi challenge?

Caboose.

oh - thanks for the help on the MCAT section, Q. 🙂

 

[j-med]

Glycoproteins' sugar residues are found exclusively in the extracellular domain, yes?

If so, is there a reason for this?

 

[DrChandy]

Glycoproteins' sugar residues are found exclusively in the extracellular domain, yes?

If so, is there a reason for this?

Sugar residues serve as ID tags for the specific cell they are present on.
A specific cell type will have specific glycoprotein sugar residues. Sugar residues help the immune system identify self cells from non-self cells.

 

[Caboose]

" ...formation of the eye, where the optic vesicles induce the ectoderm to thicken and form the lens placode, which in turn induces the optic vesicle to form the optic cup, which in turn induces the lens placode to form the cornea and the lungs."

...Seriously? The lungs?

Caboose.

p.s. Look what I found!

 

[QofQuimica]

" ...formation of the eye, where the optic vesicles induce the ectoderm to thicken and form the lens placode, which in turn induces the optic vesicle to form the optic cup, which in turn induces the lens placode to form the cornea and the lungs."

...Seriously? The lungs?

Caboose.

p.s. Look what I found!

You can literally see yourself breathing, I suppose.

 

[Caboose]

You can literally see yourself breathing, I suppose.

Caboose.

 

[Caboose]

I love bio.
O.k., so Strontium is incorporated into bone better than Calcium. It makes a complex with phosphate, yeah? ...but Sr has a larger radius and is less electronegative than Ca - any thoughts on why osteoblasts would like to add that element more?
Caboose.

 

[rcd]

How does pyridinium tribromide react?

I mean I see an aromatic 6-membered ring with an N+ in it. I also see a "Br-Br=Br(-)". And then I magically see a Br(-) ionically on the N+, and a Br-Br. ??? So the ring supplies nucleophile Br, and the Br-Br supplies electrophile Br?

 

[Krazykritter]

Not to be too harsh here, but on the subject of Organic...You know it for the MCAT & forget it for the rest of your life. I have nearly finished my M1 year and I can honestly tell you that I haven't thought about Orgo since the MCAT. I haven't taken pharm yet, but I'm almost 100% sure that you don't need it.

So if this is a question you have in your MCAT studying, stick to the basics and you will be fine.

 

[liverotcod]

on the subject of Organic...You know it for the MCAT & forget it for the rest of your life.

At least you try to. In both biochem and physiology this (my M1) year, instructors keep talking about the "benzene rings of steroid hormones." I haven't had the nerve to disagree.

 

[bdyoon]

I have a problem about finding out a chemical structure (C16H24O2).
I have an IR, H nmr, and C nmr...
I checked all over, books, websites etc. I also tried to figure it out myself of course:
IHD:5
H-nmr shifts are :1.11, 2.49, 2.70, 3.08, 5.38 (does this mean no aromatic H's??)

Would anyone be willing to take a look at 'em?? Someone with some time to kill?

I kinda have an idea but stuck at some points (with the multiplicity, symmetry parts etc.) I learned the stuff last semester and its somewhat rusty.
If anyone who's good at this want to give it a try, please let me know. I can email you the scanned paper. I would appreciate anyone's help.
I'll try different forums as well....

stressed...

 

[bdyoon]

I have a problem about finding out a chemical structure (C16H24O2).
I have an IR, H nmr, and C nmr...
I checked all over, books, websites etc. I also tried to figure it out myself of course:
IHD:5
H-nmr shifts are :1.11, 2.49, 2.70, 3.08, 5.38 (does this mean no aromatic H's??)

Would anyone be willing to take a look at 'em?? Someone with some time to kill?

I kinda have an idea but stuck at some points (with the multiplicity, symmetry parts etc.) I learned the stuff last semester and its somewhat rusty.
If anyone who's good at this want to give it a try, please let me know. I can email you the scanned paper. I would appreciate anyone's help.
I'll try different forums as well....

stressed...

Thank you for moving this here. I didn't even know about this forum.

 

[scpod]

H-nmr shifts are :1.11, 2.49, 2.70, 3.08, 5.38 (does this mean no aromatic H's??)

With the limited information, it's tough, but I would expect to find a fatty acid. The 5.38 is probably from the COOH at one end. The 1.11 is from CH3 at the other end. The heights og the others would help you determine if you are looking at = or - bonds in the middle. Just my best guess with limited information...

 

[Dave_D]

I've got a question that I've been wondering about.(Only for my own edification.) Why does cooling slowly when doing recrystallization generally work better(less garbage) than going fast? Not a big deal since I'm done with my premed course work, I was just curious.

 

[QofQuimica]

I've got a question that I've been wondering about.(Only for my own edification.) Why does cooling slowly when doing recrystallization generally work better(less garbage) than going fast? Not a big deal since I'm done with my premed course work, I was just curious.

If you cool slowly, the molecules will selectively prefer to surround themselves with similar molecules (other product molecules) to form a regular lattice that tends to exclude the differently-shaped impurities. On the other hand, if you precipitate the product quickly, the impurities tend to crash out of solution along with the product, and they become incorporated into your crystal because the crystals form too quickly for selective lattice formation to occur.

 

[Dave_D]

If you cool slowly, the molecules will selectively prefer to surround themselves with similar molecules (other product molecules) to form a regular lattice that tends to exclude the differently-shaped impurities. On the other hand, if you precipitate the product quickly, the impurities tend to crash out of solution along with the product, and they become incorporated into your crystal because the crystals form too quickly for selective lattice formation to occur.

Geez, I should have known that already. I mean they kind of covered that on the sugar episode of Good Eats. 🙂 Thanks Q

 

[pezzang]

If presynaptic neuron is placed closed to axon, which way would AP (if any AP is produced) proceed? I believe AP in axon can travel in both directions so what would determine the direction in which the AP will proceed?
AP in axon travel because of the depolarization of Na+ channels in axon, right? So if presynaptic neuron was placed near an axon and it secretes NT that will open Na+ voltage gated channels, AP will occur. So what direction?

Thank you for your help in advance.

 

[Ishzter]

I'm not sure if I completely understand your question but I'll take a stab at it. If an action potential is high enough, meaning the NTs activate the ligand gated channels. The action potential always travels along the axon to the synapse, whether the synapse is a neuromuscular junction or another neuron. The AP cannot travel backwards because the ion channels have a brief inactivation period where the cannot be repolarized hence an intrinsic directionality in AP. I hope this helps.

 

[gridiron]

I'm not sure if I completely understand your question but I'll take a stab at it. If an action potential is high enough, meaning the NTs activate the ligand gated channels. The action potential always travels along the axon to the synapse, whether the synapse is a neuromuscular junction or another neuron. The AP cannot travel backwards because the ion channels have a brief inactivation period where the cannot be repolarized hence an intrinsic directionality in AP. I hope this helps.

You are right. If a certain threshold is met, than a action potential will travel down the axon hillock to the synapse where calcium influx will lead to the release of neurotransmitter. The threshold, usually about -50mv, is a all or nothing response--meaning, if it is met than a action potential will be generated.

 

[debz957]

how could we explain the irritation in the eyes after swimming in the sea? how would an eye drom ease this particular irritation?

 

[grapeflavorsoda]

well, there is this article on JACS(08/02/2006) about the new technique of introducing small oligonucleotides into cell by linking 15-20mers with cationic peptides(variable repetition of KKKR repeats).

i was wondering what are the common non-viral protocols that are currently used to put siRNA/miRNA into cells.

 

[Krazykritter]

If you are wondering about stuff like siRNA/miRNA then I suggest you GET A NEW HOBBY!!!

Of course I am just kidding & also of course, I have no clue about that.

 

[grapeflavorsoda]

If you are wondering about stuff like siRNA/miRNA then I suggest you GET A NEW HOBBY!!!

Of course I am just kidding & also of course, I have no clue about that.

just wondering how much this new protocol is better than traditional cationic non-viral vectors(so called "proton sponge").

 

[neurodoc]

I'm sure we all know about Maxwell's Demon, that impish little fellow who has the ability to allow faster moving molecules to pass through the trap-door he controls so as to effect a temperature change between two chambers that are initially at the same temperature. In so doing, the demon would apparently violate the Second Law of Thermodynamics. The Demon thus presents a paradox.

The paradox needs to be explained, ideally within the context of Thermodynamics. The fact that JC Maxwell came up with the paradox should make us take the paradox seriously. He (along with W. Gibbs) were the geniuses who developed Thermodynamics.

It's easy to dismiss the paradox by saying that the Demon can't exist or can't do what he's supposed to do for various reasons. For example, some have argued that the Demon would need to have unaccounted-for energy he is not allowed to have to do what is expected of him, and others have argued that quantum uncertainties make his task impossible.

Unfortunately, these explanations for the impossibility of the Demon use arguments outside of Thermodynamics. They may or may not be correct, but I find them unsatisfying because they require arguments outside of Thermodynamics.

If you stay within classical Thermodynamics, I think that it is possible to accept that the Demon can do what Maxwell said he could, without violating the Second Law...

One way to accept the Demon would be to posit that for him to be able to do what he needs to do, he would need to be internally complex. One would need to calculate his complexity into the calculation of the entropy of the system in which he operates. If we do this calculation, I think that the Demon could do what Maxwell suggested (i.e. cause a temperature change in the chambers), but only to the degree that the Demon's complexity (his negative entropic value) allowed. No "violation" of the Second Law need occur, and no need to refer to any extra-Thermodynamic explanations. I think JCM would have appreciated this approach.

Any comments from you Physicists out there?

 

[Dave_D]

Here's a question for all you chemists out there. Are nitrogen molecules really smaller than oxygen molecules? I ask because my brother was watching some show on tinkering with your car and they said that was true. But from what I remember even though oxygen atoms are smaller the difference isn't much. Further more N2 forms a triple bond and O2 a double so I'd expect nitrogen molecules to actually be smaller.(Sounded like a lot of snake oil to me but what do I know.)

 

[QofQuimica]

Here's a question for all you chemists out there. Are nitrogen molecules really smaller than oxygen molecules? I ask because my brother was watching some show on tinkering with your car and they said that was true. But from what I remember even though oxygen atoms are smaller the difference isn't much. Further more N2 forms a triple bond and O2 a double so I'd expect nitrogen molecules to actually be smaller.(Sounded like a lot of snake oil to me but what do I know.)

I'm pretty sure that oxygen molecules are still smaller than nitrogen molecules in spite of only having a double bond. That's why you don't hear about people filling their tires with oxygen; the oxygen molecules would leak out of the tires faster than the nitrogen molecules do. Here's an article that backs me up, although admittedly it's from a lay source: http://www.cbc.ca/story/news/national/2006/08/22/nitro-tires.html

 

[Dave_D]

I'm pretty sure that oxygen molecules are still smaller than nitrogen molecules in spite of only having a double bond. That's why you don't hear about people filling their tires with oxygen; the oxygen molecules would leak out of the tires faster than the nitrogen molecules do. Here's an article that backs me up, although admittedly it's from a lay source: http://www.cbc.ca/story/news/national/2006/08/22/nitro-tires.html

Actually I looked into this a bit more and you're correct. What I found out is that the b in that "Vander Waals" equation corresponds to the size of the molecules. Oxygen's b is smaller than nitrogen's b, not by much but it's smaller. So oxygen really is smaller and would diffuse a bit faster.(I'm still surprised by this result but I really don't have an intuitive sense how big an atom is or how big a bond is relative to an atom.) Still, I've never really noticed my tires leaking pressure that fast and I use air. I mean I doubt I refill them more than once a month. Still I can think of a better reason not to fill your tires with oxygen😀 (BOOMA BOOMA) Anyway thanks for answering that one Q.

 

[Krazykritter]

Dave_D,

When you are using 'air' to fill your tires I think you are fogetting the composition of air that you are using which is approximately 78% Nitrogen & 20% Oxygen. Nitrogen is the most abdunant molecule in our atmosphere & all those machines are doing is compressing the air. I do not believe that this changes the composition at all so you are still filling your tires mostly w/ Nitrogen.

 

[QofQuimica]

Actually I looked into this a bit more and you're correct. What I found out is that the b in that "Vander Waals" equation corresponds to the size of the molecules. Oxygen's b is smaller than nitrogen's b, not by much but it's smaller. So oxygen really is smaller and would diffuse a bit faster.(I'm still surprised by this result but I really don't have an intuitive sense how big an atom is or how big a bond is relative to an atom.) Still, I've never really noticed my tires leaking pressure that fast and I use air. I mean I doubt I refill them more than once a month. Still I can think of a better reason not to fill your tires with oxygen😀 (BOOMA BOOMA) Anyway thanks for answering that one Q.

I was kind of surprised to see that it makes such a big difference also, not because of the size difference between the molecules being small, but because air is mainly composed of nitrogen (78% nitrogen versus 21% oxygen). Apparently if you live in Winnipeg, though, that 21% is enough to make a difference. 😉

 

[Dave_D]

Dave_D,

When you are using 'air' to fill your tires I think you are fogetting the composition of air that you are using which is approximately 78% Nitrogen & 20% Oxygen. Nitrogen is the most abdunant molecule in our atmosphere & all those machines are doing is compressing the air. I do not believe that this changes the composition at all so you are still filling your tires mostly w/ Nitrogen.

Well I knew air was mostly nitrogen. The thing is that people sometimes fill their tires with nitrogen instead of air for some alleged benefit. Yet the main reason they give, IE gas diffusing out of the tires, I've never noticed this being a problem with my own tires and I use air.

I was kind of surprised to see that it makes such a big difference also, not because of the size difference between the molecules being small, but because air is mainly composed of nitrogen (78% nitrogen versus 21% oxygen). Apparently if you live in Winnipeg, though, that 21% is enough to make a difference. 😉

Ok, so I guess you're not buying it much either. Actually the only person I've ever remembered having problems with tires holding pressure was my mom and it went away when she got new tires.(Her old ones were starting to go bald.) It just sounded like some people who tinker with their cars too much that justified it with some scientific sounding mumbo-jumbo. I guess I could check "The straight dope" and see if they've done anything on it.

 

[QofQuimica]

Ok, so I guess you're not buying it much either. Actually the only person I've ever remembered having problems with tires holding pressure was my mom and it went away when she got new tires.(Her old ones were starting to go bald.) It just sounded like some people who tinker with their cars too much that justified it with some scientific sounding mumbo-jumbo. I guess I could check "The straight dope" and see if they've done anything on it.

I wonder if it has anything to do with climate. Gases compress a lot due to decrease in temperature (Charles's Law 🙂) I'm from FL, and I'd never heard of using nitrogen to fill your tires until you brought up this question. It just isn't an issue here because it basically doesn't ever get cold here. I can't even tell you the last time I had to fill my tires; usually they just check the pressure when I get my oil changed every four months and they leave well enough alone. You're from MA, so you could have some issues due to temperature swings, but no doubt the changes you get are nowhere near as severe as the kind of climate they have in Winnipeg.

 

[deleted106503]

Formula 1 cars use Nitrogen in their tires because they heat up faster. At least that's what the race people say...

Anyways, I bet you could have nearly 100% N2 in your tires with just using compressed air. The first set will have 70% N2. Then as Oxygen diffuses out faster, you put in more compressed air. So the volume of new "displacing" air is 70% N2 instead of 0% N2 that left (I'm doing net out, which is the diff between O2 and N2 out). So each time you refill your tires with just a little more air, you're adding more N2.

I'm not sure if this is the case for me, but it's possible. When I bought my car new, I'd refill the tires literally about once every 1 or 2 months (I drove it 3,500 miles a year). Last time I filled my tires was back in April, 3 years after I had my car from new with the original tires (only 9,000 miles at that time). Still the same air in my tires, but they're not as deflated as they used to be 5 months later. Although I haven't driven as much recent since this test last Saturday. Driving probably makes you lose more air, too.

Edit: Maybe since it's been pretty constant temp here in Orlando for the last 5 months has kept the pressure the same? Another factor.